Even knowing a lot about FP, I still found this worth skimming for the esoteric Python syntax. When it comes to constructing dictionaries with list comprehensions, I would always do something like this:
dict([n, 2 ** n] for n in range(5))
But they pointed out an actual "dict comprehension" that I didn't even realize existed:
{ n: n ** 2 for n in range(5) }
And there is a similar "set comprehension":
{ n ** 2 for n in range(5) }
Always amazes me how you can use Python for so many years and still encounter new features in the language.
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.
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:
