[Python] How to resolve 'Incompatible types in assignment' when converting dictionary values...

I am working with a dictionary in Python where a key ("expiryTime") initially holds a str value in ISO 8601 format (e.g., "2025-01-23T12:34:56"). At some point in my code, I convert this string into a datetime object using datetime.strptime. However, I encounter a mypy error during type checking:

error: Incompatible types in assignment (expression has type "datetime", target has type "str") [assignment]


Here is a simplified version of the code:

from datetime import datetime

# Initial dictionary with string value
token_dict: dict[str, str] = {"expiryTime": "2025-01-23T12:34:56"}

# Convert the string to datetime
expiry_time_dt = datetime.strptime(token_dict["expiryTime"], "%Y-%m-%dT%H:%M:%S")
token_dict["expiryTime"] = expiry_time_dt # Error here: incompatible types


I understand that mypy complains because the dictionary's value was initially declared as a str, and assigning a datetime object violates the declared type. However, I need to store the datetime object for further processing in my code.

My Question:


What is the best way to handle this situation while maintaining type safety with mypy? Should I:

1. Suppress the type-checking error with # type: ignore?
2. Refactor my code to use a Union[str, datetime] type for the dictionary values, despite the strptime error?
3. Utilize an alternate approach I may not be aware of?

The goal is to ensure the code remains type-safe while handling the necessary conversions between str and datetime. Any advice or recommendations for improving this workflow would be appreciated!

What I Have Tried:

1. 
   Suppressed the Error: I used # type: ignore to bypass the issue temporarily:
   
   token_dict["expiryTime"] = expiry_time_dt # type: ignore
   
   
   While this works, it's not a clean or type-safe solution.
   
   
2. 
   Using Union[str, datetime]: I updated the type hint of the dictionary:
   
   token_dict: dict[str, Union[str, datetime]] = {"expiryTime": "2025-01-23T12:34:56"}
   
   
   This was the only viable solution until I encountered the following mypy error:
   
   error: Argument 1 to "strptime" of "datetime" has incompatible type "Union[str, datetime]"; expected "str" [arg-type]
   
   
   The error occurs when I try to pass a Union[str, datetime] value to strptime, which expects a str. This creates additional complexity because mypy requires explicit type checks for every access to token_dict["expiryTime"], making it harder to maintain type correctness.
   
   
3. 
   Conversion to str for Mocking: When mocking requests that interact with this dictionary, I must ensure the value is serialized back to a string (since datetime is not JSON-serializable). This back-and-forth conversion is unavoidable, and I need to ensure type correctness in my code.
   

Problem Context (for Reference):


The key "expiryTime" is part of a token dictionary that represents metadata for authentication. When mocking requests, I serialize the dictionary using JSON, which does not support datetime objects. This necessitates the conversion of datetime values back to strings before serialization. The back-and-forth conversions, combined with type annotations, have made this problem challenging to address cleanly.

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