escaping - How can I selectively escape percent (%) in Python strings?

405
votes

I have the following code

test = "have it break."
selectiveEscape = "Print percent % in sentence and not %s" % test

print(selectiveEscape)

I would like to get the output:

Print percent % in sentence and not have it break.

What actually happens:

    selectiveEscape = "Use percent % in sentence and not %s" % test
TypeError: %d format: a number is required, not str
6
pythonescapingpython-2.7
Why isn't it \%? That was my guess, I'm surprised to find it's %% instead - seems pretty counterintuitive. - Demis
% i means "a decimal representation of an integer, padded left with spaces. - Antti Haapala
The escape is to the function, not the language syntax. Hence if the escape was \% it would actually be \\% when written in ordinary code. <escape><escape> is the typical pattern I've seen, and \ happens to be the most common escape character, for better or worse. - shemnon
@Demis and how do you escape \ if you had to print \\%? You are bound to require escaping through repetition of special characters, if the special characters are also not special depending on circumstances. - Sassa NF
I think it is annoying in Python that the the literal % is encoded by "%%" and not by "\%". - MasterControlProgram

6 Answers

687
votes
>>> test = "have it break."
>>> selectiveEscape = "Print percent %% in sentence and not %s" % test
>>> print selectiveEscape
Print percent % in sentence and not have it break.
60
votes

Alternatively, as of Python 2.6, you can use new string formatting (described in PEP 3101):

'Print percent % in sentence and not {0}'.format(test)

which is especially handy as your strings get more complicated.

43
votes

try using %% to print % sign .

8
votes

You can't selectively escape %, as % always has a special meaning depending on the following character.

In the documentation of Python, at the bottem of the second table in that section, it states:

'%'        No argument is converted, results in a '%' character in the result.

Therefore you should use:

selectiveEscape = "Print percent %% in sentence and not %s" % (test, )

(please note the expicit change to tuple as argument to %)

Without knowing about the above, I would have done:

selectiveEscape = "Print percent %s in sentence and not %s" % ('%', test)

with the knowledge you obviously already had.

3
votes

If the formatting template was read from a file, and you cannot ensure the content doubles the percent sign, then you probably have to detect the percent character and decide programmatically whether it is the start of a placeholder or not. Then the parser should also recognize sequences like %d (and other letters that can be used), but also %(xxx)s etc.

Similar problem can be observed with the new formats -- the text can contain curly braces.

3
votes

If you are using Python 3.6 or newer, you can use f-string:

>>> test = "have it break."
>>> selectiveEscape = f"Print percent % in sentence and not {test}"
>>> print(selectiveEscape)
... Print percent % in sentence and not have it break.