Yet another Python textbook

[Colin J. Williams]

On 22/11/2012 1:27 PM, Ian Kelly wrote:
> On Thu, Nov 22, 2012 at 5:24 AM, Colin J. Williams <cjw at 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:

Thanks, this is clear.  I wish the docs made this clearer.

You and I used __format__.  I understand that the use of double 
underscore functions is deprecated.  Is there some regular function 
which can achieve the same result?

>
>>>> '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.
>