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Comment by rectangletangle | Hacker News Reader

This is part of the generalized comprehension syntax. You can also do lazy generator comprehensions.

    a = (i for i in range(10) if i % 2 == 0)
    print(list(a))

You can omit the parenthesis, and use them in calls which expect an iterable.

    b = max(i for i in range(0, 10) if i % 2 == 0)
    print(b)

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It's not exactly the same, which is what I thought was interesting. My first example did use a generator expression inside the dict() constructor, but in that case you need to specify the key and value in a tuple or a list.

With the dictionary comprehension you can just separate the key and value with a colon, which is more natural. It might just be sugar on top of a generator expression but it is definitely a special case, syntactically speaking.

Yes, in Python 2.7 onward:

  s = {i**2 for i in range(10) if i}

and the colon is what makes the dict comprehension different from the set comprehension:

  d - {i:i**2 for i in range(10) if i}

Although Raymond Hettinger has also called them generator "comprehensions" in the early proposals, the current documentation calls them "generator expressions". And good examples.

Here's how you make those set/dict whatever comprehensions in Python 2.6, before the native syntax is used:

  s = set(i for i in xrange(10) if not i%2)

Seems like a comprehension is just a particular form of Python expression.

Here's the grammar file: https://docs.python.org/2/reference/grammar.html

Look for dictorsetmaker (for {}) testlist_comp (for generator expressions) and listmaker (for [], i.e. list comprehensions).

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