8000 gh-127833: Reword and expand the Notation section by encukou · Pull Request #134443 · python/cpython · GitHub
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encukou merged 8 commits into from
Jun 9, 2025
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gh-127833: Reword and expand the Notation section #134443

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211f83d
gh-127833: Reword and expand the Notation section
encukou May 21, 2025
ec90d40
Consolidate with the Full Grammar intro
encukou May 21, 2025
3f3a0db
Don't link the examples
encukou May 21, 2025
160ff42
Add Cut
encukou May 21, 2025
231e2ba
Note practical difference between syntactic & lexical definitions
encukou May 28, 2025
fa17c00
Apply suggestions from code review
encukou Jun 3, 2025
327b90d
Constrain `"a"..."z"` and `<...>` to lexical definitions
encukou Jun 4, 2025
3c5caa6
Update Doc/reference/introduction.rst
encukou Jun 7, 2025
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Consolidate with the Full Grammar intro 
Co-authored-by: Blaise Pabon <blaise@gmail.com>
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@encukou @blaisep
encukou and blaisep committed May 21, 2025
commit ec90d4066987534c7dbeed7e91aadbac1ff8670b
16 changes: 7 additions & 9 deletions Doc/reference/grammar.rst
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Expand Up @@ -8,15 +8,13 @@ used to generate the CPython parser (see :source:`Grammar/python.gram`).
The version here omits details related to code generation and
error recovery.

The notation is a mixture of `EBNF
<https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form>`_
and `PEG <https://en.wikipedia.org/wiki/Parsing_expression_grammar>`_.
In particular, ``&`` followed by a symbol, token or parenthesized
group indicates a positive lookahead (i.e., is required to match but
not consumed), while ``!`` indicates a negative lookahead (i.e., is
required *not* to match). We use the ``|`` separator to mean PEG's
"ordered choice" (written as ``/`` in traditional PEG grammars). See
:pep:`617` for more details on the grammar's syntax.
The notation used here is the same as in the preceding docs,
and is described in the :ref:`notation <notation>` section,
except for a few extra complications:

* ``&e``: a positive lookahead (that is, ``e`` is required to match but
not consumed)
* ``!e``: a negative lookahead (that is, ``e`` is required *not* to match)

.. literalinclude:: ../../Grammar/python.gram
:language: peg
16 changes: 11 additions & 5 deletions Doc/reference/introduction.rst
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Expand Up @@ -90,9 +90,10 @@

.. index:: BNF, grammar, syntax, notation

The descriptions of lexical analysis and syntax use a modified
`Backus–Naur form (BNF) <https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form>`_ grammar
notation. This uses the following style of definition:
The descriptions of lexical analysis use a grammar notation that is a mixture
of `EBNF <https://en.wikipedia.org/wiki/Extended_Backus%E2%80%93Naur_form>`_
and `PEG <https://en.wikipedia.org/wiki/Parsing_expression_grammar>`_.
For example:

.. grammar-snippet::
:group: notation
Expand Down Expand Up @@ -136,7 +137,11 @@
* ``e1 e2``: Items separated only by whitespace denote a sequence.
Here, ``e1`` must be followed by ``e2``.
* ``e1 | e2``: A vertical bar is used to separate alternatives.
It is the least tightly binding operator in this notation.
It denotes PEG's "ordered choice": if ``e1`` matches, ``e2`` is
not considered.
In traditional PEG grammars, this is written as a slash, ``/``, rather than
a vertical bar.
See :pep:`617` for more background and details.
* ``e*``: A star means zero or more repetitions of the preceding item.
* ``e+``: Likewise, a plus means one or more repetitions.
* ``[e]``: A phrase enclosed in square brackets means zero or
Expand All @@ -145,14 +150,15 @@
the preceding item is optional.
* ``(e)``: Parentheses are used for grouping.

The unary operators (``*``, ``+``, ``?``) bind as tightly as possible.
The unary operators (``*``, ``+``, ``?``) bind as tightly as possible;
the vertical bar (``|``) binds most loosely.

White space is only meaningful to separate tokens.

Rules are normally contained on a single line, but rules that are too long
may be wrapped:

.. grammar-snippet::

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'token' reference target not found: notation:imagnumber [ref.token]
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literal: `stringliteral` | `bytesliteral`
Expand All @@ -163,7 +169,7 @@
For example:


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