Securing a future for anonymous functions in Python

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Nick Coghlan

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Dec 30, 2004, 8:28:46 AM12/30/04
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GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
Getting rid of lambda seems like a worthy goal, but I'd prefer to see it dropped
in favour of a different syntax, rather than completely losing the ability to
have anonymous functions.

Anyway, I'm looking for feedback on a def-based syntax that came up in a recent
c.l.p discussion:
http://boredomandlaziness.skystorm.net/2004/12/anonymous-functions-in-python.html

Cheers,
Nick.

--
Nick Coghlan | ncog...@email.com | Brisbane, Australia
---------------------------------------------------------------
http://boredomandlaziness.skystorm.net

Paul Rubin

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Dec 30, 2004, 8:36:06 AM12/30/04
to
Nick Coghlan <ncog...@iinet.net.au> writes:
> Anyway, I'm looking for feedback on a def-based syntax that came up in
> a recent c.l.p discussion:

Looks like just an even more contorted version of lambda. It doesn't
fix lambda's main deficiency which is inability to have several
statements in the anonymous function.

Nick Coghlan

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Dec 30, 2004, 9:00:31 AM12/30/04
to Python List

Do you consider generator expressions or list comprehensions deficient because
they don't allow several statements in the body of the for loop?

Jp Calderone

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Dec 30, 2004, 9:06:26 AM12/30/04
to pytho...@python.org
On Fri, 31 Dec 2004 00:00:31 +1000, Nick Coghlan <ncog...@iinet.net.au> wrote:
>Paul Rubin wrote:
> > Nick Coghlan <ncog...@iinet.net.au> writes:
> >
> >>Anyway, I'm looking for feedback on a def-based syntax that came up in
> >>a recent c.l.p discussion:
> >
> >
> > Looks like just an even more contorted version of lambda. It doesn't
> > fix lambda's main deficiency which is inability to have several
> > statements in the anonymous function.
>
> Do you consider generator expressions or list comprehensions deficient because
> they don't allow several statements in the body of the for loop?
>

>>> (foo + bar
... for foo in range(13) if foo % 3 == 2
... for bar in range(16, 23) if 2 <= bar % 5 < 4)
<generator object at 0xb7dec10c>
>>>

Hmm. Two for loops and two if clauses. That's four altogether.
Does that qualify as "several"? :) Sure, they're not statements
according to the grammar, but that's entirely beside the point.

Jp

Nick Coghlan

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Dec 30, 2004, 9:19:29 AM12/30/04
to Python List

And that's an expression, and hence perfectly legal as the body of an anonymous
function.

So if we feel like abusing our anonymous functions, we use generator expressions
to get if statements and for loops, sys.stdout.write() for print statements,
sys.stdin.read() for input statements, anonymous functions for def statements,
and throw in our own functions with lazily evaluated arguments for anything that
isn't already covered by the interpreter core (which isn't much).

I just don't understand why people complain so much about the restriction to a
single expression in lambdas, yet there is nary a peep about the same
restriction for generator expressions and list comprehensions.

Jp Calderone

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Dec 30, 2004, 9:24:19 AM12/30/04
to pytho...@python.org
On Fri, 31 Dec 2004 00:19:29 +1000, Nick Coghlan <ncog...@iinet.net.au> wrote:
>Jp Calderone wrote:
> > On Fri, 31 Dec 2004 00:00:31 +1000, Nick Coghlan <ncog...@iinet.net.au> wrote:
> >
> >>Paul Rubin wrote:
> >>
> >>>Nick Coghlan <ncog...@iinet.net.au> writes:
> >>>
> >>>
> >>>>Anyway, I'm looking for feedback on a def-based syntax that came up in
> >>>>a recent c.l.p discussion:
> >>>
> >>>
> >>>Looks like just an even more contorted version of lambda. It doesn't
> >>>fix lambda's main deficiency which is inability to have several
> >>>statements in the anonymous function.
> >>
> >>Do you consider generator expressions or list comprehensions deficient because
> >>they don't allow several statements in the body of the for loop?
> >>
> >
> >
> > >>> (foo + bar
> > ... for foo in range(13) if foo % 3 == 2
> > ... for bar in range(16, 23) if 2 <= bar % 5 < 4)
> > <generator object at 0xb7dec10c>
> > >>>
> >
> > Hmm. Two for loops and two if clauses. That's four altogether.
> > Does that qualify as "several"? :) Sure, they're not statements
> > according to the grammar, but that's entirely beside the point.
> >
> > Jp
>
> And that's an expression, and hence perfectly legal as the body of an anonymous
> function.

Quoting my response again:

> Jp Calderone wrote:
> > Sure, they're not statements according to the grammar, but
> > that's entirely beside the point.

I thought this was obvious to anyone, oh well.

List and generator comprehensions have special syntax so that behavior
normally acheivable only with statements can be used to control their
execution. Lambdas don't have any similar special syntax.

I'm not saying "boo hoo lambdas are crippled fix them waah". I'm saying
"Lambdas and generator comprehensions are not comparable in this sense and
arguments based on one should not be used to support positions about the
other".

Jp

John Roth

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Dec 30, 2004, 9:39:54 AM12/30/04
to

"Nick Coghlan" <ncog...@iinet.net.au> wrote in message
news:mailman.8597.1104413...@python.org...

> GvR has commented that he want to get rid of the lambda keyword for Python
> 3.0. Getting rid of lambda seems like a worthy goal, but I'd prefer to see
> it dropped in favour of a different syntax, rather than completely losing
> the ability to have anonymous functions.
>
> Anyway, I'm looking for feedback on a def-based syntax that came up in a
> recent c.l.p discussion:
> http://boredomandlaziness.skystorm.net/2004/12/anonymous-functions-in-python.html
>
> Cheers,
> Nick.

I think it's rather baroque, and I agree with Paul Ruben
that it needs multiple statement capability.

The syntax I prefer (and I don't know if it's actually been
suggested before) is to use braces, that is { and }.

In other words, an anonymous function looks like:
{p1, p2, p3 |
stmt1
stmt2
}

There are two reasons for using braces. One is
that it's the common syntax for blocks in a large number
of languages. The other is that it should be relatively
easy to disambiguate from dictionary literals, which are
the only other current use of braces.

The parameter list is optional, as is the bar ending
the list. The reason for the bar instead of a colon
is to help the parser in the case of a single parameter,
which would look like the beginning of a dictionary
literal. If the parser doesn't need the help, then a colon
would be more consistent, hence better.

A second issue is indentation. I'd set the indentation
boundary wherever the _second_ line of the construct
starts, as long as it's to the right of the prior
indentation boundary. The unfortunate part of this, and one of the
major stumbling blocks, is that it might take some
significant reconceptualizing of the lexer and parser,
which currently isn't set up to shift from expression
back to statement mode and then return to expression
mode.

