__iadd__ doesn't work when going through a Dict or List


class A(object):
    def __init__(self):
        self.a = 0

    def __iadd__(self, a):
        # type: (int) -> A
        self.a += 1
        return self

a = A()
a += 1

b = {0: a}
b[0] += 1

c = [a]
c[0] += 1

print(a.a)

Yields the following errors:

rt1.py:15: error: Incompatible types in assignment (expression has type "int", target has type "A")
rt1.py:18: error: No overload variant of "__setitem__" of "list" matches argument types [builtins.int, builtins.int]

Thanks for reporting this bug!

On a bit of debugging, here's the relevant logic:
https://github.com/python/mypy/blob/897398eff/mypy/checker.py#L1556

    def visit_operator_assignment_stmt(self,
                                       s: OperatorAssignmentStmt) -> Type:
        """Type check an operator assignment statement, e.g. x += 1."""
        lvalue_type = self.accept(s.lvalue)
        method = infer_operator_assignment_method(lvalue_type, s.op)
        rvalue_type, method_type = self.expr_checker.check_op(
            method, lvalue_type, s.rvalue, s)

        if isinstance(s.lvalue, IndexExpr):
            self.check_indexed_assignment(s.lvalue, s.rvalue, s.rvalue)
        else:
            if not is_subtype(rvalue_type, lvalue_type):
                self.msg.incompatible_operator_assignment(s.op, s)

I think the issue is that when s.lvalue is an IndexExpr, we forget the possibility that the lvalue actually has an in-place method for this operator and we attempt to check an outright assignment. (I'm not convinced it's quite right even when there isn't an in-place method -- that check_indexed_assignment line goes and type-checks it as if we're assigning the type of the actual RHS, rather than rvalue_type which is the type the operator returns -- but that'd be a separate issue.)

Will see if I can work up an easy fix.

Respect in-place operation when operator-assigning through square bra…

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iadd doesn't work when going through a Dict or List #2046

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__iadd__ doesn't work when going through a Dict or List #2046

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gnprice commented on Aug 24, 2016

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iadd doesn't work when going through a Dict or List #2046