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String formatting: % vs. .format vs. f-string literal

There are various string formatting methods:

Python <2.6:"Hello %s" % name
Python 2.6+:"Hello {}".format(name) (usesstr.format)
Python 3.6+:f"{name}" (uses f-strings)

Which is better, and for what situations?

The following methods have the same outcome, so what is the difference?

name = "Alice""Hello %s" % name"Hello {0}".format(name)f"Hello {name}"# Using named arguments:"Hello %(kwarg)s" % {'kwarg': name}"Hello {kwarg}".format(kwarg=name)f"Hello {name}"

When does string formatting run, and how do I avoid a runtime performance penalty?

_{If you are trying to close a duplicate question that is just looking for a way to format a string, please useHow do I put a variable’s value inside a string?.}

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4
Or just use'{type_names} [a-z]{{2}}'.format(type_names='triangle|square'). It's like saying.format() can help when using strings which already contain a percent character. Sure. You have to escape them then.
Alfe
– Alfe
2017-03-02 16:24:40 +00:00
CommentedMar 2, 2017 at 16:24
2
@Alfe You are right, and that is why the answer starts with"One situation where % may help is when you are formatting regex expressions." Specifically, assumea=r"[a-z]{2}" is a regex chunk that you will be used in two different final expressions (e.g.c1 = b + a andc2 = a). Assume thatc1 needs to beformated (e.g.b needs to be formatted runtime), butc2 does not. Then you needa=r"[a-z]{2}" forc2 anda=r"[a-z]{{2}}" forc1.format(...).
Jorge Leitao
– Jorge Leitao
2017-03-02 16:41:41 +00:00
CommentedMar 2, 2017 at 16:41

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String formatting: % vs. .format vs. f-string literal

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