Adding backslashes without escaping

Question

I need to escape a & (ampersand) character in a string. The problem is whenever I string = string.replace ('&', '\&') the result is '\\&'. An extra backslash is added to escape the original backslash. How do I remove this extra backslash?

Solution 1

The result '\\&' is only displayed - actually the string is \&:

>>> str = '&'
>>> new_str = str.replace('&', '\&')
>>> new_str
'\\&'
>>> print new_str
\&

Try it in a shell.

Solution 2

The extra backslash is not actually added; it's just added by the repr() function to indicate that it's a literal backslash. The Python interpreter uses the repr() function (which calls __repr__() on the object) when the result of an expression needs to be printed:

>>> '\\'
'\\'
>>> print '\\'
\
>>> print '\\'.__repr__()
'\\'

Solution 3

Python treats \ in literal string in a special way.
This is so you can type '\n' to mean newline or '\t' to mean tab
Since '\&' doesn't mean anything special to Python, instead of causing an error, the Python lexical analyser implicitly adds the extra \ for you.

Really it is better to use \\& or r'\&' instead of '\&'

The r here means raw string and means that \ isn't treated specially unless it is right before the quote character at the start of the string.

In the interactive console, Python uses repr to display the result, so that is why you see the double '\'. If you print your string or use len(string) you will see that it is really only the 2 characters

Some examples

>>> 'Here\'s a backslash: \\'
"Here's a backslash: \\"
>>> print 'Here\'s a backslash: \\'
Here's a backslash: \
>>> 'Here\'s a backslash: \\. Here\'s a double quote: ".'
'Here\'s a backslash: \\. Here\'s a double quote: ".'
>>> print 'Here\'s a backslash: \\. Here\'s a double quote: ".'
Here's a backslash: \. Here's a double quote ".

To Clarify the point Peter makes in his comment see this link

Unlike Standard C, all unrecognized escape sequences are left in the string unchanged, i.e., the backslash is left in the string. (This behavior is useful when debugging: if an escape sequence is mistyped, the resulting output is more easily recognized as broken.) It is also important to note that the escape sequences marked as “(Unicode only)” in the table above fall into the category of unrecognized escapes for non-Unicode string literals.

Solution 4

>>> '\\&' == '\&'
True
>>> len('\\&')
2
>>> print('\\&')
\&

Or in other words: '\\&' only contains one backslash. It's just escaped in the python shell's output for clarity.

Solution 5

printing a list can also cause this problem (im new in python, so it confused me a bit too):

>>>myList = ['\\']
>>>print myList
['\\']
>>>print ''.join(myList)
\ 

similarly:

>>>myList = ['\&']
>>>print myList
['\\&']
>>>print ''.join(myList)
\&

View more solutions

Comments

[Peter Hansen]

Part of this isn't correct. Python does not "implicitly add the extra `` for you". It does, however, double the backslash when you display the repr() output of the string, as at prompt, purely for presentation purposes. len("\&") is only 2, proving there is no implicit munging of your data (thank heavens!).

[erikbstack]

Why is this not marked as solution?

[Michael Mior]

@PeterHansen I think the OP was pointing out that blackslashes should normally be escaped in non-raw strings so it would normally be written doubled.

[matt_js]

The OP needs the literal string to be '\&' not '\\&'.