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Prohibit assignment to __class__ (Fixes #7724) #19180

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Prohibit assignment to __class__ (Fixes #7724) #19180

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59450e8
Prohibit assignment to __class__ inside instance methods
lsrafael13 May 30, 2025
523a168
Trigger GitHub Actions for testing
lsrafael13 May 30, 2025
fe381b2
Fix AttributeError in VarAssignVisitor and cleanup test file
lsrafael13 Jun 1, 2025
111a3c4
Fix assignment to __class__ check to use errors.report() in VarAssign…
lsrafael13 Jun 1, 2025
cd6d90b
Disallow assignment to '__class__' in TypeChecker.visit_assignment_stmt
lsrafael13 Jun 2, 2025
78a6c99
Add __class__ assignment check to TypeChecker.visit_assignment_stmt
lsrafael13 Jun 2, 2025
3a6b580
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Jun 2, 2025
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5 changes: 5 additions & 0 deletions mypy/checker.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3078,6 +3078,11 @@ def visit_assignment_stmt(self, s: AssignmentStmt) -> None:
with self.enter_final_context(s.is_final_def):
self.check_assignment(s.lvalues[-1], s.rvalue, s.type is None, s.new_syntax)

for lv in s.lvalues:
if isinstance(lv, MemberExpr):
if lv.name == "__class__":
self.fail("Assignment to '__class__' is unsafe and not allowed", lv)
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This wording is too strong - assignments are fine themselves, there's nothing inherently unsafe with doing this, it's not a security vuln. Probably something along the lines of

Suggested change
self.fail("Assignment to '__class__' is unsafe and not allowed", lv)
self.fail("Assignment to '__class__' is not supported", lv)

(probably accompanied by a self.note explaining it further) would be less intrusive.


if s.is_alias_def:
self.check_type_alias_rvalue(s)

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22 changes: 22 additions & 0 deletions test-data/unit/check-assign-to-class.test
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Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
[case test_assign_to_dunder_class]
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For some historical reason, all our testcases use camelCase naming scheme. Also I don't think this deserves its own file, probably just appending to check-statements.test would be more reasonable.

class A:
def foo(self):
self.__class__ = B # E: Assignment to '__class__' is unsafe and not allowed
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Please also add testcases with non-self assignments and assignments to self.__class__ in __init__ (they're slightly different AFAIC).

I'd also love to see tests where this is assigned to various other destinations:

b = object()
b.__class__ = something

class A:
    __class__ = something

# etc.


class B:
pass

[case test_assign_to_other_dunder_attributes]
class C:
def bar(self):
self.__name__ = "NewName" # OK
self.__doc__ = "Test doc" # OK

class D:
pass

[case test_assign_to_regular_attribute]
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We certainly already have tests checking that.

class E:
x = 1
def baz(self):
self.x = 2 # OK