silberstral 0.2.3

Reveals the true shape of type vars

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