Add basic support for recursive TypeVar defaults (PEP 696) by cdce8p · Pull Request #16878 · python/mypy · GitHub

Shouldn't we .copy_modified() here?

That doesn't work, unfortunately. This here is used to link nested TypeVarTypes so if it's changed / evaluated in one location the other stays in sync.

An example from the tests

T1 = TypeVar("T1", default=str)
T2 = TypeVar("T2", default=T1)

class ClassD1(Generic[T1, T2]): ...

k = ClassD1()
reveal_type(k)  # should be `ClassD1[str, str]`

For the first pass the type variables are as follows

T1`1 = str
T2`2 = T1`1 = str

The section here now replaces the default for T2 with the tbd of the default of T1:

T1`1 = str
T2`2 = <result of T1`1>

--
With .copy_modified it would be a copy of the T1'1 thus when it's evaluated it's not used for T2. The test would still return

ClassD1[str, T1`1 = str]

At least that's how I understand it 😅

Looked at it more closely. It's basically as said above. In particular, it's called in freshend_function_type_vars. We enter this function with linked TypeVars

callee.variables[0] == callee.variables[1].default  # True

Then create new ones for each and link them again in expand_type, so that

tvs[0] == tvs[1].default  # True

# and by extension
fresh.variables[0] == fresh.variables[1].default  # True

mypy/mypy/expandtype.py

Lines 119 to 130 in 517f5ae

    
           def freshen_function_type_vars(callee: F) -> F: 
        
               """Substitute fresh type variables for generic function type variables.""" 
        
               if isinstance(callee, CallableType): 
        
                   if not callee.is_generic(): 
        
                       return cast(F, callee) 
        
                   tvs = [] 
        
                   tvmap: dict[TypeVarId, Type] = {} 
        
                   for v in callee.variables: 
        
                       tv = v.new_unification_variable(v) 
        
                       tvs.append(tv) 
        
                       tvmap[v.id] = tv 
        
                   fresh = expand_type(callee, tvmap).copy_modified(variables=tvs)

Unfortunately that's not a safe assumption

Do you have an example I could try? Mypy will already complain about this here and reject the second TypeVar call.

T1 = TypeVar("T1", default=int)
T1 = TypeVar("T1", default=str)  # error

error: Cannot redefine "T1" as a type variable
error: Invalid assignment target
error: "int" not callable

I think you have to import the other TypeVar from a different module.

That results in the same error message. The second TypeVar call would just be ignored. The logic here would still as it only compares the actual TypeVar name type_var_name = fullname.rpartition(".")[2]. Not the complete fullname.

E.g. the comparison is with T1 not __main__.T1.

I thought something like this might fail:

[case testTypeVarDefaultsMultipleFiles]

from typing import Generic, TypeVar
from file2 import T as T2

T = TypeVar('T')

class Gen(Generic[T, T2]):
    pass

def func(a: Gen, b: Gen[str], c: Gen[str, str]) -> None:
    reveal_type(a)  # N: Revealed type is "__main__.Gen[Any, builtins.int]"
    reveal_type(b)  # N: Revealed type is "__main__.Gen[builtins.str, builtins.int]"
    reveal_type(c)  # N: Revealed type is "__main__.Gen[builtins.str, builtins.str]"

[file file2.py]
from typing import TypeVar

T = TypeVar('T', default=int)

But this passes as expected, and I couldn't find a variation that fails. So I suppose this is fine.

