Enumerating formatting strings

[Peter Otten]

Bengt Richter wrote:

> Parse might be a big word for
> 
>  >> def tupreq(fmt): return sum(map(lambda s:list(s).count('%'),
>  >> fmt.split('%%')))
>  ..
>  >> tupreq('%s this %(x)s not %% but %s')
> 
> (if it works in general ;-)

Which it doesn't:

>>> def tupreq(fmt): return sum(map(lambda s:list(s).count('%'),
fmt.split('%%')))
...
>>> fmt = "%*d"
>>> fmt % ((1,) * tupreq(fmt))
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: not enough arguments for format string

> Or maybe clearer and faster:
> 
>  >>> def tupreq(fmt): return sum(1 for c in fmt.replace('%%','') if
>  >>> c=='%')
>  ...
>  >>> tupreq('%s this %(x)s not %% but %s')
>  3

Mixed formats show some "interesting" behaviour:

>>> "%s %(x)s" % (1,2)
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: format requires a mapping
>>> class D:
...     def __getitem__(self, key):
...             return "D[%s]" % key
...
>>> "%s %(x)s" % D()
'<__main__.D instance at 0x402aaf2c> D[x]'
>>> "%s %(x)s %s" % D()
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: not enough arguments for format string
>>> "%s %(x)s %(y)s" % D()
'<__main__.D instance at 0x402aad8c> D[x] D[y]'

That is as far as I got. So under what circumstances is 
'%s this %(x)s not %% but %s' a valid format string?

Peter