John Roth

Michael Spencer

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Dec 30, 2004, 1:02:14 PM12/30/04
to pytho...@python.org
Nick Coghlan wrote:
> GvR has commented that he want to get rid of the lambda keyword for
> Python 3.0. Getting rid of lambda seems like a worthy goal, but I'd
> prefer to see it dropped in favour of a different syntax, rather than
> completely losing the ability to have anonymous functions.
>
> Anyway, I'm looking for feedback on a def-based syntax that came up in a
> recent c.l.p discussion:
> http://boredomandlaziness.skystorm.net/2004/12/anonymous-functions-in-python.html
...
The proposed syntax is:
> accepts_func((def (a, b, c) to f(a) + g(b) - h(c)))
...
>
> Cheers,
> Nick.
>
From the blog Comments:
Nick said...
>
> A potentially easier to parse version moves the argument list to the end of the anonymous function and uses the existing keyword 'from'. That is:
>
> (def f(a) + g(b) - h(c) from (a, b, c))
> 11:46 PM
>
> Post a Comment

This alternative syntax seems more elegant to me because of the similarity with
generator expressions. But the 'def' still seems awkward.

<Idle speculation>

Isn't the important point that the arguments have delayed evaluation? If so,
the syntax could highlight this instead: i.e.,

(f(a) + g(b) - h(c) from args(a, b, c))

and, in general:

(expression from args(a, *args, **kw))

while we're at it, perhaps 'for' would be more appropriate:

(expression for args(a, *args, **kw))

I also wonder if a callable, args, that stored its argument list for delayed
evaluation would have other applications besides anonymous functions, but that
is another topic

</Idle speculation>

Michael Spencer


Ian Bicking

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Dec 30, 2004, 2:07:39 PM12/30/04
to John Roth, pytho...@python.org
John Roth wrote:
> The syntax I prefer (and I don't know if it's actually been
> suggested before) is to use braces, that is { and }.
>
> In other words, an anonymous function looks like:
> {p1, p2, p3 |
> stmt1
> stmt2
> }

What's the advantage of something like that over the non-anonymous
equivalent:

def some_func(p1, p2, p3):
stmt1
stmt2

I appreciate some of the motivation, but merely avoiding giving
something a name doesn't seem like a laudible goal.

The one motivation I can see for function expressions is
callback-oriented programming, like:

get_web_page(url,
when_retrieved={page |
give_page_to_other_object(munge_page(page))})

The problem with the normal function in this case is the order of
statements is reversed:

def when_retrieved_callback(page):
give_page_to_other_object(munge_page(page))
get_web_page(url, when_retrieved=when_retrieved_callback)

Oh, and you have to type the name twice, which is annoying. For most
other functional programming constructs, list (and not generator)
comprehensions work well enough, and the overhead of naming functions
just isn't that big a deal.

I think this specific use case -- defining callbacks -- should be
addressed, rather than proposing a solution to something that isn't
necessary. Which is to say, no one *needs* anonymous functions; people
may need things which anonymous functions provide, but maybe there's
other ways to provide the same thing. Decorator abuse, for instance ;)

def get_web_page_decorator(url):
def decorator(func):
return get_web_page(url, when_retrieved=func)
return decorator

@get_web_page_decorator(url)
def when_retrieved(page):
give_page_to_other_object(munge_page(page))

Or even (given a partial function as defined in some PEP, the number of
which I don't remember):

@partial(get_web_page, url)
def when_retrieved(page):
give_page_to_other_object(munge_page(page))

It's okay not to like this proposal, I don't think I'm really serious.
But I do think there's other ways to approach this. Function
expressions could get really out of hand, IMHO, and could easily lead to
twenty-line "expressions". That's aesthetically incompatible with
Python source, IMHO.

--
Ian Bicking / ia...@colorstudy.com / http://blog.ianbicking.org

Roy Smith

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Dec 30, 2004, 2:24:46 PM12/30/04
to
Ian Bicking <ia...@colorstudy.com> wrote:
> I think this specific use case -- defining callbacks -- should be
> addressed, rather than proposing a solution to something that isn't
> necessary. Which is to say, no one *needs* anonymous functions; people
> may need things which anonymous functions provide, but maybe there's
> other ways to provide the same thing. Decorator abuse, for instance ;)

I'm not a big functional programming fan, so it should not come as a
surprise that I don't often use lambda. The one place I do use it is in
unit tests, where assertRaises() requires a callable. If what you're
testing is an expression, you need to wrap it in a lambda.

I suppose you could call this a special case of a callback.

Skip Montanaro

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Dec 30, 2004, 2:35:43 PM12/30/04
to John Roth, pytho...@python.org

John> In other words, an anonymous function looks like:
John> {p1, p2, p3 |
John> stmt1
John> stmt2
John> }

John> There are two reasons for using braces. One is that it's the
John> common syntax for blocks in a large number of languages.

Yeah, but it's not how blocks are spelled in Python. As Nick pointed out on
his blog, allowing statements within expressions risks making code more
difficult to read and understand.

People keep trying to make Python something it is not. It is not
fundamentally an expression-only language like Lisp, nor is it an
expression-equals-statement language like C. There are good reasons why
Guido chose the relationship between simple statements, compound statements
and expressions that he did, readability and error avoidance being key.

Skip

Carl Banks

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Dec 30, 2004, 3:08:58 PM12/30/04
to
Nick Coghlan wrote:
> GvR has commented that he want to get rid of the lambda keyword for
Python 3.0.
> Getting rid of lambda seems like a worthy goal, but I'd prefer to see
it dropped
> in favour of a different syntax, rather than completely losing the
ability to
> have anonymous functions.

I shall either coin or reuse a new term here: "premature optimization
of the Python language."

(Please note that the word premature is not intended to be an accurate
description, but irony by analogy, so spare me any semantic
nitpicking.)

In much the same way that programmers often spend a lot of time
optimizing parts of their program that will yield very minor dividends,
while they could have spent that time working on other things that will
pay off a lot, many of the wannabe language designers here are spending
a lot of time on aspects of the language for which any improvement
would only pay small dividends.

I think the worry about anonymous functions is one of the most
widespread cases of "premature optimization of the Python Language."
One could argue about the various benefits of particular choices, maybe
even make a convincing case that one is best in accord with the design
goals of Python; but in the end, the divends are small compared to
improving other aspects of the language.


--
CARL BANKS

David Bolen

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Dec 30, 2004, 4:44:39 PM12/30/04
to
Ian Bicking <ia...@colorstudy.com> writes:

> The one motivation I can see for function expressions is
> callback-oriented programming, like:
>
> get_web_page(url,
> when_retrieved={page |
> give_page_to_other_object(munge_page(page))})

This is my primary use case for lambda's nowadays as well - typically
just to provide a way to convert the input to a callback into a call
to some other routine. I do a lot of Twisted stuff, whose deferred
objects make heavy use of single parameter callbacks, and often you
just want to call the next method in sequence, with some minor change
(or to ignore) the last result.

So for example, an asynchronous sequence of operations might be like:

d = some_deferred_function()
d.addCallback(lambda x: next_function())
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(lambda x: last_function())

which to me is more readable (in terms of seeing the sequence of
operations being performed in their proper order), then something like:

def cb_next(x):
return next_function()
def cb_third(blah, otherargs):
return third_function(otherargs, blah)
def cb_last(x):
return last_function()

d = some_deferred_function()
d.addCallback(cb_next)
d.addCallback(cb_third, otherargs)
d.addCallback(cb_next)

which has an extra layer of naming (the callback functions), and
requires more effort to follow the flow of what is really just a simple
sequence of three functions being called.

> I think this specific use case -- defining callbacks -- should be
> addressed, rather than proposing a solution to something that isn't

> necessary. (...)

