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[PEP 696] Fix swapping TypeVars with defaults.#19449

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Merged
ilevkivskyi merged 12 commits intopython:masterfromrandolf-scholz:fix_typevar_default
Oct 27, 2025
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510a89d
initial commit + revert debugging
randolf-scholzJul 15, 2025
4b34af1
added second test
randolf-scholzJul 15, 2025
ee4dcc9
Merge branch 'python:master' into fix_typevar_default
randolf-scholzJul 15, 2025
dc7f73a
Merge branch 'master' into fix_typevar_default
randolf-scholzJul 17, 2025
665d3ff
Merge branch 'master' into fix_typevar_default
randolf-scholzJul 19, 2025
70fbf96
Merge branch 'master' into fix_typevar_default
randolf-scholzJul 27, 2025
14ece7b
Merge branch 'master' into fix_typevar_default
randolf-scholzJul 28, 2025
fd366cc
Merge branch 'master' into fix_typevar_default
randolf-scholzAug 4, 2025
76c0123
Merge branch 'master' into fix_typevar_default
randolf-scholzSep 16, 2025
1a3957a
Merge branch 'master' into fix_typevar_default
randolf-scholzSep 19, 2025
818175d
Merge branch 'master' into fix_typevar_default
randolf-scholzOct 6, 2025
8a088dc
Merge branch 'master' into fix_typevar_default
randolf-scholzOct 14, 2025
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6 changes: 3 additions & 3 deletionsmypy/expandtype.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -251,9 +251,9 @@ def visit_type_var(self, t: TypeVarType) -> Type:
if (tvar_id := repl.id) in self.recursive_tvar_guard:
return self.recursive_tvar_guard[tvar_id] or repl
self.recursive_tvar_guard[tvar_id] = None
repl = repl.accept(self)
if isinstance(repl, TypeVarType):
repl.default = repl.default.accept(self)
repl.default = repl.default.accept(self)
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Types should benever modified in place. I know you didn't introduce it, but this should be fixed ASAP. (I think there is also a chance some of the problems you try to fix may be caused by this).

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Types should be never modified in place.

So, withcopy_modified then? If stuff should never be modified in place, it surprises me that the variables are not annotated asFinal in the first place.

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Hm, If I change this then now 2 tests fail:testTypeVarDefaultsClassRecursive1 andtestTypeVarDefaultsClassRecursiveMultipleFiles

