Python way of printing: with 'format' or percent form?

Question

In Python there seem to be two different ways of generating formatted output:

user = "Alex"
number = 38746
print("%s asked %d questions on stackoverflow.com" % (user, number))
print("{0} asked {1} questions on stackoverflow.com".format(user, number))

Is there one way to be preferred over the other? Are they equivalent, what is the difference? What form should be used, especially for Python3?

Solution 1

Use the format method, especially if you're concerned about Python 3 and the future. From the documentation:

The formatting operations described here are modelled on C's printf() syntax. They only support formatting of certain builtin types. The use of a binary operator means that care may be needed in order to format tuples and dictionaries correctly. As the new :ref:string-formatting syntax is more flexible and handles tuples and dictionaries naturally, it is recommended for new code. However, there are no current plans to deprecate printf-style formatting.

Solution 2

.format was introduced in Python2.6

If you need backward compatibility with earlier Python, you should use %

For Python3 and newer you should use .format for sure

.format is more powerful than %. Porting % to .format is easy but the other way round can be non trivial

Solution 3

The docs say that the format method is preferred for new code. There are currently no plans to remove % formatting, though.

Solution 4

You can use both .No one said % formatting expression is deprecated.However,as stated before the format method call is a tad more powerful. Also note that the % expressions are bit more concise and easier to code.Try them and see what suits you best

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Comments

[guettli]

New documentation says this about the percent operator: "The formatting operations described here exhibit a variety of quirks that lead to a number of common errors ..." but it is not deprecated. docs.python.org/3/library/…

[BrenBarn]

It's not officially deprecated, but you're encouraged not to use it.

[guettli]

Related: bugs.python.org/issue14123

[yossi]

how is it more powerful ?

[John La Rooy]

@yossi Here's one example stackoverflow.com/a/3228928/174728. It also lets you do key/index lookups import sys;"{[version]}".format(vars(sys))

[sshilovsky]

It's probably worth noting that logging module uses %-like syntax, so it may be preferred for consistency reasons.

[Pax0r]

how about: import sys;"%(version)s" % (vars(sys))

[John La Rooy]

@Pax0r: The linked example is '{num:{fill}{width}}'.format(num=123, fill='0', width=6). You can't do this with % unless you "double escape" and apply it twice

[Qi Fan]

Is .format significantly slower than % ?

[joelostblom]

From Python 3.6, you can use f-strings to access previously defined variables: print(f"{user} asked {number} questions on stackoverflow.com"

[Sean Bradley]

'%s' % (str(123).rjust(6, '0'))

[John La Rooy]

@Sean, that's formatting a string, not a number. If I just manipulate a value into a string someway there is not need for an extra layer of formatting at all. Indeed '%s'% adds nothing extra to the string you already created.

[Sean Bradley]

ideone.com/FsIZFy