I saw this Python snippet on Twitter and was quite confused by the output:
>>> a, b = a[b] = {}, 5
>>> a
{5: ({...}, 5)}
What is going on here?
Answers:
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Method 1
From the Assignment statements documentation:
An assignment statement evaluates the expression list (remember that this can be a single expression or a comma-separated list, the latter yielding a tuple) and assigns the single resulting object to each of the target lists, from left to right.
You have two assignment target lists; a, b, and a[b], the value {}, 5 is assigned to those two targets from left to right.
First the {}, 5 tuple is unpacked to a, b. You now have a = {} and b = 5. Note that {} is mutable.
Next you assign the same dictionary and integer to a[b], where a evaluates to the dictionary, and b evaluates to 5, so you are setting the key 5 in the dictionary to the tuple ({}, 5) creating a circular reference. The {...} thus refers to the same object that a is already referencing.
Because assignment takes place from left to right, you can break this down to:
a, b = {}, 5
a[b] = a, b
so a[b][0] is the same object as a:
>>> a, b = {}, 5
>>> a[b] = a, b
>>> a
{5: ({...}, 5)}
>>> a[b][0] is a
True
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