Details
____________________________________________________________________________________________________________________________________ testTypeVarDefaultsClassRecursive1 _____________________________________________________________________________________________________________________________________[gw0] linux -- Python 3.13.8 /home/rscholz/github/CONTRIBUTING/mypy/.venv/bin/python3data: /home/rscholz/github/CONTRIBUTING/mypy/test-data/unit/check-typevar-defaults.test:451:Failed: Unexpected type checker output (/home/rscholz/github/CONTRIBUTING/mypy/test-data/unit/check-typevar-defaults.test, line 451)------------------------------------------------------------------------------------------------------------------------------------------- Captured stderr call --------------------------------------------------------------------------------------------------------------------------------------------Expected:  main:16: note: Revealed type is "__main__.ClassD1[builtins.str, builtins.str]"  main:17: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.int]"  main:18: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.float]"  main:21: note: Revealed type is "__main__.ClassD1[builtins.str, builtins.str]" (diff)  main:23: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.int]"  main:25: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.float]"  main:35: note: Revealed type is "__main__.ClassD2[builtins.str, builtins.str, builtins.str]"  main:36: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.int, builtins.int]"  main:37: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.float]"  main:38: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.str]"  main:41: note: Revealed type is "__main__.ClassD2[builtins.str, builtins.str, builtins.str]" (diff)  main:43: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.int, builtins.int]" (diff)  main:45: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.float]"  main:47: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.str]"  main:56: note: Revealed type is "__main__.ClassD3[builtins.str, builtins.list[builtins.str]]"  ...  main:58: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.float]"  main:63: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.list[builtins.int]]"  main:65: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.float]"Actual:  main:16: note: Revealed type is "__main__.ClassD1[builtins.str, builtins.str]"  main:17: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.int]"  main:18: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.float]"  main:21: note: Revealed type is "__main__.ClassD1[builtins.str, T1`1 = builtins.str]" (diff)  main:23: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.int]"  main:25: note: Revealed type is "__main__.ClassD1[builtins.int, builtins.float]"  main:35: note: Revealed type is "__main__.ClassD2[builtins.str, builtins.str, builtins.str]"  main:36: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.int, builtins.int]"  main:37: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.float]"  main:38: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.str]"  main:41: note: Revealed type is "__main__.ClassD2[builtins.str, T1`1 = builtins.str, T2`2 = T1`1 = builtins.str]" (diff)  main:43: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.int, T2`2 = builtins.int]" (diff)  main:45: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.float]"  main:47: note: Revealed type is "__main__.ClassD2[builtins.int, builtins.float, builtins.str]"  main:56: note: Revealed type is "__main__.ClassD3[builtins.str, builtins.list[builtins.str]]"  ...  main:58: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.float]"  main:63: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.list[builtins.int]]"  main:65: note: Revealed type is "__main__.ClassD3[builtins.int, builtins.float]"Alignment of first line difference:  E: ..._.ClassD1[builtins.str, builtins.str]"  A: ..._.ClassD1[builtins.str, T1`1 = builtins.str]"                                ^Update the test output using --update-data (implies -n0; you can additionally use the -k selector to update only specific tests)______________________________________________________________________________________________________________________________ testTypeVarDefaultsClassRecursiveMultipleFiles _______________________________________________________________________________________________________________________________[gw0] linux -- Python 3.13.8 /home/rscholz/github/CONTRIBUTING/mypy/.venv/bin/python3data: /home/rscholz/github/CONTRIBUTING/mypy/test-data/unit/check-typevar-defaults.test:518:Failed: Unexpected type checker output (/home/rscholz/github/CONTRIBUTING/mypy/test-data/unit/check-typevar-defaults.test, line 518)------------------------------------------------------------------------------------------------------------------------------------------- Captured stderr call --------------------------------------------------------------------------------------------------------------------------------------------Expected:  main:15: note: Revealed type is "__main__.ClassG1[builtins.int, builtins.int]"  main:16: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.str]"  main:17: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.float]"  main:20: note: Revealed type is "__main__.ClassG1[builtins.int, builtins.int]" (diff)  main:22: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.str]"  main:24: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.float]"Actual:  main:15: note: Revealed type is "__main__.ClassG1[builtins.int, builtins.int]"  main:16: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.str]"  main:17: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.float]"  main:20: note: Revealed type is "__main__.ClassG1[builtins.int, T2`1 = builtins.int]" (diff)  main:22: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.str]"  main:24: note: Revealed type is "__main__.ClassG1[builtins.str, builtins.float]"Alignment of first line difference:  E: ..._.ClassG1[builtins.int, builtins.int]"  A: ..._.ClassG1[builtins.int, T2`1 = builtins.int]"                                ^Update the test output using --update-data (implies -n0; you can additionally use the -k selector to update only specific tests)========================================================================================================================================== short test summary info ==========================================================================================================================================FAILED mypy/test/testcheck.py::TypeCheckSuite::check-typevar-defaults.test::testTypeVarDefaultsClassRecursive1FAILED mypy/test/testcheck.py::TypeCheckSuite::check-typevar-defaults.test::testTypeVarDefaultsClassRecursiveMultipleFiles

I noticed thatTypeVarType.__hash__ is given byhash((self.id, self.upper_bound, tuple(self.values))), so changingdefault does not change the hash.

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Yes. For historical reasons some types are constructed "gradually", so we can't make type attributes final. Maybe we will at some point, but that would be a big refactoring. For now, mypy contributors should remember a simple rule: types always new, symbols always original.

