What I want to understand is why does the following code
for x, y in [1,2], [3,4]:
print(x, y, x + y)
prints
1 2 3
3 4 7
instead of
1 3 4
2 4 6
Now, I know zip does this work and not using zip to iterate over a pair of lists could be considered an anti-pattern, but I still need an explanation for this default behavior.
From my point of view, the code above should intuitively work just like the builtin zip function
for (x, y) in zip([1,2], [3,4]):
print(x, y, x + y)
which prints
1 3 4
2 4 6
From memory I remember seeing the explanation for this technique a long time ago (and I guess that's why this solution comes to mind first), but I've done searches now, including on the whole section 5 of Python 3 documentation but I couldn't find any explanations for this behavior, not even on section 5.6. (Looping Techniques).
Is this a 4th Gotcha?
I think this is the expected behavior!
Consider that [1,2], [3,4] is a tuple literal, equivalent to the tuple ([1,2], [3,4]). (You might be using this without even noticing, for instance when assigning multiple values with a, b, c = 10, 20, 30 disregarding the ()...).
So in your example, the loop iterates through this list as follows:
# First iteration we get:
x, y = [1, 2]
# Which is the same as:
x = 1
y = 2
# Which would print:
print(x, y, x+y)
>> 1, 2, 3
# Second iteration we get:
x, y = [3, 4]
# Which is the same as:
x = 3
y = 4
# Which would print:
print(x, y, x+y)
>> 3, 4, 7
Does that make more sense now?
Considering zip: If zip would do the same thing, then I'd be suspicious! How is there this very common and considered-useful but completely redundant thing lying around? Hasn't anyone noticed? So you shouldn't expect them to do the same! ;-)
Edit due to a request for a pointer to documentation.
From 5.3 Tuples and Sequences:
A tuple consists of a number of values separated by commas, for instance: ...
As you see, on output tuples are always enclosed in parentheses, so that nested tuples are interpreted correctly; they may be input with or without surrounding parentheses, although often parentheses are necessary anyway (if the tuple is part of a larger expression).