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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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5
This looks more unreadable than necessary compared to the equivalent'x is {0}, y is {1}'.format(a.x, a.y). Should only be used when thea.x operation is very costly.
dtheodor
– dtheodor
2015-03-29 14:03:21 +00:00
CommentedMar 29, 2015 at 14:03
13
@dtheodor With a tweak to use a keyword argument instead of positional argument...'x is {a.x}, y is {a.y}'.format(a=a). More readable than both examples.
CivFan
– CivFan
2015-04-17 21:11:54 +00:00
CommentedApr 17, 2015 at 21:11
1
@CivFan Or, if you have more than one object,'x is {a.x}, y is {a.y}'.format(**vars())
Jacklynn
– Jacklynn
2015-06-18 17:02:03 +00:00
CommentedJun 18, 2015 at 17:02
1
Also note this one in the same fashion:'{foo[bar]}'.format(foo={'bar': 'baz'}).
Antoine Pinsard
– Antoine Pinsard
2016-07-23 22:01:44 +00:00
CommentedJul 23, 2016 at 22:01
5
This is incredibly useful for customer-facing applications, where your application supplies a standard set of formatting options with a user-supplied format string. I use this all the time. The configuration file, for instance, will have some "messagestring" property, which the user can supply withYour order, number {order[number]} was processed at {now:%Y-%m-%d %H:%M:%S}, will be ready at about {order[eta]:%H:%M:%S} or whatever they wish. This is far cleaner than trying to offer the same functionality with the old formatter. It makes user-supplied format strings way more powerful.
Taywee
– Taywee
2016-09-30 21:33:04 +00:00
CommentedSep 30, 2016 at 21:33

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

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