I'd be interested in this approach too, especially if it made it simpler
to handle simple manipulation of callback arguments (e.g., since I often
ignore a successful prior result in a callback in order to just move on
to the next function in sequence).

-- David

Paul L. Du Bois

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Dec 30, 2004, 5:01:34 PM12/30/04
to
Jp Calderone wrote:
> I'm not saying "boo hoo lambdas are crippled fix them waah". I'm
saying
> "Lambdas and generator comprehensions are not comparable in this
sense and
> arguments based on one should not be used to support positions about
the
> other".

This post and Michael Spencer's post which proposed (expr for args(a,
*b, **c)) got me wondering whether this could be done already today.
My solution is unfortuantely implementation-specific. I can't find a
way for a generator (let alone a generator expression) to refer to
itself; neither do generators have writable attributes. With those
features, one can imagine a more portable (and more verbose) solution.

I find that I don't really like the generator-like syntax. To me, it
makes sense for an unnamed function to look like a named function
without a name, eg:

callbacks['foo'] = def (a,b): ... expr or statement? ...

It seems to work in Lua (not that I'm a big fan of Lua). Thus, I'm not
sure what my point is with this code; maybe just that there are
workarounds if lambda ceases to exist. Also, with this sort of
solution, nobody can complain that genexps/listcomps/"lambdas" get
preferential treatment <wink>

Anyway, this is tested in Win32 python 2.4. It adds a function "fn"
(name stolen from Arc) which turns a specially-written generator
expression into an anonymous function. Default args, *args, *kwargs
are all unsupported. I'm sorry if google groups eats my leading
whitespace; I've one-lined things to reduce the effect.

def fn(gen):
"""Turns a generator expression into a callable."""
def anonymous(*args): return gen.next()
return anonymous

def args():
"""Works with fn(); yields args passed to anonymous()."""
while True: yield sys._getframe(2).f_locals['args']

args = args()

foo = fn(a + b * c for (a,b,c) in args)
assert foo(3,4,5) == 3+4*5
assert foo(4,5,6) == 4+5*6

Bengt Richter

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Dec 30, 2004, 5:08:37 PM12/30/04
to
On Thu, 30 Dec 2004 23:28:46 +1000, Nick Coghlan <ncog...@iinet.net.au> wrote:

>GvR has commented that he want to get rid of the lambda keyword for Python 3.0.
>Getting rid of lambda seems like a worthy goal, but I'd prefer to see it dropped
>in favour of a different syntax, rather than completely losing the ability to
>have anonymous functions.
>
>Anyway, I'm looking for feedback on a def-based syntax that came up in a recent
>c.l.p discussion:
>http://boredomandlaziness.skystorm.net/2004/12/anonymous-functions-in-python.html
>

Nit: You didn't try the code you posted ;-)

>>> funcs = [(lambda x: x + i) for i in range(10)]
>>>
>>> def incrementors():
... for i in range(10):
... def incrementor(x):
... return x + i
... yield incrementor
...
>>> #funcs = list(incrementors)
... funcs2 = list(incrementors())
>>>
>>> for f in funcs: print f(0),
...
9 9 9 9 9 9 9 9 9 9
>>> for f in funcs2: print f(0),
...
9 9 9 9 9 9 9 9 9 9

This is an easy trap to fall into, so if the new lambda-substitute could
provide a prettier current-closure-variable-value capture than passing a dummy default
value or nesting another def and passing the value in, to provide a private closure for each,
that might be something to think about.

IMO an anonymous def that exactly duplicates ordinary def except for leaving out
the function name and having a local indentation context would maximize flexibility
and also re-use of compiler code. People could abuse it, but that's already true
of many Python features.

From your web page:
----
def either(condition, true_case, false_case):
if condition:
return true_case()
else:
return false_case()

print either(A == B, (def "A equals B"), (def "A does not equal B"))
either(thefile, (def thefile.close()), (def 0))
----

I'd rather see (:something) than (def something) for this special case,
but the full-fledged anonymous def would spell it thus:

print either(A == B, (def():return "A equals B"), (def(): return "A does not equal B"))
either(thefile, (def(): return thefile.close()), (def(): return 0))

BTW,

funcs = [(lambda x: x + i) for i in range(10)]

might be spelled

funcs = [(def(x, i=i): return x + i) for i in range(10)]

or optionally

funcs = [(
def(x, i=i):
return x + i
) for i in range(10)]

or

funcs = [(def(x, i=i):
return x + i) for i in range(10)]
or
funcs = [(def(x, i=i):
return x + i)
for i in range(10)]
or
funcs = [def(x, i=i):
return x + i
for i in range(10)]
or
funcs = [
def(x, i=i):
return x + i
for i in range(10)]

and so on. (the def defines the indentation base if the suite is indented, and the closing ')'
terminates the anonymous def explicitly, or a dedent to the level of the def or less can do it,
as in the last two examples).

This one

(def f(a) + g(b) - h(c) from (a, b, c))

would be spelled (if I undestand your example)

(def(a, b, c): return f(a)+g(b)+h(c))

which seems to me familiar and easy to understand.

BTW, there are old threads where this and other formats were discussed. I am still
partial to the full anonymous def with nesting indentation rules. Syntactic sugar
could be provided for useful abbreviations (that could possibly be expanded by the
tokenizer -- re which possibilities I haven't seen any discussion than my own
recent post, BTW), but I'd like the full capability.

Regards,
Bengt Richter

John Roth

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Dec 30, 2004, 5:24:03 PM12/30/04
to

"Ian Bicking" <ia...@colorstudy.com> wrote in message
news:mailman.8620.1104433...@python.org...

> John Roth wrote:
>> The syntax I prefer (and I don't know if it's actually been
>> suggested before) is to use braces, that is { and }.
>>
>> In other words, an anonymous function looks like:
>> {p1, p2, p3 |
>> stmt1
>> stmt2
>> }
>
> What's the advantage of something like that over the non-anonymous
> equivalent:
>
> def some_func(p1, p2, p3):
> stmt1
> stmt2
>
> I appreciate some of the motivation, but merely avoiding giving something
> a name doesn't seem like a laudible goal.

Actually, it is a laudable goal. It's always easier to understand
something when it's right in front of your face than if it's
off somewhere else.

This, of course, trades off with two other forces: avoiding
repetition and making the whole thing small enough to
understand.

So the niche is small, single use functions. The problem
with lambdas is that they're restricted to one expression,
which is too small.

Languages that are designed with anonymous functions
in mind use them very heavily. Smalltalk is the standard
example, and it's also one of the major (possibly the
only) attraction Ruby has over Python.

Python isn't designed that way, which restricts their
utility to a much smaller niche than otherwise.

> The one motivation I can see for function expressions is callback-oriented

> programming ...

Well, that's true, but that's a very global statement:
when you pass a function into another routine, it's
essentially a callback.

...

> Function expressions could get really out of hand, IMHO, and could easily
> lead to twenty-line "expressions". That's aesthetically incompatible with
> Python source, IMHO.

Anything can get out of hand; there's no way of legislating
good style without restricting the language so much that it
becomes unusable for anything really interesting. Even then
it doesn't work: see COBOL as a really good example of
good intentions gone seriously wrong.

Have you ever programmed in a language that does use
anonymous functions extensively like Smalltalk?

