It’s even more disappointing because this isn’t just an oversight. The authors have deliberately made having additional keys an error. Apparently, this even a divergence from how TypeScript checks dictionaries.
from typing import TypedDict, Unpack, NotRequired
class Movie(TypedDict):
name: str
year: NotRequired[int]
def foo(**kwargs: Unpack[Movie]) -> None: ...
# Simple case, any halfway-decent type system should be able to handle this.
def inner(*, i): pass
def middle(*, m, **kwargs): inner(**kwargs)
def outer(*, o, **kwargs): middle(**kwargs)
kwargs = ...; outer(**kwargs)
# More complicated case, but fairly common in Python code.
class A:
def __init__(self, *, a):
pass
class B(A):
def __init__(self, *, b, **kwargs):
super().__init__(**kwargs)
class C(A):
def __init__(self, *, c, **kwargs):
super().__init__(**kwargs)
class D(B, C):
def __init__(self, *, d, **kwargs):
super().__init__(**kwargs)
kwargs = ...; D(**kwargs)
# An even more complicated case involves `kwargs.pop()`, forwarding without `**`.
Specifically: absent optional keys + extra keys is fundamentally indistinguishable from miseptl keys.
TypedDicts are disappointing because you can't partially define a type? That seems like a success
In go you would need to define all fields in your struct and if you needed unstructured data you would have to define a map, which even then is partially typed
How should extra keys behave in "typed python?"
Python is a dynamic language where one of its major features is structural subtyping, aka duck typing. It’s effectively an alternative to inheritance. Features have been added to help support this, like TypedDicts and Protocols already. They don’t go far enough.
I want to be able to say “this function argument is a dictionary that has at least the keys a, b, and c.” It gives the contract that the function only accesses those keys, and others will be ignored. The type checker can check that the function doesn’t access undeclared keys and the annotation helps communicate to client code what the interface is supposed to be.
Lots of Python’s bolted on type checking seems to be straight jacketed to match what’s in C, Java, Go, etc. Those languages don’t contain the only possible type systems or static checkers. There’s a serious lack of imagination. Python’s type system should be designed around enabling and making safer what the language is already good at.
Do we agree that this is the behavior of regular dicts in python? How should TypedDicts be different?
Surely, the goal of typing in python should not be to match behavior without