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Using repr() with escape sequences

 
 
nummertolv
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      02-23-2006
Hi,

My application is receiving strings, representing windows paths, from
an external source. When using these paths, by for instance printing
them using str() (print path), the backslashes are naturally
interpreted as escape characters.

>>> print "d:\thedir"

d: hedir

The solution is to use repr() instead of str():

>>> print repr("d:\thedir")

'd:\thedir'

What I have not been able to figure out is how to handle escape
sequences like \a, \b, \f, \v and \{any number} inside the paths. Using
repr() on these escape sequences either prints the hex value of the
character (if "unprintable" i guess) or some character ( like in the
last example below).

>>> print repr("d:\thedir\10")

'd:\thedir\x08'

>>> print repr("d:\thedir\foo")

'd:\thedir\x0coo'

>>> print repr("d:\thedir\100")

'd:\thedir@'

Could someone clear this out for me and let me know how I can find the
"real" path that I am trying to receive?

/Henrik

 
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Steven D'Aprano
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      02-23-2006
On Thu, 23 Feb 2006 07:32:36 -0800, nummertolv wrote:

> Hi,
>
> My application is receiving strings, representing windows paths, from
> an external source. When using these paths, by for instance printing
> them using str() (print path), the backslashes are naturally
> interpreted as escape characters.
>
>>>> print "d:\thedir"

> d: hedir


No. What is happening here is not what you think is happening.

> The solution is to use repr() instead of str():


The solution to what? What is the problem? The way the strings are
DISPLAYED is surely not the issue, is it?

>>>> print repr("d:\thedir")

> 'd:\thedir'



You have created a string object: "d:\thedir"

That string object is NOT a Windows path. It contains a tab character,
just like the print statement shows -- didn't you wonder about the large
blank space in the string?

Python uses backslashes for character escapes. \t means a tab character.
When you enter "d:\thedir" you are embedding a tab between the colon and
the h.

The solutions to this problem are:

(1) Escape the backslash: "d:\\thedir"

(2) Use raw strings that don't use char escapes: r"d:\thedir"

(3) Use forward slashes, and let Windows automatically handle them:
"d:/thedir"

However, if you are receiving strings from an external source, as you say,
and reading them from a file, this should not be an issue. If you read a
file containing "d:\thedir", and print the string you have just read, the
print statement uses repr() and you will see that the string is just
what you expect:

d:\thedir

You can also check for yourself that the string is correct by looking at
its length: nine characters.


--
Steven.

 
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nummertolv
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      02-23-2006
I think I might have misused the terms "escape character" and/or
"escape sequence" or been unclear in some other way because I seem to
have confused you. In any case you don't seem to be addressing my
problem.

I know that the \t in the example path is interpreted as the tab
character (that was part of the point of the example) and what the
strings are representing is irrelevant. And yes, the way the strings
are displayed is part of the issue.

So let me try to be clearer by boiling the problem down to this:

- Consider a string variable containing backslashes.
- One or more of the backslashes are followed by one of the letters
a,b,f,v or a number.

myString = "bar\foo\12foobar"

How do I print this string so that the output is as below?

bar\foo\12foobar

typing 'print myString' prints the following:

bar oo
foobar

and typing print repr(myString) prints this:

'bar\x0coo\nfoobar'


Hope this makes it clearer. I guess there is a simple solution to this
but I have not been able to find it. Thanks.

/H

 
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Sybren Stuvel
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      02-23-2006
nummertolv enlightened us with:
> myString = "bar\foo\12foobar"


Are the interpretations of the escape characters on purpose?

> How do I print this string so that the output is as below?
>
> bar\foo\12foobar


Why do you want to?

> typing 'print myString' prints the following:
>
> bar oo
> foobar


Which is correct.

Sybren
--
The problem with the world is stupidity. Not saying there should be a
capital punishment for stupidity, but why don't we just take the
safety labels off of everything and let the problem solve itself?
Frank Zappa
 
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Daniel Dittmar
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      02-23-2006
nummertolv wrote:
> - Consider a string variable containing backslashes.
> - One or more of the backslashes are followed by one of the letters
> a,b,f,v or a number.
>
> myString = "bar\foo\12foobar"
>
> How do I print this string so that the output is as below?
>
> bar\foo\12foobar
>
> typing 'print myString' prints the following:
>
> bar oo
> foobar
>
> and typing print repr(myString) prints this:
>
> 'bar\x0coo\nfoobar'
>


The interpretation of escape sequences happens when the Python compiler
reads the string "bar\foo\12foobar". You'll see that when you do
something like
>>> map (ord, "bar\foo\12foobar")

[98, 97, 114, 12, 111, 111, 10, 102, 111, 111, 98, 97, 114]
This displays the ASCII values of all the characters.

If you want to use a string literal containing backslashes, use r'' strings:
>>> myString = r'bar\foo\12foobar'
>>> map (ord, myString)

[98, 97, 114, 92, 102, 111, 111, 92, 49, 50, 102, 111, 111, 98, 97, 114]
>>> print myString

bar\foo\12foobar
>>> print repr (myString)

'bar\\foo\\12foobar'

If you get the strings from an external source as suggested by your
original post, then you really have no problem at all. No interpretation
of escape sequences takes place when you read a string from a file.

Daniel
 
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Dennis Lee Bieber
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      02-23-2006
On 23 Feb 2006 08:49:37 -0800, "nummertolv" <(E-Mail Removed)>
declaimed the following in comp.lang.python:

>
> myString = "bar\foo\12foobar"
>

This is a string literal INSIDE the Python program. The Python
interpreter sees the escape characters.

C:\Documents and Settings\Dennis Lee Bieber>python
ActivePython 2.3.5 Build 236 (ActiveState Corp.) based on
Python 2.3.5 (#62, Feb 9 2005, 16:17:0 [MSC v.1200 32 bit (Intel)] on
win32
Type "help", "copyright", "credits" or "license" for more information.
>>> aString = raw_input()

bar\foo\12foobar
>>> print aString

bar\foo\12foobar
>>> bString = "bar\foo\12foobar"
>>> print bString

bar?oo
foobar
>>> print repr(aString)

'bar\\foo\\12foobar'
>>> print repr(bString)

'bar\x0coo\nfoobar'
>>>



This one is one that was read from an outside source (I typed just
bar\foo\12foobar and nothing else). No escape characters are seen
because the input system does not parse data to create escapes. Each
character came in as expected.
--
> ================================================== ============ <
> http://www.velocityreviews.com/forums/(E-Mail Removed) | Wulfraed Dennis Lee Bieber KD6MOG <
> (E-Mail Removed) | Bestiaria Support Staff <
> ================================================== ============ <
> Home Page: <http://www.dm.net/~wulfraed/> <
> Overflow Page: <http://wlfraed.home.netcom.com/> <

 
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nummertolv
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      02-27-2006
myString = "bar\foo\12foobar"
print repr(myString)

My "problem" was that I wanted to know if there is a way of printing
"unraw" strings like myString so that the escape characters are written
like a backslash and a letter or number. My understanding was that
repr() did this and it does in most cases (\n and \t for instance). In
the cases of \a,\b,\f and \v however, it prints hexadecimal numbers.
But I guess I'll just have to live with that and as you point out, it
doesn't have to be a problem anyway.

 
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