keyword for introducing generators

[Neil Girdhar]

I'm sure this has been suggested before, but I just spent two days trying to figure out why a method wasn't being called only to find that I'd accidentally pasted a yield into the function.  What is the argument against a different keyword for introducing generator functions/methods?

If it's backward compatibility, then my suggestion to have a from __future__ and then make it real in Python 4.

Neil

[Chris Angelico]

On Sun, Aug 17, 2014 at 7:46 AM, Neil Girdhar <miste...@gmail.com> wrote:
> I'm sure this has been suggested before, but I just spent two days trying to
> figure out why a method wasn't being called only to find that I'd
> accidentally pasted a yield into the function. What is the argument against
> a different keyword for introducing generator functions/methods?

There are quite a few changes to a function based on its body, like
how the presence of assignment causes a name to be local unless
explicitly declared otherwise. It's not necessary to predeclare
everything.

But if you're having trouble with a function like that, maybe a little
decorator would help:

def announce(f):
def inner(*a,**kw):
print("Calling:",f.__name__)
ret=f(*a,**kw)
print("Return value is a",type(ret))
return ret
return inner

Decorate a function @announce, and it'll tell you (a) that it's being
called, and (b) what type its return value is. If that's a generator,
well, there's your answer.

ChrisA
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[Neil Girdhar]

This is only a good solution once I realize that the method isn't being called after which, I now know what to look for.  The whole point is to defensively design a language so that I don't get into this problem in the first place.  Since I always know when I'm writing a function whether I want it to be automatically generator-returning using the yield keyword or not, then why not let me specify that?  That way if the function gets long and I forget yield or accidentally include it, I get a reasonable error.

You're right that it's not necessary to predeclare everything, but a different keyword is hardly more work.  (And with respect to local, nonlocal and global variables, we often do predeclare those.)

Best,

Neil

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[Jonas Wielicki]

For what it’s worth, I know this problem very well, and it can take
hours to figure out whats wrong.

regards,
jwi

>
> Neil

[Andrew Barnert]

On Aug 16, 2014, at 14:55, Chris Angelico <ros...@gmail.com> wrote:

> On Sun, Aug 17, 2014 at 7:46 AM, Neil Girdhar <miste...@gmail.com> wrote:
>> I'm sure this has been suggested before, but I just spent two days trying to
>> figure out why a method wasn't being called only to find that I'd
>> accidentally pasted a yield into the function. What is the argument against
>> a different keyword for introducing generator functions/methods?
>
> There are quite a few changes to a function based on its body, like
> how the presence of assignment causes a name to be local unless
> explicitly declared otherwise. It's not necessary to predeclare
> everything.
>
> But if you're having trouble with a function like that, maybe a little
> decorator would help:
>
> def announce(f):
> def inner(*a,**kw):
> print("Calling:",f.__name__)
> ret=f(*a,**kw)
> print("Return value is a",type(ret))
> return ret
> return inner

I think it would be both simpler and more useful for him to write:

def generator(f):
assert inspect.isgeneratorfunction(f)
return f

def function(f):
asset not inspect.isgeneratorfunction(f)
return f

Then he can just declare his functions as @generator or @function as appropriate and get an error at definition time if he accidentally got something wrong.

[Guido van Rossum]

On Sat, Aug 16, 2014 at 3:17 PM, Jonas Wielicki <j.wie...@sotecware.net> wrote:

On 16.08.2014 23:46, Neil Girdhar wrote:
> I'm sure this has been suggested before, but I just spent two days trying
> to figure out why a method wasn't being called only to find that I'd
> accidentally pasted a yield into the function.  What is the argument
> against a different keyword for introducing generator functions/methods?
>
> If it's backward compatibility, then my suggestion to have a from
> __future__ and then make it real in Python 4.

For what it’s worth, I know this problem very well, and it can take
hours to figure out whats wrong.

A linter should be able to figure this out. For example, mypy will insist that a generator has a return type of Iterable[...]. So maybe you won't have to wait for Python 4; if the mypy proposal goes forward you will be able to use type annotations to distinguish generators.

--
--Guido van Rossum (python.org/~guido)

[Neil Girdhar]

Sorry for the late reply. If I mark a function as returning Iterable, the linter can check that I use yield in the function, but if I don't mark the function as returning anything, the linter should also check that I didn't accidentally use yield, which is the time-consuming bug that we're talking about.  So this works for me.

Best,

Neil

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