Mypy: Using `Literal` typed variables as keys for TypedDict causes "TypedDict key must be a string literal"

[chungwu]

Please provide more information to help us understand the issue:

• Are you reporting a bug, or opening a feature request?
  Bug

• Please insert below the code you are checking with mypy,
  or a mock-up repro if the source is private. We would appreciate
  if you try to simplify your case to a minimal repro.

from mypy_extensions import TypedDict
from typing_extensions import Literal

class Test(TypedDict):
  a: str
  b: str

TestKey = Literal['a', 'b']

def read(test: Test, key: TestKey):
  return test[key]

• What is the actual behavior/output?
  mypy.py:13: error: TypedDict key must be a string literal; expected one of ('a', 'b')

• What is the behavior/output you expect?
  No type check errors

• What are the versions of mypy and Python you are using?
  mypy -- 0.660
  python -- 3.6.3

  Do you see the same issue after installing mypy from Git master?
  Yes

• What are the mypy flags you are using? (For example --strict-optional)
  None

• If mypy crashed with a traceback, please paste
  the full traceback below.

(You can freely edit this text, please remove all the lines
you believe are unnecessary.)

[Michael0x2a]

Thanks for the bug report!

What's happening in this case is that mypy doesn't yet understand how to handle indexing TypedDicts using Unions of Literals -- it can currently handle indexing using only Literals. It probably should, though. I'll look into making a fix when I find some time later this week/next week.

[chungwu]

Thanks! This is very useful for typing JSON blobs, and having functions that extract information from the JSON blobs using specific keys.

It would also be nice to extract a keys type from a TypedDict, like so :-)

class Test(TypedDict):
  a: str
  b: str

TestKey = Test.keys()  # same as Literal['a', 'b']

[b0g3r]

I have a similar problem, but I try to use a variable as a key:

from mypy_extensions import TypedDict
from typing_extensions import Literal, Final


class MyDict(TypedDict):
    field_name: int

d: MyDict
d = {'field_name': 1}

name_in_var = 'field_name'
d = {name_in_var: 1}

name_in_literal2: Literal['field_name'] = 'field_name'
d = {name_in_literal2: 1}

name_in_final: Final = 'field_name'
d = {name_in_final: 1}

And then get errors:

Dima Boger, [04.10.19 21:51]
test.py:13: error: Expected TypedDict key to be string literal
test.py:16: error: Expected TypedDict key to be string literal
test.py:19: error: Expected TypedDict key to be string literal
test.py:22: error: Expected TypedDict key to be string literal

@Michael0x2a It is expected behavior or bug linked to this issue?

[JukkaL]

@b0g3r I created the follow-up issue #7644 about your problem.