Python string formatting: % vs. .format
Python 2.6 introduced the str.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?
The following uses each method and has the same outcome, so what is the difference?
#!/usr/bin/python
sub1 = "python string!"
sub2 = "an arg"
a = "i am a %s" % sub1
b = "i am a {0}".format(sub1)
c = "with %(kwarg)s!" % {'kwarg':sub2}
d = "with {kwarg}!".format(kwarg=sub2)
print a # "i am a python string!"
print b # "i am a python string!"
print c # "with an arg!"
print d # "with an arg!"
Furthermore when does string formatting occur in Python? For example, if my logging level is set to HIGH will I still take a hit for performing the following % operation? And if so, is there a way to avoid this?
log.debug("some debug info: %s" % some_info)
To answer your first question... .format just seems more sophisticated in many ways. An annoying thing about % is also how it can either take a variable or a tuple. You'd think the following would always work:
"hi there %s" % name
yet, if name happens to be (1, 2, 3) , it will throw a TypeError . To guarantee that it always prints, you'd need to do
"hi there %s" % (name,) # supply the single argument as a single-item tuple
which is just ugly. .format doesn't have those issues. Also in the second example you gave, the .format example is much cleaner looking.
Why would you not use it?
To answer your second question, string formatting happens at the same time as any other operation - when the string formatting expression is evaluated. And Python, not being a lazy language, evaluates expressions before calling functions, so in your log.debug example, the expression "some debug info: %s"%some_info will first evaluate to, eg "some debug info: roflcopters are active" , then that string will be passed to log.debug() .
Something that the modulo operator ( % ) can't do, afaik:
tu = (12,45,22222,103,6)
print '{0} {2} {1} {2} {3} {2} {4} {2}'.format(*tu)
result
12 22222 45 22222 103 22222 6 22222
Very useful.
Another point: format() , being a function, can be used as an argument in other functions:
li = [12,45,78,784,2,69,1254,4785,984]
print map('the number is {}'.format,li)
print
from datetime import datetime,timedelta
once_upon_a_time = datetime(2010, 7, 1, 12, 0, 0)
delta = timedelta(days=13, hours=8, minutes=20)
gen =(once_upon_a_time +x*delta for x in xrange(20))
print 'n'.join(map('{:%Y-%m-%d %H:%M:%S}'.format, gen))
Results in:
['the number is 12', 'the number is 45', 'the number is 78', 'the number is 784', 'the number is 2', 'the number is 69', 'the number is 1254', 'the number is 4785', 'the number is 984']
2010-07-01 12:00:00
2010-07-14 20:20:00
2010-07-28 04:40:00
2010-08-10 13:00:00
2010-08-23 21:20:00
2010-09-06 05:40:00
2010-09-19 14:00:00
2010-10-02 22:20:00
2010-10-16 06:40:00
2010-10-29 15:00:00
2010-11-11 23:20:00
2010-11-25 07:40:00
2010-12-08 16:00:00
2010-12-22 00:20:00
2011-01-04 08:40:00
2011-01-17 17:00:00
2011-01-31 01:20:00
2011-02-13 09:40:00
2011-02-26 18:00:00
2011-03-12 02:20:00
Assuming you're using Python's logging module, you can pass the string formatting arguments as arguments to the .debug() method rather than doing the formatting yourself:
log.debug("some debug info: %s", some_info)
which avoids doing the formatting unless the logger actually logs something.
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