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[docs] lexical_analysis: Expand the text on `_` (GH-28903)

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Also:
* Expand the discussion into its own entry. (Even before this,
  text on ``_`` was longet than the text on ``_*``.)

* Briefly note the other common convention for `_`: naming unused
  variables.

Co-authored-by: Brandt Bucher <brandtbucher@gmail.com>
Co-authored-by: Ken Jin <28750310+Fidget-Spinner@users.noreply.github.com>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>
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Petr Viktorin 2021-10-13 18:34:01 +02:00 • committed by GitHub
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20
Doc/reference/lexical_analysis.rst
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@ -385,10 +385,20 @@ classes are identified by the patterns of leading and trailing underscore
characters:
``_*``
Not imported by ``from module import *``. The special identifier ``_`` is used
in the interactive interpreter to store the result of the last evaluation; it is
stored in the :mod:`builtins` module. When not in interactive mode, ``_``
has no special meaning and is not defined. See section :ref:`import`.
Not imported by ``from module import *``.
``_``
In a ``case`` pattern within a :keyword:`match` statement, ``_`` is a
:ref:`soft keyword <soft-keywords>` that denotes a
:ref:`wildcard <wildcard-patterns>`.
Separately, the interactive interpreter makes the result of the last evaluation
available in the variable ``_``.
(It is stored in the :mod:`builtins` module, alongside built-in
functions like ``print``.)
Elsewhere, ``_`` is a regular identifier. It is often used to name
"special" items, but it is not special to Python itself.
.. note::
@ -396,6 +406,8 @@ characters:
refer to the documentation for the :mod:`gettext` module for more
information on this convention.
It is also commonly used for unused variables.
``__*__``
System-defined names, informally known as "dunder" names. These names are
defined by the interpreter and its implementation (including the standard library).