Unexpected error using Union[] as the argument to the value field of a dictionary

[sborisch]

Unexpected error using Union[] as the argument to the value field of a dictionary

Bug Report

Hello,

I am trying to use mypy to validate my use of type hints where I have a dictionary where the value fields in the dictionary can be either: str, bool, or int. That is:

Union[str,bool,int]

However, if I do this, I get

Incompatible types in assignment (expression has type "str | int | bool", variable has type "str") [assignment]

To Reproduce

Example code:

  flac_song_dict : dict[str,Union[str,int,bool]] 
  for (dummy_key,flac_song_dict) in flac_album_dict.items():

    flac_filename      : str = flac_song_dict["FILENAME"]  
    flac_ext           : str = flac_song_dict["FILEEXT"]   
    flac_track_num     : int = flac_song_dict["TRACK_NUM"] 
    flac_song          : str = flac_song_dict["SONG"]      
    flac_filepath      : str = flac_song_dict["FILEPATH"]    

to avoid this error, I need to change

  flac_song_dict : dict[str,Union[str,int,bool]] 

to
flac_song_dict : dict[str,any]

Expected Behavior

I would have expected the above to not produce an error.
IMO, the use of "any" is less optimal than Union[str,int,bool]

Actual Behavior

Incompatible types in assignment (expression has type "str | int | bool", variable has type "str") [assignment]

Your Environment

• Mypy version used: 1.16.1
• Mypy command-line flags (none):
• Mypy configuration options from mypy.ini (and other config files): As far as I can tell, this file does not exist for me. I do not see it in the directory tree where mypy is installed.
• Python version used: 3.10.12

[JelleZijlstra]

This error is correct. Your type annotation indicates that every value in the dict may be a str, an int, or a bool. Therefore, assigning a value from the dict to a variable declared to be a str is unsafe.

You may be better off using TypedDict.

[brianschubert]

Hi! This is the expected behavior. The type dict[str, str | int | bool] tells mypy that the dictionary entries will be of type str, int, or bool, but it doesn't tell mypy which entries will have which type (for all mypy knows, FILENAME could be a bool, for example). Hence, any operations on them must be valid for all of those types. To teach mypy which entries have which type, you can try using a TypedDict. Something like the following may be what you're looking for:

from typing import TypedDict

class SongDict(TypedDict):
    FILENAME: str
    FILEEXT: str   
    TRACK_NUM: int 
    SONG: str      
    FILEPATH: str

flac_song_dict: SongDict

For more on how to use TypedDict, you can refer to:

• https://typing.python.org/en/latest/spec/typeddict.html#typed-dictionaries
• https://mypy.readthedocs.io/en/stable/typed_dict.html

[sborisch]

Thanks for the recommendation. That sounds fine.