Improve inference by enabling `--protocols` by default.

[google]

Improve inference by enabling `--protocols` by default.

[matthiaskramm]

This seems to only happen under Python 3. Under Python 2 (with backported type annotations), the inference looks correct, i.e., balance and amount are, and deposit returns, a Union[complex, float].

In general, our Python 3 compatibility is still WIP.

[gvanrossum]

(CC @ddfisher :-)

[rchen152]

This no longer seems to be Python 3-specific, since I now get identical results in 2 and in 3:

from typing import Any, Dict

class Account:
    __doc__ = ...  # type: str
    balance = ...  # type: Any
    def __init__(self, opening_balance: float = ...) -> None: ...
    def deposit(self, amount) -> Any: ...

class Bank:
    _accounts = ...  # type: Dict[Any, Account]
    def __init__(self, bank_owner = ...) -> None: ...
    def deposit(self, name, amount) -> Any: ...


def main() -> None: ...

The incorrect Iterable and list are gone, but balance is still Any. That's happening because we stopped trying to infer parameter types, so doing
self.balance += amount, where amount is an unknown, causes the type of balance to become unknown as well.

Passing the --protocols flag gets us more precise types:

from typing import Any, Dict, Optional, Union

class Account:
    __doc__ = ...  # type: str
    balance = ...  # type: Union[complex, float]
    def __init__(self, opening_balance: float = ...) -> None: ...
    def deposit(self, amount: complex) -> Union[complex, float]: ...

class Bank:
    _accounts = ...  # type: Dict[Any, Account]
    def __init__(self, bank_owner = ...) -> None: ...
    def deposit(self, name, amount: complex) -> Optional[Union[complex, float]]: ...


def main() -> None: ...

So I think the way to fix this would be to make --protocols the default.