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Why Python Type System Sucks

Unpredictable Type Inference

Even with all possible type hints, Python type system don't allow passing TypedDict value to param.

py

from collections.abc import Mapping, MutableMapping
from typing import TypedDict

def my_func(v: Mapping[str, str] | dict[str, str] | MutableMapping[str, str]):  ...


class MyTypedDict(TypedDict): ...


val: MyTypedDict = {}
my_func(val) # 🚨 ERROR: "MyTypedDict" is not assignable

Nested Types are hard

Python type system is not good at handling nested types. We've to define a new type for each nested type.

py

from typing import Any, TypedDict

class ContextNode(TypedDict):
    output: Any


class Context(TypedDict):
    nodes: ContextNode

In typescript this is as simple as:

ts

interface Context {
  nodes: {
    output: any
  }
}

Beware with `Sequence`

py

from collections.abc import Sequence


def my_func(errors: Sequence[str]): ...

my_func(["Some Error"])
my_func("Some Error") # Valid, Because strings are sequence as well