Attached is another patch for file Objects/stringobject.c(relative to 
Python 2.6b1) which supports the double %f and long double %LF formats.  

However, %Lf is included only if symbol  LDBL_DIG is defined in header 
file <float.h> on the underlying platform.

This patch also handles the long and size_t flags correctly in the first 
loop to determine the size of the buffer under
    ...
    case 'd': case 'u': case 'i': case 'x':
    ...

Btw, it does not make sense to handle float since there is not formnat 
specification for floats.