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Escapeism

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Kay Schluehr

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Sep 30, 2006, 5:18:04 AM9/30/06
to
Usually I struggle a short while with \ and either succeed or give up.
Today I'm in a different mood and don't give up. So here is my
question:

You have an unknown character string c such as '\n' , '\a' , '\7' etc.

How do you echo them using print?

print_str( c ) prints representation '\a' to stdout for c = '\a'
print_str( c ) prints representation '\n' for c = '\n'
...

It is required that not a beep or a linebreak shall be printed.

First of all it has be remarked that it is impossible to a certain
extent. That's because e.g. c = '\a' and c = '\7' do represent the same
string but this ambiguity doesn't occur for many numbers. But lets
weaken the requirement and fix a canonical representation in case of
ambiguity. I'm still getting stuck here.

Peter Otten

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Sep 30, 2006, 5:33:45 AM9/30/06
to
Kay Schluehr wrote:

I don't understand the question. Wouldn't the canonical representation be
repr(c) or repr(c)[1:-1]?

Peter

Sybren Stuvel

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Sep 30, 2006, 5:36:39 AM9/30/06
to
Kay Schluehr enlightened us with:

> Usually I struggle a short while with \ and either succeed or give up.
> Today I'm in a different mood and don't give up. So here is my
> question:
>
> You have an unknown character string c such as '\n' , '\a' , '\7' etc.
>
> How do you echo them using print?
>
> print_str( c ) prints representation '\a' to stdout for c = '\a'
> print_str( c ) prints representation '\n' for c = '\n'
> ...
>
> It is required that not a beep or a linebreak shall be printed.

try "print repr(c)".

Sybren
--
Sybren Stüvel
Stüvel IT - http://www.stuvel.eu/

Kay Schluehr

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Sep 30, 2006, 1:02:07 PM9/30/06
to
Sybren Stuvel wrote:
> Kay Schluehr enlightened us with:
> > Usually I struggle a short while with \ and either succeed or give up.
> > Today I'm in a different mood and don't give up. So here is my
> > question:
> >
> > You have an unknown character string c such as '\n' , '\a' , '\7' etc.
> >
> > How do you echo them using print?
> >
> > print_str( c ) prints representation '\a' to stdout for c = '\a'
> > print_str( c ) prints representation '\n' for c = '\n'
> > ...
> >
> > It is required that not a beep or a linebreak shall be printed.
>
> try "print repr(c)".

This yields the hexadecimal representation of the ASCII character and
does not simply echo the keystrokes '\' and 'a' for '\a' ignoring the
escape semantics. One way to achieve this naturally is by prefixing
'\a' with r where r'\a' indicates a "raw" string. But unfortunately
"rawrification" applies only to string literals and not to string
objects ( such as c ). I consider creating a table consisting of pairs
{'\0': r'\0','\1': r'\1',...} i.e. a handcrafted mapping but maybe
I've overlooked some simple function or trick that does the same for
me.

Kay

Duncan Booth

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Sep 30, 2006, 1:23:52 PM9/30/06
to
"Kay Schluehr" <kay.sc...@gmx.net> wrote:

>> try "print repr(c)".
>
> This yields the hexadecimal representation of the ASCII character and
> does not simply echo the keystrokes '\' and 'a' for '\a' ignoring the
> escape semantics.

But you yourself noted earlier that '\a' and '\x07' are the same string
and said:

> But lets weaken the requirement and fix a canonical representation in
> case of ambiguity.

That's exactly what repr(c) does, it uses a canonical representation
with '\t', '\r', '\n', '\\', (and when it has to "\'", '\"') using the
short escape form (because they are so commonly used), and the all the
other (more obscure) escape sequences using the hexadecimal form.

BTW, c.encode('string_escape') is another way to convert a string to almost
the same escaped form (except for a minor difference in the treatment of
quote characters).

> But unfortunately "rawrification" applies only to string literals and
> not to string objects ( such as c ).

Oh dear, the fact that you could even consider writing that sentence seems
to show a fundamental misunderstanding of what a raw string literal means.

Steve Holden

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Sep 30, 2006, 2:51:09 PM9/30/06
to pytho...@python.org
No, you've overlooked the fact that if you print the string containing
the two characters "backslash" and "lower case a" then it will print
exactly those two characters. See:

In [30]: c = "\\a"

In [31]: len(c)
Out[31]: 2

In [32]: print c
\a

regards
Steve
--
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Holden Web LLC/Ltd http://www.holdenweb.com
Skype: holdenweb http://holdenweb.blogspot.com
Recent Ramblings http://del.icio.us/steve.holden

Kay Schluehr

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Sep 30, 2006, 3:01:04 PM9/30/06
to

I'm interested in the transition between two literals from which one is
a string literal containing \ as a "meta character" s.t. '\a' has
actually length 1 and is beep when printed to stdout and its "raw" form
without a meta character interpretation of \ that leads to the result
you described. Using the string prefix r to '\a' indicates the raw form
to the compiler. But there seems to be no runtime counterpart. I've
suggested a naive implementation such as

def rawform(c):
return {'\a': r'\a'}[c]

Here the function returns for the single input character '\a' the two
character raw form by means of escaping \ ( and raises a KeyError
exception otherwise ).

