Consider this code?
b = [[5], [3]]
a = b[1]
b[-1] += b.pop()
print(a)
This gives [3,3].
You can't explain it by expanding b[-1] += b.pop() to b[-1] = b[-1] + b.pop() it seems.
Why do you get this output?
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Bhargav Rao
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Simd
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What part specifically are you asking about? – Carcigenicate Mar 13 '17 at 12:24
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@Carcigenicate At the end `b=[[3,3]]`. So how does `a = [3,3]` when `a = b[1]`? – Simd Mar 13 '17 at 12:25
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`a` is the list that was at `b[1]` when `a` was initialized. It isn't linked to the second element in `b`. – Eric Duminil Mar 13 '17 at 12:27
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Related: [Why do Python emit STORE_SUBSTR when there's already an INPLACE_ADD?](http://stackoverflow.com/q/31039969/846892) – Ashwini Chaudhary Mar 13 '17 at 12:44
3 Answers
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Because if you work with b[-1] the reference to the second list is only fetched once, next the operation += is done and finally it is stored back into the list. So b[-1] += b.pop() is basically equivalent to:
tmp = b[-1] # tmp = [3]
tmp += b.pop() # tmp = [3,3], b = [[5]]
b[-1] = tmp # tmp = [3,3], b = [[3,3]]
(but of course there is tmp, the above fragment is done at interpreter level)
Now tmp is a refence to the second list in b (so tmp is a). So this means that you extend inplace a with b.pop(). b.pop() is [3]. So what you do is you extend tmp (which is [3] at that moment) with tmp. And so tmp (and thus a becomes [3,3]). So b is now [[3,3]].
Mind that x += y for lists is not equivalent to x = x+y. Indeed, if you write code like:
>>> a = [1]
>>> b = a
>>> b = b + [2]
>>> print(a,b)
[1] [1, 2]
this will not update a (since it is not done inplace). Whereas for:
>>> a = [1]
>>> b = a
>>> b += [2]
>>> print(a,b)
[1, 2] [1, 2]
will result in a and b being both [1,2]. Of course, usually += is supposed to behave like adding, but the adding is done inplace which can have some effects.
Willem Van Onsem
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Try printing b[-1] right before you print a. b[-1] refers to the last element in b, which you are adding another element to. It appears that this happens because when you do a = b[1], it must be assigned by reference since a is updated when b[1] (same as b[-1] in this case) is updated.
Dylan Hamilton
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First there are only 2 elements in the list b.
b = [[5], [3]]
a = b[1]
print b[1]
# gives [3]
print b[-1]
# also gives [3] because here -1 refers to the last element and there are
# only two elements.
b[-1] += b.pop()
# here you are appending 3 to [3] of b
print b[-1]
print a
# that is why when you print a it gives [3,3] as the b[1] value was
# appended to a in the second step itself and you are just printing here.
Hope you got what I am trying to say..
