Enhance reportInvalidTypeVarUse to report situations where class-scoped TypeVars may go unsolved in constructor

[erictraut]

This is similar to #3495 but applies to constructors and class-scoped TypeVars.

[erictraut]

@JelleZijlstra, I've implemented this expanded check. It reports an additional 32 errors within typeshed (in addition to the 37 errors reported for function-scoped TypeVars that may go unsolved).

Here's a full list of the errors, which includes both of the above:

/typeshed/stdlib/_weakrefset.pyi
  /typeshed/stdlib/_weakrefset.pyi:15:39 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "data"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/argparse.pyi
  /typeshed/stdlib/argparse.pyi:215:32 - error: Type variable "_ArgumentParserT" may go unsolved if caller supplies no argument for parameter "parser_class"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/array.pyi
  /typeshed/stdlib/array.pyi:24:94 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "__initializer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/array.pyi:26:98 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "__initializer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/array.pyi:28:98 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "__initializer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/array.pyi:30:71 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "__initializer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/builtins.pyi
  /typeshed/stdlib/builtins.pyi:965:45 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "__iterable"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/builtins.pyi:1001:45 - error: Type variable "_T_co" may go unsolved if caller supplies no argument for parameter "__iterable"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/contextvars.pyi
  /typeshed/stdlib/contextvars.pyi:17:47 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "default"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/difflib.pyi
  /typeshed/stdlib/difflib.pyi:32:33 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "isjunk"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/fileinput.pyi
  /typeshed/stdlib/fileinput.pyi:32:54 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "openhook"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/fileinput.pyi:75:58 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "openhook"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/gettext.pyi
  /typeshed/stdlib/gettext.pyi:157:22 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "class_"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/graphlib.pyi
  /typeshed/stdlib/graphlib.pyi:14:45 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "graph"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/mailbox.pyi
  /typeshed/stdlib/mailbox.pyi:50:66 - error: Type variable "_MessageT" may go unsolved if caller supplies no argument for parameter "factory"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/optparse.pyi
  /typeshed/stdlib/optparse.pyi:243:41 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "args"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/subprocess.pyi
  /typeshed/stdlib/subprocess.pyi:135:61 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "stdout"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/tempfile.pyi
  /typeshed/stdlib/tempfile.pyi:366:21 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "suffix"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/tempfile.pyi:387:13 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "suffix"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/tempfile.pyi:392:21 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "suffix"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/tempfile.pyi:396:20 - error: Type variable "AnyStr" may go unsolved if caller supplies no argument for parameter "suffix"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/collections/__init__.pyi
  /typeshed/stdlib/collections/__init__.pyi:85:43 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "initlist"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/collections/__init__.pyi:217:43 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "iterable"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/ctypes/__init__.pyi
  /typeshed/stdlib/ctypes/__init__.pyi:167:29 - error: Type variable "_CT" may go unsolved if caller supplies no argument for parameter "arg"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/ctypes/__init__.pyi:193:31 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "value"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/multiprocessing/managers.pyi
  /typeshed/stdlib/multiprocessing/managers.pyi:127:38 - error: Type variable "_KT" may go unsolved if caller supplies no argument for parameter "sequence"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/multiprocessing/managers.pyi:127:43 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "sequence"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stdlib/multiprocessing/managers.pyi:128:39 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "sequence"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stdlib/multiprocessing/shared_memory.pyi
  /typeshed/stdlib/multiprocessing/shared_memory.pyi:26:43 - error: Type variable "_SLT" may go unsolved if caller supplies no argument for parameter "sequence"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/SQLAlchemy/sqlalchemy/orm/decl_api.pyi
  /typeshed/stubs/SQLAlchemy/sqlalchemy/orm/decl_api.pyi:117:76 - error: Type variable "_DeclT" may go unsolved if caller supplies no argument for parameter "metaclass"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/SQLAlchemy/sqlalchemy/orm/decl_api.pyi:128:37 - error: Type variable "_DeclT" may go unsolved if caller supplies no argument for parameter "metaclass"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/SQLAlchemy/sqlalchemy/util/_collections.pyi
  /typeshed/stubs/SQLAlchemy/sqlalchemy/util/_collections.pyi:94:36 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "d"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/beautifulsoup4/bs4/element.pyi
  /typeshed/stubs/beautifulsoup4/bs4/element.pyi:354:63 - error: Type variable "_PageElementT" may go unsolved if caller supplies no argument for parameter "result"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/cachetools/cachetools/__init__.pyi
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:14:61 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:34:61 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:37:61 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:40:61 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:43:61 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:47:58 - error: Type variable "_KT" may go unsolved if caller supplies no argument for parameter "choice"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:47:105 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:54:86 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/cachetools/cachetools/__init__.pyi:70:98 - error: Type variable "_VT" may go unsolved if caller supplies no argument for parameter "getsizeof"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/contextvars/contextvars.pyi
  /typeshed/stubs/contextvars/contextvars.pyi:14:47 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "default"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/flake8-plugin-utils/flake8_plugin_utils/plugin.pyi
  /typeshed/stubs/flake8-plugin-utils/flake8_plugin_utils/plugin.pyi:21:32 - error: Type variable "TConfig" may go unsolved if caller supplies no argument for parameter "config"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/invoke/invoke/tasks.pyi
  /typeshed/stubs/invoke/invoke/tasks.pyi:75:17 - error: Type variable "_TaskT" may go unsolved if caller supplies no argument for parameter "klass"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/mock/mock/mock.pyi
  /typeshed/stubs/mock/mock/mock.pyi:233:14 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "new"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/mypy-extensions/mypy_extensions.pyi
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:24:15 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:25:22 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:26:20 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:27:27 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:28:18 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/mypy-extensions/mypy_extensions.pyi:29:17 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "type"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/psycopg2/psycopg2/_psycopg.pyi
  /typeshed/stubs/psycopg2/psycopg2/_psycopg.pyi:409:62 - error: Type variable "_T_cur" may go unsolved if caller supplies no argument for parameter "cursor_factory"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/redis/redis/commands/core.pyi
  /typeshed/stubs/redis/redis/commands/core.pyi:644:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:651:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:682:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:689:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:859:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:874:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:901:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:935:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:958:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:1023:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:1038:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:1065:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:1099:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/redis/redis/commands/core.pyi:1122:47 - error: Type variable "_ScoreCastFuncReturn" may go unsolved if caller supplies no argument for parameter "score_cast_func"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
/typeshed/stubs/setuptools/pkg_resources/__init__.pyi
  /typeshed/stubs/setuptools/pkg_resources/__init__.pyi:63:104 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "installer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
  /typeshed/stubs/setuptools/pkg_resources/__init__.pyi:67:83 - error: Type variable "_T" may go unsolved if caller supplies no argument for parameter "installer"
    Provide an overload that specifies the return type when the argument is not supplied (reportInvalidTypeVarUse)
69 errors, 0 warnings, 0 informations 

