Wrapping class with icontract invariant decorator causes PyLance to incorrectly detect type.

[ciresnave]

Environment data

• Language Server version: 2021.8.2-pre.1
• OS and version: win32 x64
• Python version (and distribution if applicable, e.g. Anaconda): Python 3.9.6
• python.analysis.indexing: undefined
• python.analysis.typeCheckingMode: strict

Expected behaviour

When using icontract to add an invariant to a class, both mypy and Python's own type() command show the type of a class wrapped by icontract to be the same as one that was never wrapped. PyLance shouldn't have an issue with using a wrapped class in place of the bare class for type hinting.

Actual behaviour

TestClass1 is a bare class.
TestClass2 is a wrapped class.
(see below for the actual code)

PyLance states that TestClass1 is of type "Type[TestClass1]" while it shows that TestClass2 is of type "type".
PyLance states that TestClass2 can't be used to type annotate a dataclass field because it expected a Class but got a type instead. Further, because of that issue, PyLance states that variables within TestClass2 are of unknown type and can't be referenced. The code works fine.

Mypy states:

src\stock_trader\invariant_type_change.py:9:13: note: Revealed type is "def () -> stock_trader.invariant_type_change.TestClass1"
src\stock_trader\invariant_type_change.py:18:13: note: Revealed type is "def () -> stock_trader.invariant_type_change.TestClass2"

..and printing the type of each from Python shows:

<class '__main__.TestClass1'>
<class '__main__.TestClass2'>

Logs

Python Language Server Log

Background analysis message: getDiagnosticsForRange
Background analysis message: getDiagnosticsForRange
Background analysis message: getDiagnosticsForRange
Background analysis message: getDiagnosticsForRange
Background analysis message: markFilesDirty
Background analysis message: markFilesDirty
Background analysis message: analyze
Background analysis message: markFilesDirty
Background analysis message: analyze
Background analysis message: getDiagnosticsForRange
Background analysis message: getDiagnosticsForRange
Background analysis message: markFilesDirty
Background analysis message: invalidateAndForceReanalysis
Background analysis message: invalidateAndForceReanalysis
Background analysis message: markFilesDirty
[Info  - 12:45:04 PM] Searching for source files
[Info  - 12:45:04 PM] Auto-excluding c:\Users\cires\Documents\GitHub\stock_trader\.venv
[Info  - 12:45:04 PM] Found 9 source files
Background analysis message: setTrackedFiles
Background analysis message: markAllFilesDirty
Background analysis message: analyze
[BG(1)] analyzing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py ...
[BG(1)]   parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (142ms)
[BG(1)]   parsing: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\builtins.pyi [fs read 0ms] (23ms)
[BG(1)]   binding: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\builtins.pyi (19ms)
[BG(1)]   binding: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (0ms)
[BG(1)]   checking: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py ...
[BG(1)]     parsing: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\dataclasses.pyi [fs read 0ms] (4ms)
[BG(1)]     binding: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\dataclasses.pyi (1ms)
[BG(1)]   checking: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (5ms)
[BG(1)] analyzing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (189ms)
[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\noxfile.py [fs read 1ms] (7ms)
[BG(1)] binding: c:\Users\cires\Documents\GitHub\stock_trader\noxfile.py (2ms)
[BG(1)] indexing: c:\Users\cires\Documents\GitHub\stock_trader\noxfile.py (0ms)
Background analysis message: markFilesDirty
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Background analysis message: analyze
[BG(1)] analyzing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py ...
[BG(1)]   checking: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py ...
[BG(1)]     parsing: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\typing.pyi [fs read 0ms] (11ms)
[BG(1)]     binding: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\typing.pyi (5ms)
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[BG(1)]     binding: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\_typeshed\__init__.pyi (1ms)
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[BG(1)]     parsing: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\__init__.py [fs read 1ms] (8ms)
[BG(1)]     binding: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\__init__.py (0ms)
[BG(1)]     parsing: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\_decorators.py [fs read 1ms] (8ms)
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[BG(1)]     parsing: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\_globals.py [fs read 0ms] (1ms)
[BG(1)]     binding: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\_globals.py (0ms)
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[BG(1)]     parsing: c:\Users\cires\Documents\GitHub\stock_trader\.venv\Lib\site-packages\icontract\_checkers.py [fs read 1ms] (10ms)
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[BG(1)]   checking: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (106ms)
[BG(1)] analyzing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (106ms)
Background analysis message: resumeAnalysis
[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\docs\conf.py [fs read 0ms] (1ms)
[BG(1)] binding: c:\Users\cires\Documents\GitHub\stock_trader\docs\conf.py (0ms)
[BG(1)] indexing: c:\Users\cires\Documents\GitHub\stock_trader\docs\conf.py [found 6] (0ms)
Indexing Done: c:\Users\cires\Documents\GitHub\stock_trader\docs\conf.py
[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\Quote.py [fs read 0ms] (1ms)
[BG(1)] binding: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\Quote.py (0ms)
[BG(1)] indexing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\Quote.py [found 4] (0ms)
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[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\__init__.py [fs read 1ms] (1ms)
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[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\__main__.py [fs read 0ms] (1ms)
[BG(1)] binding: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\__main__.py (0ms)
[BG(1)] indexing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\__main__.py [found 2] (0ms)
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[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\broker_protocols.py [fs read 1ms] (4ms)
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[BG(1)] indexing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py [found 6] (0ms)
Indexing Done: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py
[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\tests\__init__.py [fs read 1ms] (1ms)
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[BG(1)] parsing: c:\Users\cires\Documents\GitHub\stock_trader\tests\test_main.py [fs read 0ms] (2ms)
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Indexing Done: c:\Users\cires\Documents\GitHub\stock_trader\tests\test_main.py
Background analysis message: getDiagnosticsForRange
Background analysis message: getDiagnosticsForRange
[FG] parsing: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (139ms)
[FG] parsing: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\builtins.pyi [fs read 1ms] (77ms)
[FG] binding: c:\Users\cires\.vscode\extensions\ms-python.vscode-pylance-2021.8.2-pre.1\dist\typeshed-fallback\stdlib\builtins.pyi (23ms)
[FG] binding: c:\Users\cires\Documents\GitHub\stock_trader\src\stock_trader\invariant_type_change.py (0ms)

