dictionary - objects as keys in python dictionaries -

i'm trying use object key in python dictionary, it's behaving in way can't quite understand.

first create dictionary object key:

package_disseminators = {   contenttype("application", "zip", "http://other/property") : "one",   contenttype("application", "zip") : "two" } 

now create object "the same" 1 key.

content_type = contenttype("application", "zip", "http://other/property") 

i have given contenttype object custom __eq__ , custom __str__ methods, such __eq__ method compares __str__ values.

now, interactive python:

>>> key in package_disseminators: ...     if key == content_type: ...             print "match" ...     else: ...             print "no match" ...  no match match  >>> content_type in package_disseminators.keys() true 

ok, looks object being identified key, so:

>>> package_disseminators[content_type] traceback (most recent call last):   file "<stdin>", line 1, in <module> keyerror: (& (type="application/zip") (packaging="http://other/property") ) 

er ... ok? content_type in package_disseminators.keys() list, isn't key?

>>> package_disseminators.has_key(content_type) false 

apparently not.

i presume comparison process python uses determin equality differs between straight "in" statement on list , looking key in dict, don't know how. tips or insights?

from python documentation:

a dictionary’s keys arbitrary values. values not hashable, is, values containing lists, dictionaries or other mutable types (that compared value rather object identity) may not used keys.

hashable defined follows

an object hashable if has hash value never changes during lifetime (it needs __hash__() method), , can compared other objects (it needs __eq__() or __cmp__() method). hashable objects compare equal must have same hash value.

hashability makes object usable dictionary key , set member, because these data structures use hash value internally.

so if want this, need override default __hash__() method on object (see comment steven rumbalski below further explanation).

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>>> content_type in package_disseminators.keys() true 

i suppose works because dict.keys() returns list, , __contains__ checks equality, not same hashes.