Python: for instruction behavior
I am assuming you are running this from the interactive console and have already defined y
with a value of 2 (otherwise you will get NameError: name 'y' is not defined
). This will lead to what you notice.
This is because it is for x in range(4), y in range(4):
actually equivalent to the following when evaluated:
for x in (range(4), y in range(4)):
which boils down to ...
for x in ([0,1,2,3], 2 in range(4)):
which again boils down to ...
for x in ([0,1,2,3], True):
This then results in two iterations of the loop for
as it iterates over each element of the tuple:
-
x = [0,1,2,3]
-
x = True
...
(And of course y
it doesn't care 2.)
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You have created a strange, strange thing.
>>> y = 2
>>> range(4), y in range(4)
([0, 1, 2, 3], True)
y in range(4)
is a membership test.
range(4), y in range(4)
- a couple of elements; tuple.
The variable is x
set to range (4) and then the result y in range(4)
.
The variable y
just fits with the value; it is not specified by the operator for
.
This only works on the command line, typing random things from the y
left.
This is not sane Python code at all.
[And yes, the word in
has two meanings. So there are a ()
few other syntaxes.]
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It seems like before running this code you have y
. What you are range
iterating over is a two-way tuple: the first item is the generated list, the second is True
which is the result y in range(4)
:
>>> y = 2
>>> for x in range(4), y in range(4):
print x, 'x'
print y, 'y'
[0, 1, 2, 3] x
2 y
True x
2 y
What I suspect you are trying to do is loop over two variables from two lists. Use zip
for this.
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Been nailed long ago as to why the syntax you wrote doesn't work. Here are the syntaxes that work for what you are probably trying to do:
If you want all 4 x 4 combinations for x and y, you want 2 nested loops:
for x in range(4):
for y in range(4):
print x, y
Or if you really want to use a single loop:
import itertools
for (x, y) in itertools.product(range(4), range(4)):
print x, y
itertools.product()
generates all possible combinations:
It's less readable than 2 loops in this simple case, but the itertools module has many other powerful features and is worth knowing ...
If you want to x
, and y
advancing in parallel on two sequences (aka iteration of "lock-step"):
for (x, y) in zip(range(4), range(4)):
print x, y
# `zip(range(4), range(4))` is silly since you get x == y;
# would be useful for different sequences, e.g.
# zip(range(4), 'abcd')
[Background: the name zip
comes from Haskell; think about how lightning takes one tooth from here and one from there:
zip()
cut to the length of the shortest sequence; the itertools module has other options ...]
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