[issue27984] singledispatch register should typecheck its argument

[Xiang Zhang]

Xiang Zhang added the comment:

It's also better to add the typecheck to dispatch. Otherwise sometimes it can generate not obvious exception message.

>>> from enum import IntEnum
>>> from functools import singledispatch
>>> IS = IntEnum("IS", "a, b")
>>> @singledispatch
... def foo(x):
...     pass
... 
>>> foo.dispatch(IS.a)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/angwer/cpython/Lib/functools.py", line 718, in dispatch
    impl = dispatch_cache[cls]
  File "/home/angwer/cpython/Lib/weakref.py", line 365, in __getitem__
    return self.data[ref(key)]
TypeError: cannot create weak reference to 'IS' object
>>>

----------
keywords: +patch
nosy: +xiang.zhang
Added file: http://bugs.python.org/file44428/issue27984.patch

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<http://bugs.python.org/issue27984>
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