Why doesn't my idea work in python2?
Here is an idea for a dict subclass that can mutate keys. This is a simple self contained example that's just like a dict but is case insensitive for str keys.
from functools import wraps
def key_fix_decorator(f):
@wraps(f)
def wrapped(self, *args, **kwargs):
if args and isinstance(args[0], str):
args = (args[0].lower(),) + args[1:]
return f(self, *args, **kwargs)
return wrapped
class LowerDict(dict):
pass
for method_name in '__setitem__', '__getitem__', '__delitem__', '__contains__', 'get', 'pop', 'setdefault':
new_method = key_fix_decorator(getattr(LowerDict, method_name))
setattr(LowerDict, method_name, new_method)
dev note: if you copy my code for your own uses, you should implement LowerDict.__init__ to check for any key collisions - I haven't bothered to include that for the purposes of this question
On python3 it all seems to works fine:
>>> d = LowerDict(potato=123, spam='eggs')
>>> d['poTATo']
123
>>> d.pop('SPAm')
'eggs'
>>> d['A']
# KeyError: 'a'
In python2 it doesn't even import, here is the traceback:
File "/tmp/thing.py", line 15, in <module>
new_method = key_fix_decorator(getattr(LowerDict, method_name))
File "/tmp/thing.py", line 4, in key_fix_decorator
@wraps(f)
File "/usr/lib/python2.7/functools.py", line 33, in update_wrapper
setattr(wrapper, attr, getattr(wrapped, attr))
AttributeError: 'wrapper_descriptor' object has no attribute '__module__'
What could be the problem? I can't see any version-specific code except for the str / basestring thing, which is just a minor detail not a code-breaking issue.
The functools.wraps() version in Python 3 can handle function objects with some of the attributes it copies across missing; the one in Python 2 cannot. This is was because issue #3445 was fixed only for Python 3; the methods of dict are defined in C code and have no __module__ attribute.
Omitting the @wraps(f) decorator makes everything work in Python 2 too:
>>> def key_fix_decorator(f):
... def wrapped(self, *args, **kwargs):
... if args and isinstance(args[0], str):
... args = (args[0].lower(),) + args[1:]
... return f(self, *args, **kwargs)
... return wrapped
...
>>> class LowerDict(dict):
... pass
...
>>> for method_name in '__setitem__', '__getitem__', '__delitem__', '__contains__', 'get', 'pop', 'setdefault':
... new_method = key_fix_decorator(getattr(LowerDict, method_name))
... setattr(LowerDict, method_name, new_method)
...
>>> d = LowerDict(potato=123, spam='eggs')
>>> d['poTATo']
123
>>> d.pop('SPAm')
'eggs'
>>> d['A']
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in wrapped
KeyError: 'a'
You can replicate enough of what wraps does manually:
def key_fix_decorator(f):
def wrapped(self, *args, **kwargs):
if args and isinstance(args[0], str):
args = (args[0].lower(),) + args[1:]
return f(self, *args, **kwargs)
wrapped.__name__ = f.__name__
wrapped.__doc__ = f.__doc__
return wrapped
or limit the attributes that wraps tries to copy across:
def key_fix_decorator(f):
@wraps(f, assigned=('__name__', '__doc__'))
def wrapped(self, *args, **kwargs):
if args and isinstance(args[0], str):
args = (args[0].lower(),) + args[1:]
return f(self, *args, **kwargs)
return wrapped
You don't really need to update __module__ attribute here; that is mostly useful only for introspection.