[erictraut]

@JelleZijlstra, I'm having second thoughts about this expanded check. I think it may result in less type safe code.

The problem is that if you make changes to typeshed to eliminate the above errors, I presume you'll do so by explicitly supplying an Any type argument in many of these cases. That will propagate Any types, which will go undetected even in pyright's strict type checking mode. In contrast, today the unsolved TypeVar will result in an Unknown (implicit Any) type argument, and the user will be alerted to the type checking hole. They can then provide an explicit specialization for the class when calling the constructor.

For example, here's the current constructor for UserList

class UserList(MutableSequence[_T]):
    def __init__(self, initlist: Iterable[_T] | None = ...) -> None: ...

To eliminate the new error, it will need to be changed to:

class UserList(MutableSequence[_T]):
    @overload
    def __init__(self, initlist: Iterable[_T]) -> None: ...
    @overload
    def __init__(self: UserList[Any], initlist: None = ...) -> None: ...

Previously, a user could have used:

x1 = UserList()
reveal_type(x1) # UserList[Unknown]
x2 = UserList[int]()
reveal_type(x2) # UserList[int]

Now, this will result in the following:

x1 = UserList()
reveal_type(x1) # UserList[Any]
x2 = UserList[int]()
reveal_type(x2) # UserList[Any]

This seems like a step backward, so I'm inclined to revert this change. What do you think?

I suppose the error could be eliminated without adding an explicit Any:

class UserList(MutableSequence[_T]):
    @overload
    def __init__(self, initlist: Iterable[_T]) -> None: ...
    @overload
    def __init__(self, initlist: None = ...) -> None: ...

But this will still result in an unsolved TypeVar, which defeats the purpose of adding an overload and flagging the original constructor declaration as an inappropriate use of a TypeVar.

[JelleZijlstra]

Yes, empty collections are a tricky situation. Looking at the collection builtins, there are three options:

list has an overload that doesn't use its TypeVar (https://github.com/python/typeshed/blob/c1aba8059f8386230ec7d467e40c86b31eba8c7f/stdlib/builtins.pyi#L865):

    @overload
    def __init__(self) -> None: ...

dict has an overload that sets its TypeVars but leaves them unconstrained:

    @overload
    def __init__(self: dict[_KT, _VT]) -> None: ...

set and frozenset use a defaulted parameter with a TypeVar (https://github.com/python/typeshed/blob/c1aba8059f8386230ec7d467e40c86b31eba8c7f/stdlib/builtins.pyi#L965):

    def __init__(self, __iterable: Iterable[_T] = ...) -> None: ...

Only the set option gets flagged by your new check, but pyright's reportUnknownVariableType check flags all of x = list(), x = dict(), x = set(). So if we switch to the list or dict option, we can fix the unconstrained TypeVar while keeping the same degree of type safety.

Do you have a preference between the list and dict option? They appear to behave the same in both pyright and mypy; I'd favor the list option for being less code.

[saaketp]

@JelleZijlstra If after making the change, there would still be unsolved TypeVar anyway, then we could just ignore the error too, instead of adding overloads.

And is it only empty collections where this will be an issue?
I kinda suspect a lot of times there won't be any solution other than Any for the T, so this check will lead to lots of false positives, and require unnecessary overloads that still leave the T unsolved, if we want to avoid Any.
(Plus adding overloads has other costs, like incompatibility with ParamSpec and performance (I guess?) and a bit too too verbose (for inline annotations at least)).

[JelleZijlstra]

Many cases (e.g., FileInput) do have correct solutions when the default is used. I think it's worth it to add this check to help us catch such cases.

[saaketp]

Okay well at least it is a separate rule, so can be enabled/disabled by choice without affecting other rules.
So worth having definitely.
Just may not be a good idea to be enabled by default.

[erictraut]

we can fix the unconstrained TypeVar while keeping the same degree of type safety.

OK, you've convinced me that we should leave this new check in place. I reserve the right to revert it if we get a backlash from pyright users, but I'm willing to publish a new version with this check in place.

Do you have a preference between the list and dict option?

I don't have a preference between these two. They both seem fine to me.