John Roth

Jeff Shannon

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Dec 30, 2004, 6:15:51 PM12/30/04
to
David Bolen wrote:

>>I think this specific use case -- defining callbacks -- should be
>>addressed, rather than proposing a solution to something that isn't
>>necessary. (...)
>>
>>
>
>I'd be interested in this approach too, especially if it made it simpler
>to handle simple manipulation of callback arguments (e.g., since I often
>ignore a successful prior result in a callback in order to just move on
>to the next function in sequence).
>
>

It seems to me that what most people *actually* want, when asking for
lambdas, is a quicker and more convenient way to get closures. (At
least, that's what the vast majority of examples of lambda use seem to
be for.) Perhaps one could get a bit more traction by looking for
improved closure syntax instead of focusing on the anonymous function
aspect.

All of the suggestions for anonymous multiline functions (with embedded
indentation) seem to me to miss what seems to me to be the only
significant benefit of lambda -- its ability to be used in-line without
creating a huge ugly tangle. (I'd argue that lambdas create a *small*
ugly tangle, but apparently that's just me. ;) ) Lambdas do have some
value in defining callbacks, but that value derives almost exclusively
from the fact that they are in-line (rather than a few lines above).
Mimicking function-def indentation inside of another function's arglist
strikes me as an abomination just waiting to happen; in comparison, the
need to type a name twice seems trivial.

As a result, it seems to me that, rather than generalize lambdas into
"full" anonymous functions (with most of the negatives and few of the
positives of lambda), it would be much better to specialize them further
into inline-closure-creators, where they can serve a valuable purpose
without quite as much risk of code pollution.

Jeff Shannon
Technician/Programmer
Credit International

Bengt Richter

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Dec 30, 2004, 7:31:45 PM12/30/04
to
On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je...@ccvcorp.com> wrote:

>David Bolen wrote:
>
>>>I think this specific use case -- defining callbacks -- should be
>>>addressed, rather than proposing a solution to something that isn't
>>>necessary. (...)
>>>
>>>
>>
>>I'd be interested in this approach too, especially if it made it simpler
>>to handle simple manipulation of callback arguments (e.g., since I often
>>ignore a successful prior result in a callback in order to just move on
>>to the next function in sequence).
>>
>>
>
>It seems to me that what most people *actually* want, when asking for
>lambdas, is a quicker and more convenient way to get closures. (At
>least, that's what the vast majority of examples of lambda use seem to
>be for.) Perhaps one could get a bit more traction by looking for
>improved closure syntax instead of focusing on the anonymous function
>aspect.

Not sure what you mean by closure here. To me it means the necessary
external environment needed to be captured for use by a function definition
getting exported from its definition environment. I.e., it is something
a function uses, and part of the function definition, but it isn't the
function itself. I would compare a closure more to a callable class instance's
self attributes, except that the latter are more flexible.

In fact, for a callback, a constructor call creating a suitable
callable class instance could sometimes work well as a substitute
for a lambda expression, ISTM. (I.e., when it is not important to
show the code in line, and the differences are in initialization parameters
rather than code).

>
>All of the suggestions for anonymous multiline functions (with embedded
>indentation) seem to me to miss what seems to me to be the only
>significant benefit of lambda -- its ability to be used in-line without
>creating a huge ugly tangle. (I'd argue that lambdas create a *small*
>ugly tangle, but apparently that's just me. ;) ) Lambdas do have some
>value in defining callbacks, but that value derives almost exclusively
>from the fact that they are in-line (rather than a few lines above).

They do let you define _code_ inline, which a constructor call doesn't do
(unless you pass a string to compile etc -- not cool).



>Mimicking function-def indentation inside of another function's arglist
>strikes me as an abomination just waiting to happen; in comparison, the
>need to type a name twice seems trivial.

Self-restraint can avoid abominations ;-)

>
>As a result, it seems to me that, rather than generalize lambdas into
>"full" anonymous functions (with most of the negatives and few of the
>positives of lambda), it would be much better to specialize them further
>into inline-closure-creators, where they can serve a valuable purpose
>without quite as much risk of code pollution.

There's always the temptation to be enforcer when being persuader
is not the easiest ;-)

(BTW, again, by closure, do you really mean deferred-action-thingie?)

Regards,
Bengt Richter

Jeff Shannon

unread,
Dec 30, 2004, 8:39:06 PM12/30/04
to
Bengt Richter wrote:

> On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je...@ccvcorp.com> wrote:
>
>>Mimicking function-def indentation inside of another function's arglist
>>strikes me as an abomination just waiting to happen; in comparison, the
>>need to type a name twice seems trivial.
>
> Self-restraint can avoid abominations ;-)

It can, but given the prevalence of lambda abominations (such as the
many that the Martellibot described among Cookbook submissions), is
there any reason to believe that there would be *more* restraint in
using a less-constrained feature?? :)

>>As a result, it seems to me that, rather than generalize lambdas into
>>"full" anonymous functions (with most of the negatives and few of the
>>positives of lambda), it would be much better to specialize them further
>>into inline-closure-creators, where they can serve a valuable purpose
>>without quite as much risk of code pollution.
>

> (BTW, again, by closure, do you really mean deferred-action-thingie?)

My understanding of "closure" (which may well be wrong, as I've
inferred it entirely from past discussions on this newsgroup) is that
it's a callable with some or all of its parameters already set; e.g.,

def one_and(foo):
def closure(arg):
return foo(1, arg)
return closure

incr = one_and(operator.add)

The function one_and() returns a closure, here bound to incr. It is
essentially a (partially) deferred action thingy, if you want to use
technical terms. ;) I suppose that one could look at it as the
environment in which to call a given function, exported for later use...

My thesis here is that one of the most common (legitimate) uses of
lambda is as an adapter, to create an intermediary that allows a
callable with a given signature to be used in places where a different
signature is expected -- that is, altering the number or order of
arguments passed to a given callable (and possibly also capturing the
current value of some other variable in the process). I feel that
it's more fruitful to focus on this "adapter" quality rather than
focusing on the "anonymous function" quality.

Nick Coghlan

unread,
Dec 30, 2004, 9:38:24 PM12/30/04
to Python List
Batista, Facundo wrote:
> [Nick Coghlan]
>
> #- I just don't understand why people complain so much about
> #- the restriction to a
> #- single expression in lambdas, yet there is nary a peep about
> #- the same
> #- restriction for generator expressions and list comprehensions.
>
> What *I* don't understand (and it could perfectly be plain ignorance, as
> never studied languages or computer sciences) is why anonymous functions
> are so important.

Speaking for myself, I find lazy evaluation of arguments is impractical without
them. Defining 2 or 3 named functions in order to make a single function call
seems rather unnecessarily verbose. I don't need to use lazy evaluation very
often, but when I do use it, it is generally because other approaches would be
much more complicated (or significantly less efficient).

This usage pattern is also why I do not find the restriction to a single
expression a problem in practice - since I'm merely delaying the evaluation of
what I would have supplied as a function argument anyway, I'm already working
with an expression.

Cheers,
Nick.

Nick Coghlan

unread,
Dec 30, 2004, 9:50:14 PM12/30/04
to Python List
Bengt Richter wrote:
> This is an easy trap to fall into, so if the new lambda-substitute could
> provide a prettier current-closure-variable-value capture than passing a dummy default
> value or nesting another def and passing the value in, to provide a private closure for each,
> that might be something to think about.

