CODEWITHSUNDEEP
python
Angelika
Angelika

Reputation: 216

How to correct type errors with mypy library

I check my code using mypy. I got these errors:

  1. I have an object names: List[str], for len(names) I get

    Argument 1 to "len" has incompatible type "Optional[List[str]]"; expected "Sized"

    When I try to index like names[i] I get:

    Value of type "Optional[List[str]]" is not indexable

  2. I have an object matrix: List[List[int]], similarly matrix[i][j]

    Value of type "Optional[List[List[int]]]" is not indexable

  3. I have an object

     def func() -> Dict[str, List[str]]
     g = {"1": ["2"],
     "2": ["3"]}
     return g

    annotated as Dict[str, List[str]], I get an error:

    Incompatible return value type (got "Dict[str, object]", expected "Dict[str, List[str]]")

    I don't understand, why I got this type. If I change it to Dict[str, object], I get another errors in my code like in 4.

  4. When I try to use my object:

     d: DefaultDict[str, List[str]] = defaultdict(list)
 for obj in g:
     d[obj].extend(g.get(obj))

    I get this error:

    Argument 1 to "extend" of "list" has incompatible type "Optional[List[str]]"; expected "Iterable[str]"

I am new in python and don't understand principles of working with mypy- how should I annotate types so as not to get similar errors?

Upvotes: 2

Views: 4801

Answers (1)

ar d
ar d

Reputation: 69

In number 4. when you use .get() method the return type becomes optional i.e. if you were expecting to get List[str] it would be Optional[list[str]] this is equivalent to [None, List[str]]. To overcome this you could do following:-

if g.get(obj) is None:
     raise RunTimeError("Not found")

Upvotes: 2

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