Asking "is hashable" about a Python value

def hashable(v):
    """Determine whether `v` can be hashed."""
    try:
        hash(v)
    except TypeError:
        return False
    return True

Since Python 2.6 you can use the abstract base class collections.Hashable:

>>> import collections
>>> isinstance({}, collections.Hashable)
False
>>> isinstance(0, collections.Hashable)
True

This approach is also mentioned briefly in the documentation for __hash__.

Doing so means that not only will instances of the class raise an appropriate TypeError when a program attempts to retrieve their hash value, but they will also be correctly identified as unhashable when checking isinstance(obj, collections.Hashable) (unlike classes which define their own __hash__() to explicitly raise TypeError).