Silberstral

Reveal the true shape of type vars

Python's typing system is weak and lacks features well known in other languages. For example, templating in C++ allows you to instantiate a new object of the templated class T via T(..) which is not possible in Python. The Silberstral package provides remedy with a simple utility to obtain the actual type that a generic type var refers to:

C++

template<typename T>
class DefaultContainer {
    
    T get(int idx) {
        defaultElement = T();  // <- in C++, we can access the actual class of T
        ...
    }
    
}

Python

_T = TypeVar('_T')
class DefaultContainer(Generic[_T]):
    
    def get(self, idx: int) -> _T:
        default_element =  _T()  # <- DOES NOT WORK
        ...

Python + Silberstral

from silberstral import reveal_type_var

_T = TypeVar('_T')
class DefaultList(Generic[_T]):
    
    def get(self, idx: int) -> _T:
        T_cls = reveal_type_var(self, _T)  # <- Reveals the actual class of _T, e.g., int, str, ...
        default_element = T_cls()
        ...

Installation

The package is available at pypi:

pip install silberstral

Usage

reveal_type_var(obj_or_cls, type_var): Finds the actual type that type_var was instantiated to in obj_or_cls.

Example:

from typing import TypeVar, Generic
from silberstral import reveal_type_var

_T = TypeVar('_T')
class List(Generic[_T]):
    pass

reveal_type_var(List[int], _T)
>>> int

str_list = List[str]()
reveal_type_var(str_list, _T)
>>> str

reveal_type_vars(obj_or_cls): Lists all type vars and their corresponding instantiations of obj_or_cls

Example:

from typing import TypeVar, Generic
from silberstral import reveal_type_vars

_K = TypeVar('_K')
_V = TypeVar('_V')
class Dict(Generic[_K, _V]):
    pass

reveal_type_vars(Dict[int, str])
>>> {_K: int, _V: str}

is_type_var_instantiated(obj_or_cls, type_var): Checks whether type_var was instantiated with an actual class in obj_or_cls

Example:

from typing import TypeVar, Generic
from silberstral import is_type_var_instantiated

_T = TypeVar('_T')
class List(Generic[_T]):
    pass

is_type_var_instantiated(List, _T)
>>> False

is_type_var_instantiated(List[int], _T)
>>> True