3.0 Comprehension Syntax Summary

We’ve been focusing on list comprehensions and generators in this chapter, but keep in mind that there are two other comprehension expression forms: set and dictionary comprehensions are also available as of Python 3.0. We met these briefly in Chapters 5 and 8, but with our new knowledge of comprehensions and generators, you should now be able to grasp these 3.0 extensions in full:

 

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  • For sets, the new literal form {1, 3, 2} is equivalent to set([1, 3, 2]), and the new set comprehension syntax {f(x) for x in S if P(x)} is like the generator expression set(f(x) for x in S if P(x)), where f(x) is an arbitrary expression.
  • For dictionaries, the new dictionary comprehension syntax {key: val for (key, val) in zip(keys, vals)} works like the form dict(zip(keys, vals)), and {x: f(x) for x in items} is like the generator expression dict((x, f(x)) for x in items).

Here’s a summary of all the comprehension alternatives in 3.0. The last two are new and are not available in 2.6:

>>> [x * x for x in range(10)]            # List comprehension: builds list
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]      # like list(generator expr)

>>> (x * x for x in range(10))            # Generator expression: produces items
<generator object at 0x009E7328>          # Parens are often optional

>>> {x * x for x in range(10)}            # Set comprehension, new in 3.0
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}      # {x, y} is a set in 3.0 too

>>> {x: x * x for x in range(10)}         # Dictionary comprehension, new in 3.0
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}