On Wed, Aug 05, 2015 at 09:08:16PM -0700, tom arnall wrote:
> i have read -- with what i think is a reasonable amount of attention
> -- all of the doc' i can find about string formatting in python. for
> the life of me, i cannot see how any of the other methods do more than
> you can do with, to use a concrete example:
>
> print "Here %s a number: %3d" % ("is", 1)
>
> #OR:
>
> s = "Here %s a number: %3d" % ("is", 1)
> print s
>
> what am i not getting?
%s and %r work with any object at all. Or do they? Run
this code snippet to find out:
objects = [ 1, "a", 2.5, None, [1,2], (1,2) ]
for o in objects:
print "the object is: %s" % o
With positional arguments, % formatting operates strictly left to right,
while the format() method lets you operate on arguments in any order,
and you can refer to the same item multiple times:
py> "{1} {0} {0}".format("first", "second")
'second first first'
Compared to:
"%s %s %s" % ("second", "first", "first")
which is inconvenient and annoying.
% formatting allows you to use positional arguments or keyword
arguments, but not both at the same time. The format() method allows
both at the same time:
py> "{1} {0} {spam}".format("first", "second", spam="third")
'second first third'
The format() method also supports customization via the special method
__format__, although I haven't done this and I'm not sure how it works.
But I think it means that you can invent your own formatting codes.
The format() method also supports centring with the ^ alignment option,
thousands separators with , and a few other additional features, e.g.
binary output:
py> "{:b}".format(3000)
'101110111000'
There's no doubt that the format() method is significantly more powerful
than % interpolation, but its also slower, and for many purposes good
old fashioned % is perfectly fine.
--
Steve
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