Code Snippet / Additional information

from dataclasses import dataclass
from icontract import invariant


class TestClass1:
    test_int: int = 1


reveal_type(TestClass1)
# According to PyLance: Type[TestClass1]
# According to mypy: src\stock_trader\invariant_type_change.py:9:13: note: Revealed type is "def () -> stock_trader.invariant_type_change.TestClass1"

@invariant(lambda self: self.test_int == 1)
class TestClass2:
    test_int: int = 1


reveal_type(TestClass2)
# According to PyLance: type
# According to mypy: src\stock_trader\invariant_type_change.py:18:13: note: Revealed type is "def () -> stock_trader.invariant_type_change.TestClass2"


@dataclass
class TestClass3:
    test_class4: TestClass1
    test_class5: TestClass2 # PyLance states: Expected "class" type but received type


instance_of_TestClass1 = TestClass1()
instance_of_TestClass2 = TestClass2()

print(type(instance_of_TestClass1))
# <class '__main__.TestClass1'>
print(type(instance_of_TestClass2))
# <class '__main__.TestClass2'>

print(instance_of_TestClass1.test_int)
print(instance_of_TestClass2.test_int) # PyLance states test_int is of type Any

class_holder = TestClass3(
    test_class4=instance_of_TestClass1, test_class5=instance_of_TestClass2
)
print(class_holder.test_class4.test_int)
print(class_holder.test_class5.test_int)
"""When trying to print class_holder.test_class5.test_int, PyLance has the following 3 complaints:
Type of "test_int" is unknown.
Argument type is unknown.
Cannot access member "test_int" for type "type"
"""

[erictraut]

This looks like it's a bug in the type annotations within the icontract library.

The decorator invariant is defined in icontract as a class with a __call__ method. This method returns type, which obscures the type once the decorator is applied.

    def __call__(self, cls: type) -> type:

The correct type annotation for this method is:

        def __call__(self, cls: _T) -> _T:

where `_T is defined as:

_T = TypeVar("_T", bound=type)

This preserves the type of the decorated class. If I make this change, it type checks fine in pylance and in mypy.

I recommend filing a bug or submitting a PR in the icontract repo.

[ciresnave]

I worked with the developer behind icontract and he and I verified a fix on his side is possible (see his pull request #227). I still question why Python itself plus mypy had no issue with the looser typing but PyLance did. Either way, this may be solved by a change in PyLance.

[jakebailey]

Python doesn't care about annotations at runtime, so you could put anything there and it will run.

As for mypy, that's hard to say. It could be a bug they haven't fixed or had reported to them.

[erictraut]

This appears to be a bug in mypy. They are not honoring the __call__ method when applying a class decorator. I'll file a bug in the mypy repo.

[erictraut]

It looks like this is a known bug in mypy. It has been open for more than four years: python/mypy#3135.

Here's an sample that demonstrates how pyright (the type checker that underlies pylance) applies the class decorator whereas mypy does not.

from typing import Callable, TYPE_CHECKING, Type

class replace_with_int1:
    def __init__(self):
        ...

    def __call__(self, cls: type) -> Type[int]:
        return int

@replace_with_int1()
class NotInt1:
    ...

if TYPE_CHECKING:
    reveal_type(NotInt1)  # pyright: "Type[int]"", mypy: "def () -> NotInt1"
print(NotInt1) # <class 'int'>

def replace_with_int2() -> Callable[..., Type[int]]:
    return lambda x: int

@replace_with_int2()
class NotInt2:
    ...

if TYPE_CHECKING:
    reveal_type(NotInt2)  # pyright: "Type[int]"", mypy: "def () -> NotInt2"
print(NotInt2) # <class 'int'>

[jakebailey]

I'm going to close this as needing a fix upstream; we're doing the right thing according to the type annotations here, so there's no work to do.