New submission from Mahmoud Hashemi:
While porting some old code, I found some interesting misbehavior in the
new-style string formatting. When formatting objects which support int and
float conversion, old-style percent formatting works great, but new-style
formatting explodes hard.
Here's a basic example:
class MyType(object):
def __init__(self, func):
self.func = 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__
And the same holds true for int and so forth. I would expect these behaviors to
be the same between the two formatting styles, and tangentially, expect a more
python2-like error message for the python 3 case.
----------
messages: 236192
nosy: mahmoud
priority: normal
severity: normal
status: open
title: str.format() breaks object duck typing
type: behavior
versions: Python 2.7, Python 3.2, Python 3.3, Python 3.4, Python 3.5, Python 3.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue23479>
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