I forgot about that little trap. . .

Nick Coghlan

unread,
Dec 30, 2004, 10:07:51 PM12/30/04
to Python List
Carl Banks wrote:

> Nick Coghlan wrote:
> In much the same way that programmers often spend a lot of time
> optimizing parts of their program that will yield very minor dividends,
> while they could have spent that time working on other things that will
> pay off a lot, many of the wannabe language designers here are spending
> a lot of time on aspects of the language for which any improvement
> would only pay small dividends.

Whereas I see it as wannabe language designers only being able to tinker at the
edges of Python, because GvR already got the bulk of the language right.
Anonymous functions are the major core construct that doesn't 'fit right', so a
disproportionate amount of time is spent playing with ideas about them.

Nick Coghlan

unread,
Dec 30, 2004, 10:22:35 PM12/30/04
to Python List
Michael Spencer wrote:
> while we're at it, perhaps 'for' would be more appropriate:
>
> (expression for args(a, *args, **kw))

Hmm, what about an 'abbreviated generator expression' syntax:

(<expression> for <args-tuple>)

For instance:

(f(a) + g(b) - h(c) for (a, b, c))

(the 'in' clause of the standard generator expression is missing).

That does leave the question of the correct spelling of 'no arguments', which
generator expressions don't have to deal with. Perhaps:

(f.close() for ())

It does make for an interesting interpretation of the generator expression syntax:

(<expression> for <args-tuple> in <sequence> if <condition>)

def expression_func(<args-tuple>):
return <expression>

def generator():
for <args-tuple> in <sequence>
if <condition>:
yield expression_func(<args-tuple>)

Bengt Richter

unread,
Dec 30, 2004, 10:46:02 PM12/30/04
to
On Thu, 30 Dec 2004 17:39:06 -0800, Jeff Shannon <je...@ccvcorp.com> wrote:

>Bengt Richter wrote:
>
>> On Thu, 30 Dec 2004 15:15:51 -0800, Jeff Shannon <je...@ccvcorp.com> wrote:
>>
>>>Mimicking function-def indentation inside of another function's arglist
>>>strikes me as an abomination just waiting to happen; in comparison, the
>>>need to type a name twice seems trivial.
>>
>> Self-restraint can avoid abominations ;-)
>
>It can, but given the prevalence of lambda abominations (such as the
>many that the Martellibot described among Cookbook submissions), is
>there any reason to believe that there would be *more* restraint in
>using a less-constrained feature?? :)

Maybe. If people are determined to overcome limitations that needn't exist,
they are likely to invent horrible hacks ;-)

>
>>>As a result, it seems to me that, rather than generalize lambdas into
>>>"full" anonymous functions (with most of the negatives and few of the
>>>positives of lambda), it would be much better to specialize them further
>>>into inline-closure-creators, where they can serve a valuable purpose
>>>without quite as much risk of code pollution.
>>
>> (BTW, again, by closure, do you really mean deferred-action-thingie?)
>
>My understanding of "closure" (which may well be wrong, as I've
>inferred it entirely from past discussions on this newsgroup) is that
>it's a callable with some or all of its parameters already set; e.g.,
>
> def one_and(foo):
> def closure(arg):
> return foo(1, arg)
> return closure
>
> incr = one_and(operator.add)
>
>The function one_and() returns a closure, here bound to incr. It is
>essentially a (partially) deferred action thingy, if you want to use
>technical terms. ;) I suppose that one could look at it as the
>environment in which to call a given function, exported for later use...

Quoting from "Essentials of Programming Languages" (Friedman, Wand, Haynes):
(using *xxx yyy etc* for italics)
"""
In order for a procedure to retain the bindings that its free variables had
at the time it was created, it must be a *closed* package, independent of
the environment in which it is used. Such a package is called a closure.
In order to be self-contained, a closure must contain the procedure body,
the list of formal parameters, and the bindings of its free variables.
It is convenient to store the entire creation environment, rather than
just the bindings of the free variables. We sometimes say the procedure
*is closed over* or *closed in* its creation environment. We represent
closures as records.
"""

So it looks like you infer better than I remember, and attachment to
faulty memory has led me to resist better inferences ;-/

Closure is the name for the whole thing, apparently, not just the environment
the procedure body needs, which was the aspect that I (mis)attached the name to.

(Representing closures as records doesn't really belong in that paragraph IMO,
since it is not really part of the definition there, just a choice in that stage
of exposition in the book, using scheme-oriented examples. But I quoted verbatim.).

On the subject CLtL finally (after some stuff beyond my current caffeine level) says,
"""
The distinction between closures and other kinds of functions is somewhat pointless,
actually, since Common Lisp defines no particular representation for closures and
no way to distinguish between closures and non-closure functions. All that matters
is that the rules of lexical scoping be obeyed.
"""
I guess that clinches it ;-)

Might be a good python glossary wiki entry.

>
>My thesis here is that one of the most common (legitimate) uses of
>lambda is as an adapter, to create an intermediary that allows a
>callable with a given signature to be used in places where a different
>signature is expected -- that is, altering the number or order of
>arguments passed to a given callable (and possibly also capturing the
>current value of some other variable in the process). I feel that
>it's more fruitful to focus on this "adapter" quality rather than
>focusing on the "anonymous function" quality.
>

I see what you are saying (I think), but I think I'd still like a full
anonymous def, whatever adapter you come up with. And I prefer to be persuaded ;-)

Regards,
Bengt Richter

Steven Bethard

unread,
Dec 30, 2004, 11:14:31 PM12/30/04
to
Jeff Shannon wrote:
> My thesis here is that one of the most common (legitimate) uses of
> lambda is as an adapter, to create an intermediary that allows a
> callable with a given signature to be used in places where a different
> signature is expected -- that is, altering the number or order of
> arguments passed to a given callable (and possibly also capturing the
> current value of some other variable in the process). I feel that it's
> more fruitful to focus on this "adapter" quality rather than focusing on
> the "anonymous function" quality.

Maybe the 'functional' module proposed in PEP 309[1] could provide such
functions?

py> def ignoreargs(func, nargs, *kwd_names):
... def _(*args, **kwds):
... args = args[nargs:]
... kwds = dict((k, kwds[k])
... for k in kwds if k not in kwd_names)
... return func(*args, **kwds)
... return _
...
py> def f(x, y):
... print x, y
...
py> ignoreargs(f, 2)(1, 2, 3, 4)
3 4
py> ignoreargs(f, 2, 'a', 'b')(1, 2, 3, 4, a=35, b=64)
3 4

Steve

[1] http://python.fyxm.net/peps/pep-0309.html

Paul Rubin

unread,
Dec 31, 2004, 1:28:40 AM12/31/04
to
bo...@oz.net (Bengt Richter) writes:
> print either(A == B, (def "A equals B"), (def "A does not equal B"))
> either(thefile, (def thefile.close()), (def 0))

I'd really rather have some reasonable macro facility, than to resort
to using anonymous functions and deferred evaluation for common things
like that.

Ian Bicking

unread,
Dec 31, 2004, 4:01:45 AM12/31/04
to John Roth, pytho...@python.org
John Roth wrote:
>> I appreciate some of the motivation, but merely avoiding giving
>> something a name doesn't seem like a laudible goal.
>
> Actually, it is a laudable goal. It's always easier to understand
> something when it's right in front of your face than if it's
> off somewhere else.

