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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.
from enum import auto, Enum
class MaybeSentinel(Enum):
"""
A :class:`MaybeSentinel` value is used as the default value for some attributes to
denote that when generating code (when :attr:`Module.code` is evaluated) we should
optionally include this element in order to generate valid code.
:class:`MaybeSentinel` is only used for "syntactic trivia" that most users shouldn't
care much about anyways, like commas, semicolons, and whitespace.
For example, a function call's :attr:`Arg.comma` value defaults to
:attr:`MaybeSentinel.DEFAULT`. A comma is required after every argument, except for
the last one. If a comma is required and :attr:`Arg.comma` is a
:class:`MaybeSentinel`, one is inserted.
This makes manual node construction easier, but it also means that we safely add
arguments to a preexisting function call without manually fixing the commas:
>>> import libcst as cst
>>> fn_call = cst.parse_expression("fn(1, 2)")
>>> new_fn_call = fn_call.with_changes(
... args=[*fn_call.args, cst.Arg(cst.Integer("3"))]
... )
>>> dummy_module = cst.parse_module("") # we need to use Module.code_for_node
>>> dummy_module.code_for_node(fn_call)
'fn(1, 2)'
>>> dummy_module.code_for_node(new_fn_call)
'fn(1, 2, 3)'
Notice that a comma was automatically inserted after the second argument. Since the
original second argument had no comma, it was initialized to
:attr:`MaybeSentinel.DEFAULT`. During the code generation of the second argument, a
comma was inserted to ensure that the resulting code is valid.
.. warning::
While this sentinel is used in place of nodes, it is not a :class:`CSTNode`, and
will not be visited by a :class:`CSTVisitor`.
Some other libraries, like `RedBaron`_, take other approaches to this problem.
RedBaron's tree is mutable (LibCST's tree is immutable), and so they're able to
solve this problem with `"proxy lists"
<http://redbaron.pycqa.org/en/latest/proxy_list.html>`_. Both approaches come with
different sets of tradeoffs.
.. _RedBaron: http://redbaron.pycqa.org/en/latest/index.html
"""
DEFAULT = auto()
def __repr__(self) -> str:
return str(self)
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