On Thu, Sep 1, 2011 at 10:32 AM, Richard D. Moores <rdmoo...@gmail.com>wrote:
> The glossary defines "hashable" as: > > hashable > An object is hashable if it has a hash value which never changes > during its lifetime (it needs a __hash__() method), and can be > compared to other objects (it needs an __eq__() method). Hashable > objects which compare equal must have the same hash value. > > Hashability makes an object usable as a dictionary key and a set > member, because these data structures use the hash value internally. > > > All of Python’s immutable built-in objects are hashable, while no > mutable containers (such as lists or dictionaries) are. Objects which > are instances of user-defined classes are hashable by default; they > all compare unequal, and their hash value is their id(). > > I'm trying to write a general test for hashability. How can I test if > an object has both a __hash__() method and an __eq__() method? > > Thanks, > > Dick Moores > _______________________________________________ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > To test for a method within any object you can just go like this: >>> a = () >>> type(a) <type 'tuple'> >>> if '__hash__' in dir(a): print True True >>> if '__eq__' in dir(a): print True True >>> But, I think the method you are approaching it from will only test for hashability. For example, you could do this: >>> a = [] >>> type(a) <type 'list'> >>> if '__hash__' in dir(a): print True True >>> if '__eq__' in dir(a): print True True >>> and then do this: >>> b = [] >>> c = {b:1} Traceback (most recent call last): File "<pyshell#36>", line 1, in <module> c = {b:1} TypeError: unhashable type: 'list' here is the dir for a list (not hashable): >>> dir(b) ['__add__', '__class__', '__contains__', '__delattr__', '__delitem__', '__delslice__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getslice__', '__gt__', '__hash__', '__iadd__', '__imul__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__rmul__', '__setattr__', '__setitem__', '__setslice__', '__sizeof__', '__str__', '__subclasshook__', 'append', 'count', 'extend', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort'] here is the dir for a tuple (hashable): >>> dir(()) ['__add__', '__class__', '__contains__', '__delattr__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getnewargs__', '__getslice__', '__gt__', '__hash__', '__init__', '__iter__', '__le__', '__len__', '__lt__', '__mul__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__rmul__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', 'count', 'index'] What I would probably do though is use the built in method hash so you could do something like this: >>> a = 'a' >>> b = () >>> c = [] >>> print type(a), type(b), type(c) <type 'str'> <type 'tuple'> <type 'list'> >>> print hash(a) -468864544 >>> print hash(b) 3527539 >>> print hash(c) Traceback (most recent call last): File "<pyshell#48>", line 1, in <module> print hash(c) TypeError: unhashable type: 'list' >>> You can then use try, except to catch it on an as needed basis. Not sure if this answers the question you are asking though.
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