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If I change this then now 2 tests fail

Oh well, it means there is another bug. TBH I am not sure if default should be part of the hash conceptually (and what it even means to have a type variable with same id, but a different default). IMO the support for type variable defaults may need to be re-implemented from scratch, as it is problematic on a conceptual level: regular default is not a property of an argument, it is a property of a function; same for type variables, their defaults are not property of type variable type itself, but of the function/class that uses them.

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So the problem seems to be with getting the correct ID's. In the test case, we haveclass D[T1=str, T2=T1].

>>>self.variables {1:T1`604 = builtins.str, 2: T2`605=T1`1=builtins.str}>>>self.recursive_tvar_guard{604:T1`604=builtins.str,605:None}>>>repl.default=repl.default.accept(self)>>>self.variables {1:T1`604 = builtins.str, 2: T2`605=T1`604=builtins.str}>>>self.recursive_tvar_guard{604:T1`604=builtins.str,605:None}

Modifyingrepl.default in-place ensures that in theExpandTypeVisitor.variables dictionary, the default of the dependent type variable gets updated to the new ID.

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TBH I don't understand why do we need to handle the defaults inexpand_type in the first place. As I explained, the default is not a property of a type variable, it is a property of a class/function, so the caller (e.g.applytype etc) should always provide acomplete type var -> replacement mapping. So thatexpand_type() should not be even allowed to look at the defaults.

Yeah, I just looked at the original PR that added this logic#16878, it looks broken. The full complete mapping should be computed either in:

  • semanal/typeanal for "static" application likex: C[int]
  • inapplytype for "dynamic" application likex = C[int]()

before we even callexpand_type. I think the only right way to do it is this way.

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I am not sure how to fix this at the moment, but given that this PR

  1. Fixes two very annoying bugs with PEP696 checking (essentially prevents typing bijections with defaults)
  2. Doesn't introduce inplace modification -- it was already there before (all we really do is make sure to update the default a-priori, which is crucial e.g. when the second typevar defaults to the first one and we expand the first one.)
  3. This issue probably needs to be addressed elsewhere

Maybe this can still be merged as-is?

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The downside is that this is papering over the problem instead of fixing the real root cause. But anyway, if you feel strongly about this, I can merge this, it will not be much more work to axe this all later (and the tests are useful).

expanded = repl.accept(self) # Note: `expanded is repl` may be true.
repl = repl if isinstance(expanded, TypeVarType) else expanded
self.recursive_tvar_guard[tvar_id] = repl
return repl

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32 changes: 32 additions & 0 deletionstest-data/unit/check-typevar-defaults.test
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Expand Up@@ -416,6 +416,38 @@ def func_c4(
reveal_type(m) # N: Revealed type is "__main__.ClassC4[builtins.int, builtins.float]"
[builtins fixtures/tuple.pyi]

[case testTypeVarDefaultsSwap]
from typing import TypeVar, Generic

T = TypeVar("T")
X = TypeVar("X", default=object)
Y = TypeVar("Y", default=object)


class Foo(Generic[T, Y]):
def test(self) -> None:
reveal_type( Foo[Y, T]() ) # N: Revealed type is "__main__.Foo[Y`2 = builtins.object, T`1]"


class Bar(Generic[X, Y]):
def test(self) -> None:
reveal_type( Bar[Y, X]() ) # N: Revealed type is "__main__.Bar[Y`2 = builtins.object, X`1 = builtins.object]"


[case testTypeVarDefaultsSwap2]
from typing import TypeVar, Generic

X = TypeVar("X", default=object)
Y = TypeVar("Y", default=object)
U = TypeVar("U", default=object)
V = TypeVar("V", default=object)

class Transform(Generic[X, Y]):
def invert(self) -> "Transform[Y, X]": ...

class Foo(Transform[U, V], Generic[U, V]):
def invert(self) -> "Foo[V, U]": ...

[case testTypeVarDefaultsClassRecursive1]
# flags: --disallow-any-generics
from typing import Generic, TypeVar, List
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