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Allow type checkers to infer trait types #788

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Allow type checkers to infer trait types #788

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2435520
Proof of concept for exploring
vidartf Oct 20, 2022
e4a5819
Add some workable typings, and some tests
vidartf Oct 20, 2022
7bca016
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Oct 20, 2022
3d079c3
Fix complex type
vidartf Oct 20, 2022
4fb7293
Currently failing tests
vidartf Oct 27, 2022
0d88902
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Oct 27, 2022
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3 changes: 2 additions & 1 deletion traitlets/tests/test_typing.py
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Original file line number Diff line number Diff line change
Expand Up @@ -9,12 +9,13 @@
@pytest.mark.mypy_testing
def mypy_bool_typing():
class T(HasTraits):
b = Bool()
b = Bool().tag(sync=True)

t = T()
reveal_type(t.b) # R: Union[builtins.bool, None]
# we would expect this to be Optional[Union[bool, int]], but...
t.b = "foo" # E: Incompatible types in assignment (expression has type "str", variable has type "Optional[int]") [assignment]
T.b.tag(foo=True)


@pytest.mark.mypy_testing
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4 changes: 3 additions & 1 deletion traitlets/traitlets.py
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Expand Up @@ -518,6 +518,8 @@ def instance_init(self, obj):
G = t.TypeVar("G")
S = t.TypeVar("S")

Self = t.TypeVar("Self", bound="TraitType") # Holdover waiting for typings.Self in Python 3.11


class TraitType(BaseDescriptor, t.Generic[G, S]):
"""A base class for all trait types."""
Expand Down Expand Up @@ -855,7 +857,7 @@ def set_metadata(self, key, value):
warn("Deprecated in traitlets 4.1, " + msg, DeprecationWarning, stacklevel=2)
self.metadata[key] = value

def tag(self, **metadata):
def tag(self, **metadata) -> "TraitType[G, S]":
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I think with the typing_extension package we can do this:

Suggested change
def tag(self, **metadata) -> "TraitType[G, S]":
def tag(self, **metadata) -> typing_extension.Self":

"""Sets metadata and returns self.

This allows convenient metadata tagging when initializing the trait, such as:
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