On Sat, May 18, 2013 at 2:01 PM, Steven D'Aprano
<steve+comp.lang.pyt...@pearwood.info> wrote:
> Consider if x is an arbitrary object, and you call "%s" % x:
>
> py> "%s" % 23 # works
> '23'
> py> "%s" % [23, 42] # works
> '[23, 42]'
>
> and so on for *almost* any object. But if x is a tuple, strange things
> happen
Which can be guarded against by wrapping it up in a tuple. All you're
seeing is that the shortcut notation for a single parameter can't
handle tuples.
>>> def show(x):
return "%s" % (x,)
>>> show(23)
'23'
>>> show((23,))
'(23,)'
>>> show([23,42])
'[23, 42]'
One of the biggest differences between %-formatting and str.format is
that one is an operator and the other a method. The operator is always
going to be faster, but the method can give more flexibility (not that
I've ever needed or wanted to override anything).
>>> def show_format(x):
return "{}".format(x) # Same thing using str.format
>>> dis.dis(show)
2 0 LOAD_CONST 1 ('%s')
3 LOAD_FAST 0 (x)
6 BUILD_TUPLE 1
9 BINARY_MODULO
10 RETURN_VALUE
>>> dis.dis(show_format)
2 0 LOAD_CONST 1 ('{}')
3 LOAD_ATTR 0 (format)
6 LOAD_FAST 0 (x)
9 CALL_FUNCTION 1 (1 positional, 0 keyword pair)
12 RETURN_VALUE
Attribute lookup and function call versus binary operator. Potentially
a lot of flexibility, versus basically hard-coded functionality. But
has anyone ever actually made use of it?
str.format does have some cleaner features, like naming of parameters:
>>> "{foo} vs {bar}".format(foo=1,bar=2)
'1 vs 2'
>>> "%(foo)s vs %(bar)s"%{'foo':1,'bar':2}
'1 vs 2'
Extremely handy when you're working with hugely complex format
strings, and the syntax feels a bit clunky in % (also, it's not
portable to other languages, which is one of %-formatting's biggest
features). Not a huge deal, but if you're doing a lot with that, it
might be a deciding vote.
ChrisA
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