Python string formatting when string contains "%s" without escaping

Question

When formatting a string, my string may contain a modulo "%" that I do not wish to have converted. I can escape the string and change each "%" to "%%" as a workaround.

e.g.,

'Day old bread, 50%% sale %s' % 'today!'

output:

'Day old bread, 50% sale today'

But are there any alternatives to escaping? I was hoping that using a dict would make it so Python would ignore any non-keyword conversions.
e.g.,

'Day old bread, 50% sale %(when)s' % {'when': 'today'}

but Python still sees the first modulo % and gives a:

TypeError: not enough arguments for format string

escaping % is not a workaround, it's a documented and official way of doing this. — SilentGhost, May 17 '10 at 07:41

Tim Pietzcker · Accepted Answer · 2010-05-17T08:15:31.973

24

You could (and should) use the new string .format() method (if you have Python 2.6 or higher) instead:

"Day old bread, 50% sale {0}".format("today")

The manual can be found here.

The docs also say that the old % formatting will eventually be removed from the language, although that will surely take some time. The new formatting methods are way more powerful, so that's a Good Thing.

edited May 17 '10 at 08:15

answered May 17 '10 at 07:36

Tim Pietzcker

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Unless you want to print "{{0}} = {0}".format("today"), then you won't like doubling up the curly braces. :^) – Mark Tolonen May 18 '10 at 01:27
@MarkTolonen Ugg, I specifically came here hoping to find a solution to the "{{0}}".format(...) problem... – KSletmoe Aug 02 '13 at 01:13

cfillol · Answer 2 · 2014-04-23T15:47:02.180

2

You can use regular expressions to replace % by %% where % is not followed by (

def format_with_dict(str, dictionary):
    str = re.sub(r"%([^\(])", r"%%\1", str)
    str = re.sub(r"%$", r"%%", str)  # There was a % at the end?
    return str % dictionary

This way:

print format_with_dict('Day old bread, 50% sale %(when)s', {'when': 'today'})

Will output:

Day old bread, 50% sale today

This method is useful to avoid "not enough arguments for format string" errors.

edited Apr 23 '14 at 15:47

answered Apr 23 '14 at 15:32

cfillol

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There is a problem with replacing only one % occurrence and not replacing two and more % occurrences. – alisher-matkurbanov Jul 21 '20 at 19:22

score 2 · Answer 3 · answered May 17 '10 at 07:32

2

Not really - escaping your % signs is the price you pay for using string formatting. You could use string concatenation instead: 'Day old bread, 50% sale ' + whichday if that helps...

answered May 17 '10 at 07:32

psmears

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For where I need to support Python 2.5 or prior, this is the method I'll use. – Stephen Gornick May 17 '10 at 17:23

score 2 · Answer 4 · answered May 17 '10 at 07:35

Escaping a '%' as '%%' is not a workaround. If you use String formatting that is the way to represent a '%' sign. If you don't want that, you can always do something like:

print "Day old bread, 50% sale " + "today"

e.g. not using formatting.

Please note that when using string concatenation, be sure that the variable is a string (and not e.g. None) or use str(varName). Otherwise you get something like 'Can't concatenate str and NoneType'.

Python string formatting when string contains "%s" without escaping

4 Answers4

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