Duck-typing and string formatting. Yet another example of percent formatting's superiority over newstyle .format()

duck_type_format.py

class MyType(object):
    def __init__(self, func):
        self.func = func

    def __int__(self):
        return int(self.func())

    def __float__(self):
        return float(self.func())


print '%f' % MyType(lambda: 3)

# Output (python2 and python3): 3.000000


print '{:f}'.format(MyType(lambda: 3))

# Output (python2):
# Traceback (most recent call last):
#   File "tmp.py", line 28, in <module>
#     print '{:f}'.format(MyType(lambda: 3))
# ValueError: Unknown format code 'f' for object of type 'str'
#
# Output (python3.4):
# Traceback (most recent call last):
# File "tmp.py", line 30, in <module>
#    print('{:f}'.format(MyType(lambda: 3)))
# TypeError: non-empty format string passed to object.__format__

While this is an unintuitive default, the short answer is that this is what __format__ is for, and the longer answers are here: http://bugs.python.org/issue23479