Finding the intersection of two dicts (Python recipe) by andy mckay

ActiveState Code (http://code.activestate.com/recipes/59875/)

I think the code here has a typo, it should say "has_key" instead of "has_keys"

A Python 2.2 enhancement. Note that the difference disappears in the forthcoming Python 2.2: there, "if somekey in in adict" becomes equivalent to "if adict.has_key(somekey)". The recipe should therefore be indicated as only important for Python 2.1 and earlier.

Oops. Thanks.

Alternatively...

intersect = [item for item in some_dict.keys() if another_dict.has_key(item)]

or

intersect = filter(another_dict.has_key, some_dict.keys())

Dictionaries of different size. It is also worthwhile to note that if one dictionary has considerably more elements, the smallest dictionary should be the one to loop over. However, the difference starts to get noticeable only with dictionaries that have thousands of keys.

Finding items not in both dictionaries. What if I want to find the items that are not in both dictionaries?

Finding intersections and non-intersections. ### Finding intersections and non-intersections

some_dict = { 'zope':'zzz', 'python':'rocks','1':'one','2':'two','3':'three','4':'four' }

another_dict = { 'python':'rocks', 'perl':'$','2':'two','3':'three' }

intersection = dict([(item,some_dict[item]) for item in some_dict.keys() if another_dict.has_key(item)])

non_intersection = dict([(item,some_dict[item]) for item in some_dict.keys() if not another_dict.has_key(item)])

print "a new dict containing the intersection:",intersection

print "a new dict containing the non-intersection:",non_intersection

I do not know what a non-intersection is through education but I expected a non-intersection is:

keys in some_dict but not in another_dict AND

keys in another_dict but not in some_dict.

s g's recipe:

non_intersection = dict([(item,some_dict[item]) for item in some_dict.keys() if not another_dict.has_key(item)])

Testing this code in python 2.5 and Pythonwin with "print non_interection" shows that another_dict['perl'] was not appended to non_intersection even though it is not in some_dict.

You are correct. In order to complete the non-intersection, you must reverse the dictionaries in the same command:

non_intersection1 = dict([(item,some_dict[item]) for item in some_dict.keys() if not another_dict.has_key(item)])
non_intersection2 = dict([(item,another_dict[item]) for item in another_dict.keys() if not some_dict.has_key(item)])

and then merge them together:

non_intersection = non_intersection1.update( non_intersection2 )

I think in recent versions this might be easier. (Tested with Python 3.3.0)

>>> def intersect(d1, d2):
...     'Intersect dictionaries d1 and d2 by key *and* value'
...     return dict(d1.items() & d2.items())
>>>
>>> intersect({'a': 2}, {'b': 42, 'a': 2})
{'a': 2}
>>> intersect({'a': 2}, {'b': 42, 'a': 3})
{}

If you only want to intersect keys/values use keys() or values() instead (without the dict()). (Sorry if this is old, but google still lists it on first place)