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JukkaL opened this issue Dec 29, 2020 · 3 comments · May be fixed by python/mypy#10444 or coderabbit-test/mypy#8
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Evaluation order of multiple assignment from iterable doesn't match Python #793

JukkaL opened this issue Dec 29, 2020 · 3 comments · May be fixed by python/mypy#10444 or coderabbit-test/mypy#8
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bug good first issue python compat Mypyc doesn't match CPython or documented semantics.

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@JukkaL
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JukkaL commented Dec 29, 2020

In code like this we should iterate over the rvalue first before performing any assignments to match Python semantics:

o.x, o.y = it

We currently interleave assignments and iteration in some cases. The evaluation order is correct if the rvalue is a list or a tuple, but not for arbitrary iterables. Assignments and iteration can have side effects, so in some edge cases these two approaches will produce different results.

Dealing with *x lvalues may make the implementation somewhat tricky (not sure though).

For more context, see python/mypy#9800 (review).
@JukkaL JukkaL added the bug label Dec 29, 2020
@JukkaL
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JukkaL commented Mar 20, 2021

Here's a full example where we use the wrong evaluation order:

it = iter(['x', 'y'])

def f(a) -> None:
    a.x, a.y = it

Currently f is compiled to something like this (pseudocode, simplified):

_tmp = iter(it)
a.x = next(_tmp)
a.y = next(_tmp)

We should compile it to something like this (pseudocode, simplified):

_tmp = iter(it)
_tmp2 = next(_tmp)
_tmp3 = next(_tmp)
a.x = _tmp2
a.y = _tmp3

The relevant code can be found by starting from mypyc.irbuild.statement.transform_assignment_stmt.

@starrohan999
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hii @JukkaL i want to work on this.
please assign me.

@JukkaL
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JukkaL commented Mar 21, 2021

@starrohan999 You can go ahead and work on this. We don't use issue assignments very consistently, since they get easily out of date. They mostly act as reminders for the assignees.

@JukkaL JukkaL added the python compat Mypyc doesn't match CPython or documented semantics. label Apr 19, 2021
ChetanKhanna added a commit to ChetanKhanna/mypy that referenced this issue May 7, 2021
@ChetanKhanna
[mypyc] Match evaluation order of multiple assignment from iterable w…
ff24ef3 
…ith Python (python#793)

Refactored to iterate rvalues first before performing any assignments.

Closes mypyc/mypyc#793.
@ChetanKhanna ChetanKhanna linked a pull request May 7, 2021 that will close this issue
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@arvi18 arvi18 linked a pull request Apr 25, 2025 that will close this issue
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bug good first issue python compat Mypyc doesn't match CPython or documented semantics.
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