How to use static type checking using Dict with different value types in Python 3.6?

Question

Trying to use static types in Python code, so mypy can help me with some hidden errors. It's quite simple to use with single variables

real_hour: int = lower_hour + hour_iterator

Harder to use it with lists and dictionaries, need to import additional typing library:

from typing import Dict, List
hour_dict: Dict[str, str] = {"test_key": "test_value"}

But the main problem - how to use it with Dicts with different value types, like:

hour_dict = {"test_key": "test_value", "test_keywords": ["test_1","test_2"]}

If I don't use static typing for such dictionaries - mypy shows me errors, like:

len(hour_dict['test_keywords'])
- Argument 1 to "len" has incompatible type

So, my question: how to add static types to such dictionaries? :)

Answer 1

You need a Union type, of some sort.

from typing import Dict, List, Union

# simple str values
hour_dict: Dict[str, str] = {"test_key": "test_value"}

# more complex values
hour_dict1: Dict[str, Union[str, List[str]]] = {
    "test_key": "test_value", 
    "test_keywords": ["test_1","test_2"]
}

In general, when you need an "either this or that," you need a Union. In this case, your options are str and List[str].

There are several ways to play this out. You might, for example, want to define type names to simplify inline types.

OneOrManyStrings = Union[str, List[str]]

hour_dict2: Dict[str, OneOrManyStrings] = {
    "test_key": "test_value", 
    "test_keywords": ["test_1","test_2"]
}

I might also advise for simplicity, parallelism, and regularity to make all your dict values pure List[str] even if there's only one item. This would allow you to always take the len() of a value, without prior type checking or guard conditions. But those points are nits and tweaks.