# Comparison: Membership Test

In Python, the term "membership test operators" refers to the operators `in` and `not in`. To
customize their behavior, classes can implement one of the special methods `__contains__`,
`__iter__`, or `__getitem__`.

For references, see:

- <https://docs.python.org/3/reference/expressions.html#membership-test-details>
- <https://docs.python.org/3/reference/datamodel.html#object.__contains__>
- <https://snarky.ca/unravelling-membership-testing/>

## Implements `__contains__`

Classes can support membership tests by implementing the `__contains__` method:

```py
class A:
    def __contains__(self, item: str) -> bool:
        return True

reveal_type("hello" in A())  # revealed: bool
reveal_type("hello" not in A())  # revealed: bool
# error: [unsupported-operator] "Operator `in` is not supported between objects of type `Literal[42]` and `A`"
reveal_type(42 in A())  # revealed: bool
# error: [unsupported-operator] "Operator `not in` is not supported between objects of type `Literal[42]` and `A`"
reveal_type(42 not in A())  # revealed: bool
```

## Implements `__iter__`

Classes that don't implement `__contains__`, but do implement `__iter__`, also support containment
checks; the needle will be sought in their iterated items:

```py
class StringIterator:
    def __next__(self) -> str:
        return "foo"

class A:
    def __iter__(self) -> StringIterator:
        return StringIterator()

reveal_type("hello" in A())  # revealed: bool
reveal_type("hello" not in A())  # revealed: bool
reveal_type(42 in A())  # revealed: bool
reveal_type(42 not in A())  # revealed: bool
```

## Implements `__getitems__`

The final fallback is to implement `__getitem__` for integer keys. Python will call `__getitem__`
with `0`, `1`, `2`... until either the needle is found (leading the membership test to evaluate to
`True`) or `__getitem__` raises `IndexError` (the raised exception is swallowed, but results in the
membership test evaluating to `False`).

```py
class A:
    def __getitem__(self, key: int) -> str:
        return "foo"

reveal_type("hello" in A())  # revealed: bool
reveal_type("hello" not in A())  # revealed: bool
reveal_type(42 in A())  # revealed: bool
reveal_type(42 not in A())  # revealed: bool
```

## Wrong Return Type

Python coerces the results of containment checks to `bool`, even if `__contains__` returns a
non-bool:

```py
class A:
    def __contains__(self, item: str) -> str:
        return "foo"

reveal_type("hello" in A())  # revealed: bool
reveal_type("hello" not in A())  # revealed: bool
```

## Literal Result for `in` and `not in`

`__contains__` with a literal return type may result in a `BooleanLiteral` outcome.

```py
from typing import Literal

class AlwaysTrue:
    def __contains__(self, item: int) -> Literal[1]:
        return 1

class AlwaysFalse:
    def __contains__(self, item: int) -> Literal[""]:
        return ""

reveal_type(42 in AlwaysTrue())  # revealed: Literal[True]
reveal_type(42 not in AlwaysTrue())  # revealed: Literal[False]

reveal_type(42 in AlwaysFalse())  # revealed: Literal[False]
reveal_type(42 not in AlwaysFalse())  # revealed: Literal[True]
```

## No Fallback for `__contains__`

If `__contains__` is implemented, checking membership of a type it doesn't accept is an error; it
doesn't result in a fallback to `__iter__` or `__getitem__`:

```py
class CheckContains: ...
class CheckIter: ...
class CheckGetItem: ...

class CheckIterIterator:
    def __next__(self) -> CheckIter:
        return CheckIter()

class A:
    def __contains__(self, item: CheckContains) -> bool:
        return True

    def __iter__(self) -> CheckIterIterator:
        return CheckIterIterator()

    def __getitem__(self, key: int) -> CheckGetItem:
        return CheckGetItem()

reveal_type(CheckContains() in A())  # revealed: bool

# error: [unsupported-operator] "Operator `in` is not supported between objects of type `CheckIter` and `A`"
reveal_type(CheckIter() in A())  # revealed: bool
# error: [unsupported-operator] "Operator `in` is not supported between objects of type `CheckGetItem` and `A`"
reveal_type(CheckGetItem() in A())  # revealed: bool

class B:
    def __iter__(self) -> CheckIterIterator:
        return CheckIterIterator()

    def __getitem__(self, key: int) -> CheckGetItem:
        return CheckGetItem()

reveal_type(CheckIter() in B())  # revealed: bool
# Always use `__iter__`, regardless of iterated type; there's no NotImplemented
# in this case, so there's no fallback to `__getitem__`
reveal_type(CheckGetItem() in B())  # revealed: bool
```

## Invalid Old-Style Iteration

If `__getitem__` is implemented but does not accept integer arguments, then the membership test is
not supported and should trigger a diagnostic.

```py
class A:
    def __getitem__(self, key: str) -> str:
        return "foo"

# error: [unsupported-operator] "Operator `in` is not supported between objects of type `Literal[42]` and `A`"
reveal_type(42 in A())  # revealed: bool
# error: [unsupported-operator] "Operator `in` is not supported between objects of type `Literal["hello"]` and `A`"
reveal_type("hello" in A())  # revealed: bool
```

## Return type that doesn't implement `__bool__` correctly

`in` and `not in` operations will fail at runtime if the object on the right-hand side of the
operation has a `__contains__` method that returns a type which is not convertible to `bool`. This
is because of the way these operations are handled by the Python interpreter at runtime. If we
assume that `y` is an object that has a `__contains__` method, the Python expression `x in y`
desugars to a `contains(y, x)` call, where `contains` looks something like this:

```ignore
def contains(y, x):
    return bool(type(y).__contains__(y, x))
```

where the `bool()` conversion itself implicitly calls `__bool__` under the hood.

TODO: Ideally the message would explain to the user what's wrong. E.g,

```ignore
error: [operator] cannot use `in` operator on object of type `WithContains`
    note: This is because the `in` operator implicitly calls `WithContains.__contains__`, but `WithContains.__contains__` is invalidly defined
    note: `WithContains.__contains__` is invalidly defined because it returns an instance of `NotBoolable`, which cannot be evaluated in a boolean context
    note: `NotBoolable` cannot be evaluated in a boolean context because its `__bool__` attribute is not callable
```

It may also be more appropriate to use `unsupported-operator` as the error code.

<!-- snapshot-diagnostics -->

```py
class NotBoolable:
    __bool__: int = 3

class WithContains:
    def __contains__(self, item) -> NotBoolable:
        return NotBoolable()

# error: [unsupported-bool-conversion]
10 in WithContains()
# error: [unsupported-bool-conversion]
10 not in WithContains()
```
