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bpo-43224: Forbid TypeVar substitution with Unpack #32031

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Merged
merged 1 commit into from
Apr 13, 2022
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bpo-43224: Forbid TypeVar substitution with Unpack #32031

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14 changes: 14 additions & 0 deletions Lib/test/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -506,6 +506,20 @@ class G(Generic[Unpack[Ts]]): pass
self.assertEqual(E[float, str, int, bytes],
Tuple[List[float], A[str, int], List[bytes]])

def test_bad_var_substitution(self):
Ts = TypeVarTuple('Ts')
T = TypeVar('T')
T2 = TypeVar('T2')
class G(Generic[Unpack[Ts]]): pass

for A in G, Tuple:
B = A[T, Unpack[Ts], str, T2]
with self.assertRaises(TypeError):
B[int, Unpack[Ts]]
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Remind me where PEP 646 forbids this? Assuming Unpack[Ts] === *Ts, it seems this is forbidding the following:

class G(Generic[*Ts]): ...
B = G[T, *Ts, str, T2]
X = B[int, *Ts]  # <-- TypeError here

What is wrong with X that isn't wrong with B?

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I'll rewrite it as B[int, *Ts2], so we could unambiguously refer to variables.

AFAIK PEP 646 does not cover such case. It does not specify the result.

But what can be the result of substitution B[int, *Ts2]? T is substituted with int, T2 should be substituted with the last item of *Ts2, and *T should be substituted with all but the last items of *Ts2. But we cannot express this, because Ts2 is a variable. If we substitute *Ts2 with an empty sequence of types (X[()]), there would not be a value for T2.

I think that it is better to make it an error:

def f(a: tuple[*Ts2]) -> B[int, *Ts2]: ...

You should write

def f(a: tuple[*Ts2, T3]) -> B[int, *Ts2, T3]: ...

to make it having some sense.

There is a workaround of this problem. It may even make the code more clear. You can see, that a should be a tuple containing at least 1 item. T2 in B will be substituted with the type of the last item, and *Ts in B will be substituted with the rest of types.

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@mrahtz what do you think of this case?

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Thanks for the prod, Jelle. I agree with Serhiy - looks like this is one of the cases we should forbid at runtime.

C = A[T, Unpack[Ts], str, T2]
with self.assertRaises(TypeError):
C[int, Unpack[Ts], Unpack[Ts]]

def test_repr_is_correct(self):
Ts = TypeVarTuple('Ts')
self.assertEqual(repr(Ts), 'Ts')
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2 changes: 2 additions & 0 deletions Lib/typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -912,6 +912,8 @@ def __init__(self, name, *constraints, bound=None,
def __typing_subst__(self, arg):
msg = "Parameters to generic types must be types."
arg = _type_check(arg, msg, is_argument=True)
if (isinstance(arg, _GenericAlias) and arg.__origin__ is Unpack):
raise TypeError(f"{arg} is not valid as type argument")
return arg


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