Search code examples
pythonpython-typingmypy

Specify keys for mypy in python dictionary

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:

  1. Is this possible? Can mypy validate dictionary keys?
  2. If yes, how is this done? If no, is there a preferred workaround?
Solution

  • 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.