a = b = random.random()
The initial line is the same, but:
a = b = random.random()
a += 1
a == b # False
This talk still applies despite its age: https://youtu.be/_AEJHKGk9ns?si=q5HjMOM9QS3_bFzH
A somewhat trickier example of the same issue is using [] as a default parameter value ... though there are warnings about the problem with that (it's the same list on every call) throughout the documentation.
a = b = []
b = []
a = b
Is chained assignment a pattern that comes from another language that people are applying to python?
I don’t think this is true. I think something like it is true for CPython (an argument isn’t added but the byte is kept for functions w no arg) but it’s not a statement about Python.
a, b = b[a] = 1, [0, 1, 2, 3]It's unusual, but pretty obvious. In single steps it's basically this:
a, b = 1, [0, 1, 2, 3]
b[a] = b
If someone is doing BS like this, they deserve spanking. But banning the whole concept because people are unaware or how something is to be used properly is strange. But then again, it seems people have a bit of a problem how python really works and how it's different from other languages.
0, (1, [...]), 2, 3]
[0, [...], 2, 3]
a, b = 1, [0, 1, 2, 3]
b[a] = 1, b # relies on cpython implementation detail¹
a, b = 1, [0, 1, 2, 3]
b[a] = a, bUmmm no the list is constructed once and assigned to b and then b is assigned to a. It would be crazy semantics if `a = b = ...` meant `a` was assigned `...`.
Edit: I'm wrong it's left to right not right to left, which makes the complaint in the article even dumber.
> An assignment statement evaluates the expression list and assigns the single resulting object to each of the target lists, from left to right.
Consider this:
a = [1, 2]
i = a[i] = 1
Wouldn't that require a LOAD_FAST? Also a is assigned first (from left to right) so a = ... happens either way.