>>> c = '\a'
>>> print rawform(c)
\a

This has the same effect as writing:
>>> c = r'\a'
>>> print c
\a

But there is some ambiguity due to the the fact that applying '\7' to
rawform() yields r'\a' and not r'\7'. So one needs more specification
for disambiguation using e.g. an extra parameter.

Leonhard Vogt

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Sep 30, 2006, 4:50:18 PM9/30/06
to
> But there is some ambiguity due to the the fact that applying '\7' to
> rawform() yields r'\a' and not r'\7'. So one needs more specification
> for disambiguation using e.g. an extra parameter.
>
>>> '\a'=='\7'
True

The two are actually the same thing, so how could a function decide
whether to return '\\a' or '\\7'.

It's like asking the following:
>>> c = 04
>>> print rawform(c)
04

>>> c = 4
>>> print rawform(c)
4

which is obviously not possible, neither of any use imho.

Leonhard

Steve Holden

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Sep 30, 2006, 5:52:48 PM9/30/06
to pytho...@python.org
In [33]: "\7" == "\a"
Out[33]: True

Sorry. It can't possibly know which of two alternative representation
were used to represent a particular character in a literal.

Frederic Rentsch

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Oct 1, 2006, 3:58:49 AM10/1/06
to pytho...@python.org
Kay,

This is perhaps yet another case for SE? I don't really know, because I
don't quite get what you're after. See for yourself:

>>> import SE
>>> Printabilizer = SE.SE ( '''
(1)=\\1 # All 256 octets can be written as parenthesized ascii
(2)=\\2
"\a=\\a" # (7)=\\a"
"\n=\\n" # or (10)=\\n or (10)=LF or whatever
"\r=\\r" # (13)=CR
"\f=\\f"
"\v=\\v"
# Add whatever other ones you like
# and translate them to anything you like.
''')

>>> print Printabilizer ('abd\aefg\r\nhijk\vlmnop\1\2.')
abd\aefg\r\nhijk\vlmno\1\2.

If you think this may help, you'll find SE here:
http://cheeseshop.python.org/pypi/SE/2.2%20beta


Regards

Frederic

Kay Schluehr

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Oct 1, 2006, 5:20:15 AM10/1/06
to

This looks quite good. "rawrification" or "printabalization" that's
exactly what I was looking for and I thought this problem would be so
common that someone has done an implementation already. Thanks,
Frederik!

Kay

Hendrik van Rooyen

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Oct 1, 2006, 3:39:29 AM10/1/06
to pytho...@python.org
"Kay Schluehr" <kay.sc...@gmx.net> wrote:

dumb question - is the backslash as escape character fixed or can one set its
(the escape char's)
value so that backslash is not the escape char?

seems to me that would help - or if you could turn the behaviour off - don't
know how though...

- Hendrik

Fredrik Lundh

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Oct 1, 2006, 5:46:22 AM10/1/06
to pytho...@python.org
Kay Schluehr wrote:

> This yields the hexadecimal representation of the ASCII character and
> does not simply echo the keystrokes '\' and 'a' for '\a' ignoring the
> escape semantics. One way to achieve this naturally is by prefixing
> '\a' with r where r'\a' indicates a "raw" string. But unfortunately
> "rawrification" applies only to string literals and not to string
> objects ( such as c ). I consider creating a table consisting of pairs
> {'\0': r'\0','\1': r'\1',...} i.e. a handcrafted mapping but maybe
> I've overlooked some simple function or trick that does the same for
> me.

if not else, you've missed that octal escapes consists of three digits,
not one, so translating chr(1) to r"\1" doesn't work in the general case
(e.g. len("\100") == 1, not 3)

</F>

Fredrik Lundh

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Oct 1, 2006, 6:12:36 AM10/1/06
to pytho...@python.org
Hendrik van Rooyen wrote:

> dumb question - is the backslash as escape character fixed

yes.

> seems to me that would help

help with what?

> or if you could turn the behaviour off - don't know how though...

eh? if you don't want to use repr(), you don't have to.

</F>

Matthew Warren

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Oct 3, 2006, 11:07:06 AM10/3/06
to pytho...@python.org

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Matthew Warren

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Oct 3, 2006, 11:48:23 AM10/3/06
to Matthew Warren, pytho...@python.org

((I have no idea why the following was missing from my first attempt to
post, and since posting the thread has gone over my head, but wasn't the
OP just after a way to print user-inputted strings with '\' in
them?))...

I cant test this where I am, but would the following work

We have our literal user-inputted strings in a list,

l=['\a','\b','\c'] etc..

For s in l:
for c in s:
print c,
print

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