Why does the following code produce the result? The LHS and RHS structure is different yet,this is the result..
[[w,x],[[y],z]]=[{10,20},[(30,),40]]
w
10
x
20
y
30
z
40
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1 Answer
It's actually not guaranteed that w and x will always produce the same result, in dependence of your Python interpreter. Try switching it to:
>>> [[w,x],[[y],z]]=[{20,10},[(30,),40]]
>>> w
10
That's because the first 'entry' is a set and sets are not guaranteed to be ordered. The rest will work pretty much as expected because tuples and lists are ordered. It doesn't matter of the actual type on both sides so long as there is a parity, i.e. (x, y) = [p, q] will work the same as [x, y] = (p, q).
2 Comments
metasj
Yeah,that's what I wanted to know..Is there any specific reason why parity is not required?That's so counterintuitive.
zwer
@metasj - when you do assignments this way you're just swapping 'references' - Python doesn't care what container the reference is coming from as long as you provide the same number of elements on both sides to satisfy the basic 'number of assignments on LHS has to match the RHS` rule. So, the parity of elements is required, but you don't need parity of their container types because they get discarded during the assignment anyway. If you're looking for a more specific reason it's probably a mix of a speed optimization (why would you care about the container types?) and a KISS principle.