This chage more closely matches the behavior of `get` at runtime. Users can pass whatever they want in to the default
parameter and it will be returned if the key is absent. Additionally, `get` should return an `Optional` if called with
only one parameter.
```python
z = {'a': 22}
reveal_type(z.get('b'))
reveal_type(z.get('b', 22))
reveal_type(z.get('b', 'hello'))
```
Before:
```shell
test_get_default.py:2: error: Revealed type is 'builtins.int*'
test_get_default.py:3: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Revealed type is 'builtins.int*'
test_get_default.py:4: error: Argument 2 to "get" of "dict" has incompatible type "str"; expected "int"
```
After:
```shell
test_get_default.py:2: error: Revealed type is 'Union[builtins.int*, builtins.None]'
test_get_default.py:3: error: Revealed type is 'builtins.int'
test_get_default.py:4: error: Revealed type is 'Union[builtins.int, builtins.str*]'
```
* Make Mapping covariant.
Fixes#510.
This requires a `# type: ignore` on `__getitem__` and `get` because
mypy complains about covariant parameters.
FWIW typing.py needs to be changed too. (It was covariant in the
value but invariant in the key -- typeshed was invariant in both.)
* Delete outdated comment.
* Backpeddle a bit -- Mapping key type should not be covariant.
In Python, it's possible to use the `next` builtin method on file
objects produced by `open`. This change modifies `typing.IO` so this
usage will successfully typecheck.
This reverts commit d43adbe97e.
Here's a simple example of code that breaks with this PR:
from typing import Mapping, Dict, Tuple
a = {('0', '0'): 42} # type: Mapping[Tuple[str, str], int]
b = a.get(('1', '1'), 0)
This gives an error on the last line:
error: No overload variant of "get" of "dict" matches argument types [Tuple[builtins.str, builtins.str], builtins.int]