Naming the function doesn't move it far away. It changes the order (you
have to define it before you use it), and it introduces a name.

>> The one motivation I can see for function expressions is
>> callback-oriented programming ...
>
> Well, that's true, but that's a very global statement:
> when you pass a function into another routine, it's
> essentially a callback.

Sure, technically. But I'm thinking of real use cases. One I'm
familiar with is things like map and filter. These are generally better
handled with list expressions (and MUCH more readable as such, IMHO).
Another is control structures, ala Ruby or Smalltalk. IMHO, we have all
the control structures we need -- while, for, if. Most "novel" control
structures in Ruby or Smalltalk are just another take on iterators. The
exception being callbacks, and perhaps some other lazy evaluation
situations (though outside of what I think of as "callbacks", I can't
think of any lazy evaluation situations off the top of my head).

So that's why I think callbacks are important; callbacks in the style of
Twisted Deferred, GUI events, etc.

>> Function expressions could get really out of hand, IMHO, and could
>> easily lead to twenty-line "expressions". That's aesthetically
>> incompatible with Python source, IMHO.
>
> Anything can get out of hand; there's no way of legislating
> good style without restricting the language so much that it
> becomes unusable for anything really interesting. Even then
> it doesn't work: see COBOL as a really good example of
> good intentions gone seriously wrong.

OK, I should go further -- even a two-line expression (i.e., an
expression that contains meaningful vertical whitespace) is
aesthetically incompatible with Python source. Which covers any
anonymous function that is more powerful than lambda. I'm not arguing
that it can be abused, but more that it isn't any good even when it's
not being abused.

> Have you ever programmed in a language that does use
> anonymous functions extensively like Smalltalk?

Yep, I've done a fair amount of Smalltalk and Scheme programming. I
don't expect Python to act like them. I appreciate the motivation, but
I don't think their solution is the right one for Python.

Alex Martelli

unread,
Dec 31, 2004, 12:38:24 PM12/31/04
to
Paul L. Du Bois <poly...@gmail.com> wrote:
...

> are all unsupported. I'm sorry if google groups eats my leading
> whitespace; I've one-lined things to reduce the effect.

It does/did, so let me repost while fixing it since this is truly,
deliciously evil:

> def fn(gen):
> """Turns a generator expression into a callable."""
> def anonymous(*args): return gen.next()
> return anonymous
>
> def args():
> """Works with fn(); yields args passed to anonymous()."""
> while True: yield sys._getframe(2).f_locals['args']
>
> args = args()
>
> foo = fn(a + b * c for (a,b,c) in args)
> assert foo(3,4,5) == 3+4*5
> assert foo(4,5,6) == 4+5*6

Paul, you really SHOULD have posted this BEFORE I had to send in the
files for the 2nd ed's Coobook... this gets my vote for the most
delightful abuse of sys._getframe even (and I've seen quite a few;-).

Kudos!!!


Alex

Steven Bethard

unread,
Dec 31, 2004, 1:19:59 PM12/31/04
to

So, I couldn't figure out why this worked until I started to write an
email to ask. Once I understood it, I figured it wouldn't hurt to send
my thoughts out anyway to (1) verify that I understand it right, and (2)
help anyone else who was trying to figure this out.


As I understand it sys._getframe(2).f_locals should get the names local
to the stack frame two above the current one. So, in the context of:
fn(... for ... in args)
sys._getframe(2).f_locals should be looking at the names local to the
'anonymous' function in the 'fn' function, because one stack frame up
from the 'args' function is the generator's 'next' function, and two
stack frames up is the 'anonymous' function. That means that:
sys._getframe(2).f_locals['args']
gets whatever object has been bound to 'args' in:
def anonymous(*args):
So then in:


foo = fn(a + b * c for (a,b,c) in args)

foo(3,4,5)
foo(4,5,6)
sys._getframe(2).f_locals['args'] will get (3, 4, 5) in the first foo
call, (4, 5, 6) in the second foo call, etc.


So basically the way a call like foo(3, 4, 5) works is:
(1) foo(3, 4, 5) calls gen.next() where gen is the generator expression
(2) gen.next() calls args.next()
(3) args.next() returns the (3, 4, 5) argument tuple of foo by looking
up the stack frames
(4) gen.next() binds (3, 4, 5) to the names a, b, c respectively
(5) gen.next() returns the value of "a + b * c" for these bindings
(6) foo(3, 4, 5) returns the same value (as gen.next() did)

Does that seem about right?

Steve

P.S. That's so *evilly* cool!

Scott David Daniels

unread,
Dec 31, 2004, 1:24:21 PM12/31/04
to
David Bolen wrote:
> So for example, an asynchronous sequence of operations might be like:
>
> d = some_deferred_function()
> d.addCallback(lambda x: next_function())
> d.addCallback(lambda blah: third_function(otherargs, blah))
> d.addCallback(lambda x: last_function())
>
> which to me is more readable (in terms of seeing the sequence of
> operations being performed in their proper order), then something like:
>
> def cb_next(x):
> return next_function()
> def cb_third(blah, otherargs):
> return third_function(otherargs, blah)
> def cb_last(x):
> return last_function()
>
> d = some_deferred_function()
> d.addCallback(cb_next)
> d.addCallback(cb_third, otherargs)
> d.addCallback(cb_next)
>
> which has an extra layer of naming (the callback functions), and
> requires more effort to follow the flow of what is really just a simple
> sequence of three functions being called.

But this sequence contains an error of the same form as the "fat":

while test() != False:
...code...

The right sequence using lambda is:
d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(lambda blah: third_function(otherargs, blah))
d.addCallback(last_function)

And I would write it as:

def third_function_fixed_blah(blah):
def call_third(otherargs):
return third_function(otherargs, blah)
return call_third

d = some_deferred_function()
d.addCallback(next_function)
d.addCallback(third_function_fixed_blah, otherargs)
d.addCallback(last_function)

The name gives you the chance to point out that the argument order is
tweaked. In many such cases, I use curry (ASPN recipe #52549), which
should show up in Python as "partial" in the "functional" module
according to PEP 309 <http://www.python.org/peps/pep-0309.html>
(accepted but not included). I suppose it will show up in Python 2.5.

Programming is a quest is for clear, easy-to-read code, not quick,
easy-to-write code. Choosing a name is a chance to explain what you
are doing. lambda is used too often in lieu of deciding what to write.

--Scott David Daniels
Scott....@Acm.Org

Alex Martelli

unread,
Dec 31, 2004, 1:38:24 PM12/31/04
to
Steven Bethard <steven....@gmail.com> wrote:

> Does that seem about right?

Yep!

> P.S. That's so *evilly* cool!

We should have an Evilly Cool Hack of the Year, and I nominate Paul du
Bois's one as the winner for 2004. Do I hear any second...?


Alex

Paul Rubin

unread,
Dec 31, 2004, 1:46:08 PM12/31/04
to
ale...@yahoo.com (Alex Martelli) writes:
> We should have an Evilly Cool Hack of the Year, and I nominate Paul du
> Bois's one as the winner for 2004. Do I hear any second...?

The year's not over yet :).

