Map, Filter, and Reduce over Python Dictionaries

Written by Ben on March 16, 2006 | 3 Comments

I wanted to apply some Functional Programming ideas to solve a problem in some Python code I had, which involved manipulation of data found in a couple dictionaries. Unfortunately, the built in map, filter, reduce, and list composition methods can only be used over iterable lists.

Luckily, I remembered that equivalent dictionaries in LISP are all represented as a list-of-lists: ( (key1, val1), (key2, val2) ); python map & filter functions can then be used over this converted datastructure.

After a little digging, I found that it’s really easy to do this conversion in Python:

# Uses the list composition to make the key value pairs over a dictionary.
dict2list = lambda dic: [(k, v) for (k, v) in dic.iteritems()]

# Use the built in dictionary constructor to convert the list.
list2dict = lambda lis: dict(lis)

# Example Usage follows:

dict1 = { ‘a’:1, ‘b’:2, ‘c’:3, ‘d’:4 }

# Filter function
is_even = lambda item: item[1] % 2 == 0
# Map functions
add_two = lambda item: (item[0], item[1] + 2)
add_sss = lambda item: (“%s SSS” % item[0], item[1])
# Reduce function
add_all = lambda x, y: (“%s %s” % (x[0], y[0]), x[1] + y[1])

# Make sure you have the [] around the reduce call.
list2dict([reduce(add_all, map(add_two, map(add_sss, filter(is_even, dict2list( dict1 )))))])

3 Comments

1. very cool thanks for this

   Comment by Sonny — August 9, 2009 @ 8:39 am

2. just for reference:

   >>> dict2list (d)
   is the same as
   >>> d.values()
   (i. e.: dict2list = lambda d: d.values())

   Comment by ibotty — September 22, 2009 @ 12:54 am

3. @ibotty: I think you mean the items method:
   

   >>> dict(a=1, b=2, c=3).items()
   [('a', 1), ('c', 3), ('b', 2)]

   Comment by vrde — October 19, 2009 @ 3:02 pm

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