On 10/22/2016 7:57 PM, Chris Angelico wrote:
This surprised me.
Python 3.4.2 (default, Oct 8 2014, 10:45:20)
[GCC 4.9.1] on linux
Type "help", "copyright", "credits" or "license" for more information.
y=6
[(x,y) for x in range(y) for y in range(3)]
[(0, 0), (0, 1), (0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2,
2), (3, 0), (3, 1), (3, 2), (4, 0), (4, 1), (4, 2), (5, 0), (5, 1),
(5, 2)]
[(x,y) for x in range(3) for z in range(y) for y in range(3)]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 1, in <listcomp>
UnboundLocalError: local variable 'y' referenced before assignment
Normally, a comprehension is described as being equivalent to an
unrolled loop, inside a nested function. That would be like this:
def temp():
ret = []
for x in range(y):
for y in range(3):
ret.append((x,y))
return ret
temp()
This would make the first example fail, which would not be nice.
But it seems that the first iterator (and only that one) is evaluated
in the parent context:
Because the first iterator *can* always be evaluated.
def temp(iter):
ret = []
for x in iter:
for y in range(3):
ret.append((x, y))
return ret
temp(iter(range(y)))
Why is this? It seems rather curious.
Guido explained this somewhere some time ago.
Not sure it is documented very well.
In general, subordinate clauses depend on the initial loop variable,
hence cannot be evaluated.
--
Terry Jan Reedy
--
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