Ian Bicking

unread,
Dec 31, 2004, 12:54:51 PM12/31/04
to David Bolen, pytho...@python.org
David Bolen wrote:
> Ian Bicking <ia...@colorstudy.com> writes:
>
>
>>The one motivation I can see for function expressions is
>>callback-oriented programming, like:
>>
>> get_web_page(url,
>> when_retrieved={page |
>> give_page_to_other_object(munge_page(page))})
>
>
> This is my primary use case for lambda's nowadays as well - typically
> just to provide a way to convert the input to a callback into a call
> to some other routine. I do a lot of Twisted stuff, whose deferred
> objects make heavy use of single parameter callbacks, and often you
> just want to call the next method in sequence, with some minor change
> (or to ignore) the last result.
>
> So for example, an asynchronous sequence of operations might be like:
>
> d = some_deferred_function()
> d.addCallback(lambda x: next_function())
> d.addCallback(lambda blah: third_function(otherargs, blah))
> d.addCallback(lambda x: last_function())

Steven proposed an ignoreargs function, and the partial function offers
the other side (http://www.python.org/peps/pep-0309.html). So this
would become:

d = some_deferred_function()
d.addCallback(ignoreargs(next_function, 1))
d.addCallback(partial(third_function, otherargs))
d.addCallback(ignoreargs(last_function, 1))

I'm not sure this is "better" than it is with lambda. It's actually
considerably less readable to me. Hmm... well, that makes me less
excited about those...

David Bolen

unread,
Dec 31, 2004, 2:15:59 PM12/31/04
to
Scott David Daniels <Scott....@Acm.Org> writes:

> David Bolen wrote:
> > So for example, an asynchronous sequence of operations might be like:
> > d = some_deferred_function()
> > d.addCallback(lambda x: next_function())
> > d.addCallback(lambda blah: third_function(otherargs, blah))
> > d.addCallback(lambda x: last_function())
> > which to me is more readable (in terms of seeing the sequence of
> > operations being performed in their proper order), then something like:
> > def cb_next(x):
> > return next_function()
> > def cb_third(blah, otherargs):
> > return third_function(otherargs, blah)
> > def cb_last(x):
> > return last_function()
> > d = some_deferred_function()
> > d.addCallback(cb_next)
> > d.addCallback(cb_third, otherargs)
> > d.addCallback(cb_next)
> > which has an extra layer of naming (the callback functions),
> > and
> > requires more effort to follow the flow of what is really just a simple
> > sequence of three functions being called.
>
> But this sequence contains an error of the same form as the "fat":

"this" being which of the two scenarios you quote above?

> while test() != False:
> ...code...

I'm not sure I follow the "error" in this snippet...

> The right sequence using lambda is:
> d = some_deferred_function()
> d.addCallback(next_function)
> d.addCallback(lambda blah: third_function(otherargs, blah))
> d.addCallback(last_function)

By what metric are you judging "right"?

In my scenario, the functions next_function and last_function are not
written to expect any arguments, so they can't be passed straight into
addCallback because any deferred callback will automatically receive
the result of the prior deferred callback in the chain (this is how
Twisted handles asynchronous callbacks for pending operations).
Someone has to absorb that argument (either the lambda, or
next_function itself, which if it is an existing function, needs to be
handled by a wrapper, ala my second example).

Your "right" sequence simply isn't equivalent to what I wrote.
Whether or not next_function is fixable to be used this way is a
separate point, but then you're discussing two different scenarios,
and not two ways to write one scenario.

-- David

Scott David Daniels

unread,
Dec 31, 2004, 2:58:04 PM12/31/04
to
David Bolen wrote:
> Scott David Daniels <Scott....@Acm.Org> writes:
>> while test() != False:
>> ...code...
> I'm not sure I follow the "error" in this snippet...

The code is "fat" -- clearer is:
while test():
...code...

>>The right sequence using lambda is:
>> d = some_deferred_function()
>> d.addCallback(next_function)
>> d.addCallback(lambda blah: third_function(otherargs, blah))
>> d.addCallback(last_function)
> By what metric are you judging "right"?

By a broken metric that requires you to mis-understand the original code
in the same way that I did. It was an idiotic response that required
more careful reading than I am doing this morning. The thing I've seen
in too much code (and though I saw in your code) is code like:

requires_function(lambda: function())

rather than:

requires_function(function)

It happens quite often, and I'm sure you've seen it. But I got your
code wrong, and for that I apologize.

--Scott David Daniels
Scott....@Acm.Org

Simo Melenius

unread,
Dec 31, 2004, 4:37:23 PM12/31/04
to
bo...@oz.net (Bengt Richter) writes:

> Closure is the name for the whole thing, apparently, not just the
> environment the procedure body needs, which was the aspect that I
> (mis)attached the name to.

Which brings me to the point where I'd welcome more flexibility in
writing to variables outside the local scope. This limitation most
often kicks in in closed-over code in function objects, although it's
a more general issue in Python's scoping.

As we know, you can't write to variables that are both non-local and
non-global (globals you can declare "global"). Now that effectively
makes free variables read-only (although, the objects they point to
can _still_ be mutated).

Allowing write access to variables in a closed-over lexical scope
outside the innermost scope wouldn't hurt because:

1) if you need it, you can already do it -- just practice some
cumbersome tricks make suitable arrangements (e.g. the classical
accumulator example uses an array to hold the counter value instead
of binding it directly to the free variable;

2) if you don't need or understand it, you don't have to use it;

3) and at least in function instances: if you accidentally do, it'll
change the bindings within your closure only which is definitely
less dangerous than mutating objects that are bound inside the
closure.

It must be noted, however, that such behaviour would change the way of
hiding nested variable names:

Now it's safe (though maybe lexically confusing) to use the same
variable names in inner functions. This could happen with common names
for temporary variables like "i", "x", "y".

On the other hand, one could introduce a way to declare variables from
global scope or from local scope, with default from lexical scope. (If
you want to explicitly hide an outer binding, you'd declare "local
foo", for example. You can already do "global foo".)

> I see what you are saying (I think), but I think I'd still like a
> full anonymous def, whatever adapter you come up with. And I prefer
> to be persuaded ;-)

I elaborated on this one in a post a few days ago. Indeed, it is
mostly a minor issue that _can_ be worked around(1). The problem is
that it eventually becomes irritating, when repeated all the time, to
name functions even if the name isn't used elsewhere.

It also creates an implicit dependency from the function call (one of
whose arguments points to the once-named function) to the once-named
function. That is, when you refactor some of your code, you must keep
two things paired all the time in your cut+paste maneuvers.


br,
S

(1) Everything can be worked around. In contrast: you can work around
the lack of a syntactic language by typing in machine code manually.
Sound stupid? Yes, it was done decades ago. How about using C to write
a "shell script" equivalent that you need, as a workaround to the
problem of lacking a shell? Stupid? Yes, but less -- it's been done.
How about writing callbacks by passing in a function pointer and a
data pointer, if you don't want to use a language like Python that
automates that task for you? Stupid? Yes, but not much -- it's been
done all the time. How about implementing an _anonymous_ function by
naming it, if you can't do it otherwise?

Message has been deleted

Simo Melenius

unread,
Dec 31, 2004, 4:56:09 PM12/31/04
to
Ian Bicking <ia...@colorstudy.com> writes:

> But I do think there's other ways to approach this. Function


> expressions could get really out of hand, IMHO, and could easily lead
> to twenty-line "expressions". That's aesthetically incompatible with
> Python source, IMHO.

