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Python: For/List Comprehensions and Dictionaries

Learn about playing with dictionaries and for/list comprehensions in Python.

I’ve been working through Coursera’s Linear Algebra course and since all of the exercises are in Python I’ve been playing around with it again.

One interesting thing I learnt is that you can construct dictionaries using a list comprehension type syntax.

For example, if we start with the following dictionaries:

>>> x = { "a": 1, "b":2 } 
>>> y = {1: "mark", 2: "will"}
>>> x
{'a': 1, 'b': 2}
>>> y
{1: 'mark', 2: 'will'}

We might want to create a new dictionary which links from the keys in x to the values in y. In this case we work out the mapping by finding the key in y which corresponds with each value in x.

So the map we want to see at the end should look like this:

{"a": 'mark', "b": 'will'}

We can iterate over the keys/values of a dictionary by calling Dictionary#iteritems like so:

>>> for key, value in x.iteritems():
...   print (key, value)
('a', 1)
('b', 2)

I thought I might be able to construct my new dictionary by converting this into a for comprehension:

>>> [key:value for key, value in x.iteritems()]
  File "<stdin>", line 1
    [key:value for key, value in x.iteritems()]
SyntaxError: invalid syntax

Unfortunately that didn’t work but I came across an interesting post from which I learned that using curly brackets might do the trick.

>>> {key:value for key, value in x.iteritems()}
{'a': 1, 'b': 2}
>>> type({key:value for key, value in x.iteritems()})
<type 'dict'>

I wanted to make the final dictionary take a lookup into account which we can do like this:

>>> {key:y[value] for key, value in x.iteritems()}
{'a': 'mark', 'b': 'will'}

Apparently this is known as a dictionary comprehension and has been in the language since version 2.7.

I’m sure this is old news to seasoned Python developers but I’d not come across it before so to me it’s pretty neat!

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