What does this line of Python code mean?

Question

I am translating some code from Python to C++. I came across the following:

set(x for listX in listY for x in listX)

I am quite well-versed in C++ and know some basic constructs in Python. In the above code, I know that a set is being created but I do not understand the code inside the brackets. Any help?

andrean · Accepted Answer · 2012-11-10T11:38:56.860

6

listY is probably something like the structure below, so the expanded code is:

listY = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
result = set()
for listX in listY:
    for x in listX:
        result.add(x)

notice: sets won't allow duplicate entries

edited Nov 10 '12 at 11:38

answered Sep 01 '12 at 06:01

andrean

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The generator expression has been passed to `set`, so everything is evaluated immediately - inside the `set` implementation, sure. – Karl Knechtel Sep 01 '12 at 07:24

score 3 · Answer 2 · edited May 23 '17 at 10:34

3

It is a generator comprehension, analogous to a list comprehension. See this previous question for some info comparing them.

edited May 23 '17 at 10:34

Community

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answered Sep 01 '12 at 05:58

BrenBarn

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Thanks for the link, I found a similar example in [generator comprehension](http://www.python.org/dev/peps/pep-0289/) – Mika Sep 01 '12 at 06:57

What does this line of Python code mean?

2 Answers2