Pyright is assigning Never type to a clearly defined type

[jmg-duarte]

I have the following code snippet:

DataT = TypeVar(
    "DataT", bool, int, float, str, List[Any], Dict[str, Any], DataFrame, None
)
class T:
    def f(self, path, type_, data):        
        old_data = self._read_data(path=path, type_=type_) # returns DataT

        if isinstance(data, list):
            if isinstance(old_data, list):
                new_data = old_data + data # old_data: Never
                # Operator "+" not supported for types "Never" and "List[Any]*"

I don't get why old_data is attributed type Never.

I was not able to create a minimal example and the original code is not public.

[erictraut]

I tried to create a minimal self-contained example from your code above, but I'm not able to repro the problem. There must be something more involved.

from typing import Any, Dict, List, Type, TypeVar

DataT = TypeVar("DataT", bool, int, float, str, List[Any], Dict[str, Any], None)

def read_data(path, type_: Type[DataT]) -> DataT:
    ...

class T:
    def f(self, path, type_, data):
        old_data = read_data(path=path, type_=type_)

        if isinstance(data, list):
            reveal_type(data) # list[Unknown]
            if isinstance(old_data, list):
                reveal_type(old_data) # list[Unknown]
                new_data = old_data + data

Any further thoughts or insights?

[jmg-duarte]

Is there any way I can dive in and try to get a better understanding? Like some inference trace?

[erictraut]

The best approach is to try to find a minimal reproducible case.

You can use reveal_type statements to reveal the evaluated type of an expression at various places in the code. For example, I'm curious the type of old_data immediately prior to the isinstance conditional.

[jmg-duarte]

[erictraut]

Based on that, the old_data is interpreted as an instantiable type (a class), not an object (an instance of a class). You can see that its type is Type[T] rather than just T. For that reason, isinstance(old_data, list) will always be False.

I don't have enough visibility into the rest of the code to say whether the type of old_data should be an instantiable type at that point, but at least we understand why the type of old_data is narrowed to Never in this case.