Request: Optional config to warn when a parameter annotation could use Mapping instead of Dict or Sequence instead of List

[ckarnell]

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

Basically, I think it would be nice if mypy could warn when the user could be giving their function immutable versions of the argument types instead of mutable ones. For example:

def len_greater_than_three(input_list: List[int]) -> bool: # Warning: Could use read only type Sequence instead of List for parameter 1.
    return len(input_list) > 3

Being prompted to use immutable types when you're not mutating the input could improve the usability of mypy quite a bit since it would result in functions who's parameters are covariant much more often. It's also convenient to know when a function isn't mutating its arguments based on its annotation, and the desire to default to using immutable types wherever possible probably makes sense for a lot of users.

[ilevkivskyi]

In principle think can be a useful feature. But I think it is relatively low priority.