Eryk Sun added the comment:

It's consistent with the behavior of generator expressions:

    Variables used in the generator expression are evaluated lazily
    when the __next__() method is called for the generator object
    (in the same fashion as normal generators). However, the
    leftmost for clause is immediately evaluated, so that an error
    produced by it can be seen before any other possible error in
    the code that handles the generator expression. Subsequent for
    clauses cannot be evaluated immediately since they may depend
    on the previous for loop. For example: (x*y for x in range(10)
    for y in bar(x)).


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