8000 Make it an error to use a class-attribute type var outside a type by sixolet · Pull Request #3105 · python/mypy · GitHub
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Make it an error to use a class-attribute type var outside a type #3105

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Merged
merged 7 commits into from
Apr 3, 2017
Merged

Make it an error to use a class-attribute type var outside a type #3105

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d54708d
Make it an error to use a class-attribute type var outside a type
sixolet Apr 1, 2017
fe6eee1
Fix the error message to handle more cases
sixolet Apr 1, 2017
7b8e409
oops accidentally changed a file
sixolet Apr 1, 2017
8bfd1a7
Typo fix
sixolet Apr 1, 2017
7d658c6
more checking things in tests
sixolet Apr 1, 2017
54ae7aa
Change error message
sixolet Apr 1, 2017
0aa5a70
Change error message
sixolet Apr 1, 2017
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Fix the error message to handle more cases
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@sixolet
sixolet committed Apr 1, 2017
commit fe6eee173d611bb574ef5e2f1200919da0f0e125
4 changes: 2 additions & 2 deletions mypy/checkmember.py
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Expand Up @@ -406,8 +406,8 @@ def analyze_class_attribute_access(itype: Instance,
return AnyType()

if isinstance(node.node, TypeVarExpr):
msg.fail('Type variable "{}" is invalid as target for type alias'.format(
node.node.fullname() or node.node.name()), context)
msg.fail('Type variable "{}" is invalid except as a parameter to another type'.format(
node.node.fullname() or node.node.name()), context)
return AnyType()

if isinstance(node.node, TypeInfo):
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1 change: 1 addition & 0 deletions test-data/unit/check-generics.test
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Expand Up @@ -14,6 +14,7 @@ class A(Generic[T]):

class B: pass
class C: pass

[out]
main:4: error: Incompatible types in assignment (expression has type "B", variable has type "C")

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3 changes: 2 additions & 1 deletion test-data/unit/check-typevar-values.test
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Expand Up @@ -503,7 +503,8 @@ class C:
L = List[C.T] # a valid type alias
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Would doing y: C.T = x be valid?

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Yes, and good point -- that's not "as a parameter to a type" that's "as a type"

How to express "Don't try and use this at runtime"?

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I guess "Is only valid when used as a type or a parameter to a type in a type alias" covers all the bases, but is wicked confusing if I don't already know exactly what it's talking about.

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Some time ago we had "... is invalid in runtime context" for subscripted generics. Maybe it is better to say "... can not be used in runtime context, only in type context".

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Oh! How about

  • "Type variable 'C.T' is not a valid expression"
  • "Type variable 'C.T' cannot be used as an expression"
    ?

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The second option sounds good.

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Ok, rolling with @ilevkivskyi's suggestion for now.

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... as I incur more and more test changes as I can't decide. I think I like how it is now.

l: L = []
l.append(x)
A = C.T # E: Type variable "T" is invalid as target for type alias
C.T # E: Type variable "T" is invalid except as a parameter to another type
A = C.T # E: Type variable "T" is invalid except as a parameter to another type
return l[0]

[builtins fixtures/list.pyi]
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