On Thu, Nov 22, 2012 at 5:24 AM, Colin J. Williams <c...@ncf.ca> wrote:
> From my reading of the docs, it seems to me that the three following should
> be equivalent:
>
> (a) formattingStr.format(values)
> with
> (b) format(values, formattingStr)
> or
> (c) tupleOfValues.__format__(formattingStr
>
> Example:
> print('{:-^14f}{:^14d}'.format(-25.61, 95 ))
> print(format((-25.61, 95), '{:-^14f}{:^14d}'))
> (-25.61, 95 ).__format__('{:-^14f}{:^14d}')
>
> The second fails, perhaps because values can only be a single value.
> The third fails, the reason is unclear.
The latter two (which are more or less equivalent) fail because they are
intended for invoking the formatting rules of a single value. The
string argument to each of them is not a format string, but a "format
specification", which in a format string is only the part that goes
inside the curly braces and after the optional colon. For example, in
this format string:
>>> 'Hello world {0!s:_>4s}'.format(42)
'Hello world __42'
The format specifier here is "_>4s":
>>> format('42', '_>4s')
'__42'
The valid format specifiers depend upon the type of the object being formatted:
>>> format(42, '04x')
'002a'
>>> format(datetime(2012, 11, 22, 11, 17, 0), 'The time is %Y %d %m %H:%M:%S')
'The time is 2012 22 11 11:17:00'
Custom types can implement custom format specifications by overriding
the __format__ method:
>>> class Foo:
... def __init__(self, value):
... self.value = value
... def __format__(self, spec):
... if spec == 'a':
... return str(self.value)
... if spec == 'b':
... return ''.join(reversed(str(self.value)))
... raise ValueError("Unknown format code {!r}".format(spec))
...
>>> format(Foo(42), 'a')
'42'
>>> format(Foo(42), 'b')
'24'
The same format specifications can then also be passed to str.format:
>>> '{0:a} reversed is {0:b}'.format(Foo(42))
'42 reversed is 24'
Unfortunately, there does not seem to be a good reference to the
format specifications available for built-in types beyond basic
strings and numbers. I only knew about the datetime example because
it is used in an example in the str.format docs. The
datetime.__format__ implementation (which seems to be just a thin
wrapper of datetime.strftime) does not seem to be documented anywhere
in the datetime module docs.
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