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Suppose I have some code like
def get_x(d: dict) -> int:
d["x"]
However, I want to tell mypy that d should only contain certain keys (only the "x" key for example). That way, if I make a mistake lower in the code trying to reference an invalid key of d, mypy will trigger an error.
My question is:
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In Python, the keys() and items() methods of dictionaries dict can be used to perform set operations on keys and key-value pairs. For example, you can generate a dictionary consisting of elements (keys and values) common to multiple dictionaries.
For example, you can use an integer, float, string, or Boolean as a dictionary key. However, neither a list nor another dictionary can serve as a dictionary key, because lists and dictionaries are mutable.
The keys of a dictionary can be any kind of immutable type, which includes: strings, numbers, and tuples: mydict = {"hello": "world", 0: "a", 1: "b", "2": "not a number" (1, 2, 3): "a tuple!"}
Ran into this problem too when trying to put some typing around AWS API Gateway/Lambda integration.
As pointed out by @Michael0x2a in the comments, TypedDict would seem to be the way to go, especially because it does not require any conversions yourself before passing into a function (a requirement for API responses, like you mentioned).
from mypy_extensions import TypedDict
DictWithOnlyX = TypedDict('DictWithOnlyX', {"x": str})
# error: Extra key 'y' for TypedDict "DictWithOnlyX@58"
dx1: DictWithOnlyX = { "y": 123 }
# error: Incompatible types (expression has type "int",
# TypedDict item "x" has type "str")
dx2: DictWithOnlyX = { "x": 123 }
# GOOD!
dx3: DictWithOnlyX = { "x": "123" }
Should also note that, if dependencies are segregated between production and dev/test, then this makes mypy a production dependency.
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