objects as keys in python dictionaries

Question

I'm trying to use an object as a key in a python dictionary, but it's behaving in a way that I can't quite understand.

First I create a dictionary with my object as the key:

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

Now create another object that is "the same" as one which is a key.

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

I have given the ContentType object custom __eq__ and custom __str__ methods, such that the __eq__ method compares the __str__ values.

Now, some interactive python:

>>> for 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, so it looks like my object is definitely being identified properly as a 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? So content_type is in the package_disseminators.keys() list, but isn't a key?

>>> package_disseminators.has_key(content_type)
False

Apparently not.

I presume that the comparison process that Python uses to determin equality differs between a straight "in" statement on a list and actually looking up a key in a dict, but I don't know how. Any tips or insights?

Reiner Gerecke · Accepted Answer · 2011-02-09 20:56:07Z

52

From the python documentation:

A dictionary’s keys are almost arbitrary values. Values that are not hashable, that is, values containing lists, dictionaries or other mutable types (that are compared by value rather than by object identity) may not be used as keys.

Hashable is defined as follows

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__() or __cmp__() 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.

So if you want to do this, you need to override the default __hash__() method on your object (see the comment from Steven Rumbalski below for further explanation).

>>> content_type in package_disseminators.keys()
True

I suppose this works because dict.keys() returns a list, and __contains__ probably checks for equality, but not for the same hashes.

edited Feb 9, 2011 at 20:56

answered Feb 9, 2011 at 20:41

Reiner Gerecke

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1 Comment

Steven Rumbalski Over a year ago

Some further clarification: Your object already has a __hash__ method inherited from object. But the default implementation returns a unique value for each instance, so two equal instances will have different hashes unless you provide a better implementation. has_key compares hash values, in checks for equality, that's why has_key fails while in succeeds in your examples.

nosklo · Accepted Answer · 2011-02-09 20:42:39Z

38

Since dicts are hash tables under the hood, you need to define both __eq__ and __hash__ for that to work.

The basic rule of thumb is:

For objects that __eq__ compares equal, __hash__ must return the same hash.

From your description, something like

def __hash__(self):
    return hash(str(self))

should work.

answered Feb 9, 2011 at 20:42

nosklo

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1 Comment

Richard J Over a year ago

Thanks for the implementation of __hash__! Unfortunately I can't allocate two right answers to a question, although the combination of yours and the other are everything that I need. Cheers, R.

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