You can already write unaesthetic hundred-line Python functions, if
you want to. Python's syntax doesn't yet impose a restriction on the
number of sequential statements :-) It sounds artificial to impose
such restrictions on these hypothetical "inline blocks", even if by
only allowing them to be plain expressions.

IMHO, the most pythonic way to write an "inline-block" is by reusing
existing keywords, using Python-like start-of-blocks and ending it by
indentation rules:

map (def x:
if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)
return baz_3 (hab),
[1,2,3,4,5,6])

and for one-liners:

map (def x: return x**2, [1,2,3,4,5,6])

As a side-effect, we also

- got rid of the "lambda" keyword;

- strenghtened the semantics of "def": a "def" already defines a
function so it's only logical to use it to define anonymous
functions, too;

- unified the semantics: function is always a function, and functions
return values by using "return". When learning Python, I learned the
hard way that "lambda"s are expressions, not functions. I'd pay the
burden of writing "return" more often in exchange for better
consistency.


my two cents,
br,
S

Steven Bethard

unread,
Dec 31, 2004, 5:26:38 PM12/31/04
to
Simo Melenius wrote:
> map (def x:
> if foo (x):
> return baz_1 (x)
> elif bar (x):
> return baz_2 (x)
> else:
> global hab
> hab.append (x)
> return baz_3 (hab),
> [1,2,3,4,5,6])

I think this would probably have to be written as:

map (def x:
if foo(x):
return baz_1(x)
elif bar(x):
return baz_2(x)
else:
global hab
hab.append(x)


return baz_3(hab)
, [1,2,3,4,5,6])

or:

map (def x:
if foo(x):
return baz_1(x)
elif bar(x):
return baz_2(x)
else:
global hab
hab.append(x)


return baz_3(hab)
,
[1,2,3,4,5,6])

Note the placement of the comma. As it is,
return baz_3(hab),
returns the tuple containing the result of calling baz_3(hab):

py> def f(x):
... return float(x),
...
py> f(1)
(1.0,)

It's not horrible to have to put the comma on the next line, but it
isn't as pretty as your version that doesn't. Unfortunately, I don't
think anyone's gonna want to revise the return statement syntax just to
introduce anonymous functions.

Steve

Simo Melenius

unread,
Dec 31, 2004, 5:56:30 PM12/31/04
to
Steven Bethard <steven....@gmail.com> writes:

> Simo Melenius wrote:
> > map (def x:
> > if foo (x):
> > return baz_1 (x)
> > elif bar (x):
> > return baz_2 (x)
> > else:
> > global hab
> > hab.append (x)
> > return baz_3 (hab),
> > [1,2,3,4,5,6])
>
> I think this would probably have to be written as:

...


> return baz_3(hab)
> , [1,2,3,4,5,6])
> or:

...


> return baz_3(hab)
> ,
> [1,2,3,4,5,6])
>
> Note the placement of the comma. As it is,
> return baz_3(hab),
> returns the tuple containing the result of calling baz_3(hab):

That one didn't occur to me; creating a one-item tuple with (foo,) has
been odd enough for me: only few times I've seen also the parentheses
omitted.

I did ponder the unambiguousness of the last line, though. One could
suggest a new keyword like "end", but keyword bloat is bad.

(Of course, if we trade the "lambda" keyword for another, new keyword
we're not exactly _adding_ keywords... :))

> It's not horrible to have to put the comma on the next line, but it
> isn't as pretty as your version that doesn't. Unfortunately, I don't
> think anyone's gonna want to revise the return statement syntax just
> to introduce anonymous functions.

There might not be a return statement: the anonymous function might
conditionally return earlier and have side-effects at the end of the
block (to implicitly return None). So the block-ending would need to
fit after any statement and be strictly unambiguous.


br,
S

Doug Holton

unread,
Dec 31, 2004, 6:09:53 PM12/31/04
to
Steven Bethard wrote:

> Simo Melenius wrote:
>
>> map (def x:
>> if foo (x):
>> return baz_1 (x)
>> elif bar (x):
>> return baz_2 (x)
>> else:
>> global hab
>> hab.append (x)
>> return baz_3 (hab),
>> [1,2,3,4,5,6])
>
>
> I think this would probably have to be written as:

Right the comma plus other things make this difficult for a parser to
handle correctly. Other people have already come up with working solutions.

We have a special way to pass a multiline closure as a parameter to a
function. Put it outside the parameter list.

First, the single-line way using curly braces:

newlist = map({x as int | return x*x*x}, [1,2,3,4,5,6])

Then the multi-line way. I had to add an overload of map to support
reversing the order of parameters (list first, then the closure):

newlist = map([1,2,3,4,5,6]) def (x as int):
return x*x*x

for item in newlist:
print item

Paul L. Du Bois

unread,
Dec 31, 2004, 6:41:41 PM12/31/04
to
Alex Martelli wrote:
> We should have an Evilly Cool Hack of the Year, and I nominate
> Paul Du Bois's one as the winner for 2004. Do I hear any second...?

Oh bother, I just realized I sent my first reply using a good email
address. Hope that cancel goes through quickly.

Anyway, thank you! I've submitted it as a recipe. It includes a
less-evil portable version, which might be improved further if PEP 288
ever comes through.

p

The recipe:
http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/361452

Steven Bethard

unread,
Dec 31, 2004, 7:00:22 PM12/31/04
to

Ok, now that we're past 0:00:00 UTC, I'll second that nomination! ;)

Steve

P.S. Happy New Year all!

Bengt Richter

unread,
Dec 31, 2004, 9:56:33 PM12/31/04
to

ISTM you don't need "end" -- just put the def expression in parens,
and let the closing paren end it, e.g.:

map((def x:


if foo (x):
return baz_1 (x)
elif bar (x):
return baz_2 (x)
else:
global hab
hab.append (x)

return baz_3 (hab)), [1,2,3,4,5,6])

>
>(Of course, if we trade the "lambda" keyword for another, new keyword
>we're not exactly _adding_ keywords... :))
>
>> It's not horrible to have to put the comma on the next line, but it
>> isn't as pretty as your version that doesn't. Unfortunately, I don't
>> think anyone's gonna want to revise the return statement syntax just
>> to introduce anonymous functions.
>
>There might not be a return statement: the anonymous function might
>conditionally return earlier and have side-effects at the end of the
>block (to implicitly return None). So the block-ending would need to
>fit after any statement and be strictly unambiguous.

Just use parens as necessary or when in doubt ;-)

Regards,
Bengt Richter

Terry Reedy

unread,
Dec 31, 2004, 10:25:40 PM12/31/04
to pytho...@python.org

"Simo Melenius" <firstname...@iki.fi-spam> wrote in message
news:m24qi2w3...@geist.local...

> bo...@oz.net (Bengt Richter) writes:
> Which brings me to the point where I'd welcome more flexibility in
> writing to variables outside the local scope.

This idea was discussed extensively on PyDev perhaps 2 years ago. (You can
check the archived summaries if interested enough ;-) As I remember, there
was no consensus either on the desireability of such a mechanism or on the
syntax (several were proposed).

Terry J. Reedy

Simo Melenius

unread,
Jan 1, 2005, 4:26:59 PM1/1/05