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# Copyright (c) Meta Platforms, Inc. and affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
"""
Used by visitors. This is hoisted into a separate module to avoid some circular
dependencies in the definition of CSTNode.
"""
from enum import auto, Enum
class RemovalSentinel(Enum):
"""
A :attr:`RemovalSentinel.REMOVE` value should be returned by a
:meth:`CSTTransformer.on_leave` method when we want to remove that child from its
parent. As a convenience, this can be constructed by calling
:func:`libcst.RemoveFromParent`.
The parent node should make a best-effort to remove the child, but may raise an
exception when removing the child doesn't make sense, or could change the semantics
in an unexpected way. For example, a function definition with no name doesn't make
sense, but removing one of the arguments is valid.
In we can't automatically remove the child, the developer should instead remove the
child by constructing a new parent in the parent's :meth:`~CSTTransformer.on_leave`
call.
We use this instead of ``None`` to force developers to be explicit about deletions.
Because ``None`` is the default return value for a function with no return
statement, it would be too easy to accidentally delete nodes from the tree by
forgetting to return a value.
"""
REMOVE = auto()
def RemoveFromParent() -> RemovalSentinel:
"""
A convenience method for requesting that this node be removed by its parent.
Use this in place of returning :class:`RemovalSentinel` directly.
For example, to remove all arguments unconditionally::
def leave_Arg(
self, original_node: cst.Arg, updated_node: cst.Arg
) -> Union[cst.Arg, cst.RemovalSentinel]:
return RemoveFromParent()
"""
return RemovalSentinel.REMOVE
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