This variation fails:

[case testTypeVarDefaultsMultipleFiles]

from typing import Generic, TypeVar
from file2 import T as T2

T = TypeVar('T', default=T2)

class Gen(Generic[T2, T]):
    pass

def func(a: Gen, b: Gen[str], c: Gen[str, float]) -> None:
    reveal_type(a)  # N: Revealed type is "__main__.Gen[builtins.int, builtins.int]"
    reveal_type(b)  # N: Revealed type is "__main__.Gen[builtins.str, builtins.str]"
    reveal_type(c)  # N: Revealed type is "__main__.Gen[builtins.str, builtins.float]"

[file file2.py]
from typing import TypeVar

T = TypeVar('T', default=int)

Expected:
  main:11: note: Revealed type is "__main__.Gen[builtins.int, builtins.int]" (diff)
  main:12: note: Revealed type is "__main__.Gen[builtins.str, builtins.str]" (diff)
  main:13: note: Revealed type is "__main__.Gen[builtins.str, builtins.float]"
Actual:
  main:11: note: Revealed type is "__main__.Gen[builtins.int, T2?]" (diff)
  main:12: note: Revealed type is "__main__.Gen[builtins.str, T2?]" (diff)
  main:13: note: Revealed type is "__main__.Gen[builtins.str, builtins.float]"

Not code I'd expect anyone to actually write, but it does seem like it should be legal.

Thanks for the test, that helped! Tbh I could also have though about this earlier but it's actually quite simple to add a name lookup here 😅

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/ext/commands/commands.py:851: error: Overloaded function implementation does not accept all possible arguments of signature 3  [misc]
+ steam/ext/commands/commands.py:851: error: Overloaded function implementation cannot produce return type of signature 3  [misc]

Add basic support for recursive TypeVar defaults (PEP 696)

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cdce8p requested a review from JelleZijlstra February 6, 2024 14:37

JelleZijlstra reviewed Feb 6, 2024

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cdce8p requested a review from JelleZijlstra February 7, 2024 06:38

cdce8p added the topic-pep-696 TypeVar defaults label Feb 8, 2024

Add name lookup

408e406

JelleZijlstra merged commit 2e5174c into python:master Feb 16, 2024

cdce8p deleted the TypeVar-11-recurrsive-basic branch February 16, 2024 14:06

This was referenced Mar 20, 2024

Make slice generic python/typeshed#11637

Merged

Support PEP 696 – Type defaults for TypeVarLikes #14851

Closed

ilevkivskyi mentioned this pull request Oct 15, 2025

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

Merged

+                          self.recursive_tvar_guard[tvar_id] = None
+                          repl = repl.accept(self)
+                          if isinstance(repl, TypeVarType):
+                              repl.default = repl.default.accept(self)

                       for name, tvar_expr in declared_tvars:
+                          tvar_expr_default = tvar_expr.default
+                          if isinstance(tvar_expr_default, UnboundType):
+                              # Assumption here is that the names cannot be duplicated

Add basic support for recursive TypeVar defaults (PEP 696) by cdce8p · Pull Request #16878 · python/mypy · GitHub

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Add basic support for recursive TypeVar defaults (PEP 696) - #16878

Add basic support for recursive TypeVar defaults (PEP 696)#16878
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	def freshen_function_type_vars(callee: F) -> F:
	"""Substitute fresh type variables for generic function type variables."""
	if isinstance(callee, CallableType):
	if not callee.is_generic():
	return cast(F, callee)
	tvs = []
	tvmap: dict[TypeVarId, Type] = {}
	for v in callee.variables:
	tv = v.new_unification_variable(v)
	tvs.append(tv)
	tvmap[v.id] = tv
	fresh = expand_type(callee, tvmap).copy_modified(variables=tvs)

Add basic support for recursive TypeVar defaults (PEP 696) by cdce8p · Pull Request #16878 · python/mypy · GitHub

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Add basic support for recursive TypeVar defaults (PEP 696) - #16878

Add basic support for recursive TypeVar defaults (PEP 696)#16878JelleZijlstra merged 2 commits intopython:masterpython/mypy:masterfrom cdce8p:TypeVar-11-recurrsive-basiccdce8p/mypy:TypeVar-11-recurrsive-basicCopy head branch name to clipboard

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Add basic support for recursive TypeVar defaults (PEP 696)#16878
JelleZijlstra merged 2 commits into
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cdce8p:TypeVar-11-recurrsive-basic

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