Understanding nested list comprehension

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Indeed, you are correct. This is described in detail in the Expressions section in the Python Language Reference.

Note especially the order of nesting of several fors in a single list comprehension, which is always left-to-right:

>>> matrix = [[1, 2], [3, 4]]
>>> [item for item in row for row in matrix] # oops!
Traceback (most recent call last):
  File "<pyshell#1>", line 1, in <module>
    [item for item in row for row in matrix]
NameError: name 'row' is not defined
>>> [item for row in matrix for item in row] # nesting is in left-to-right order
[1, 2, 3, 4]

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