How to filter a dictionary according to an arbitrary condition function?

python dictionary filter
Adam Matan picture Adam Matan · May 16, 2010 · Viewed 205.9k times · Source

I have a dictionary of points, say:

>>> points={'a':(3,4), 'b':(1,2), 'c':(5,5), 'd':(3,3)}

I want to create a new dictionary with all the points whose x and y value is smaller than 5, i.e. points 'a', 'b' and 'd'.

According to the the book, each dictionary has the items() function, which returns a list of (key, pair) tuple:

>>> points.items()
[('a', (3, 4)), ('c', (5, 5)), ('b', (1, 2)), ('d', (3, 3))]

So I have written this:

>>> for item in [i for i in points.items() if i[1][0]<5 and i[1][1]<5]:
...     points_small[item[0]]=item[1]
...
>>> points_small
{'a': (3, 4), 'b': (1, 2), 'd': (3, 3)}

Is there a more elegant way? I was expecting Python to have some super-awesome dictionary.filter(f) function...

Answer

Thomas picture Thomas · May 16, 2013

Nowadays, in Python 2.7 and up, you can use a dict comprehension:

{k: v for k, v in points.iteritems() if v[0] < 5 and v[1] < 5}

And in Python 3:

{k: v for k, v in points.items() if v[0] < 5 and v[1] < 5}

Related questions

Python: Check if one dictionary is a subset of another larger dictionary

I'm trying to write a custom filter method that takes an arbitrary number of kwargs and returns a list containing the elements of a database-like list that contain those kwargs. For example, suppose d1 = {'a':'2', 'b':'3'} …

Read More
Iterating over dictionaries using 'for' loops

I am a bit puzzled by the following code: d = {'x': 1, 'y': 2, 'z': 3} for key in d: print key, 'corresponds to', d[key] What I don't understand is the key portion. How does Python recognize that it needs only to …

Read More
Check if a given key already exists in a dictionary

I wanted to test if a key exists in a dictionary before updating the value for the key. I wrote the following code: if 'key1' in dict.keys(): print "blah" else: print "boo" I think this is not the …

Read More