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bpo-35975: Support parsing earlier minor versions of Python 3 (GH-12086)
This adds a `feature_version` flag to `ast.parse()` (documented) and `compile()` (hidden) that allow tweaking the parser to support older versions of the grammar. In particular if `feature_version` is 5 or 6, the hacks for the `async` and `await` keyword from PEP 492 are reinstated. (For 7 or higher, these are unconditionally treated as keywords, but they are still special tokens rather than `NAME` tokens that the parser driver recognizes.) https://bugs.python.org/issue35975
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‎Doc/library/ast.rst‎

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@@ -126,7 +126,7 @@ The abstract grammar is currently defined as follows:
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Apart from the node classes, the :mod:`ast` module defines these utility functions
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and classes for traversing abstract syntax trees:
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.. function:: parse(source, filename='<unknown>', mode='exec', *, type_comments=False)
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.. function:: parse(source, filename='<unknown>', mode='exec', *, type_comments=False, feature_version=-1)
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Parse the source into an AST node. Equivalent to ``compile(source,
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filename, mode, ast.PyCF_ONLY_AST)``.
@@ -145,13 +145,19 @@ and classes for traversing abstract syntax trees:
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modified to correspond to :pep:`484` "signature type comments",
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e.g. ``(str, int) -> List[str]``.
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Also, setting ``feature_version`` to the minor version of an
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earlier Python 3 version will attempt to parse using that version's
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grammar. For example, setting ``feature_version=4`` will allow
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the use of ``async`` and ``await`` as variable names. The lowest
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supported value is 4; the highest is ``sys.version_info[1]``.
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.. warning::
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It is possible to crash the Python interpreter with a
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sufficiently large/complex string due to stack depth limitations
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in Python's AST compiler.
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.. versionchanged:: 3.8
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Added ``type_comments=True`` and ``mode='func_type'``.
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Added ``type_comments``, ``mode='func_type'`` and ``feature_version``.
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.. function:: literal_eval(node_or_string)

‎Doc/library/token-list.inc‎

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‎Doc/library/token.rst‎

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@@ -88,3 +88,6 @@ the :mod:`tokenize` module.
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.. versionchanged:: 3.8
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Added :data:`TYPE_COMMENT`.
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Added :data:`AWAIT` and :data:`ASYNC` tokens back (they're needed
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to support parsing older Python versions for :func:`ast.parse` with
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``feature_version`` set to 6 or lower).

‎Grammar/Grammar‎

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@@ -18,7 +18,7 @@ decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE
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decorators: decorator+
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decorated: decorators (classdef | funcdef | async_funcdef)
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async_funcdef: 'async' funcdef
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async_funcdef: ASYNC funcdef
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funcdef: 'def' NAME parameters ['->' test] ':' [TYPE_COMMENT] func_body_suite
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parameters: '(' [typedargslist] ')'
@@ -70,7 +70,7 @@ nonlocal_stmt: 'nonlocal' NAME (',' NAME)*
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assert_stmt: 'assert' test [',' test]
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compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
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async_stmt: 'async' (funcdef | with_stmt | for_stmt)
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async_stmt: ASYNC (funcdef | with_stmt | for_stmt)
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if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)* ['else' ':' suite]
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while_stmt: 'while' namedexpr_test ':' suite ['else' ':' suite]
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for_stmt: 'for' exprlist 'in' testlist ':' [TYPE_COMMENT] suite ['else' ':' suite]
@@ -106,7 +106,7 @@ arith_expr: term (('+'|'-') term)*
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term: factor (('*'|'@'|'/'|'%'|'//') factor)*
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factor: ('+'|'-'|'~') factor | power
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power: atom_expr ['**' factor]
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atom_expr: ['await'] atom trailer*
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atom_expr: [AWAIT] atom trailer*
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atom: ('(' [yield_expr|testlist_comp] ')' |
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'[' [testlist_comp] ']' |
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'{' [dictorsetmaker] '}' |
@@ -144,7 +144,7 @@ argument: ( test [comp_for] |
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comp_iter: comp_for | comp_if
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sync_comp_for: 'for' exprlist 'in' or_test [comp_iter]
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comp_for: ['async'] sync_comp_for
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comp_for: [ASYNC] sync_comp_for
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comp_if: 'if' test_nocond [comp_iter]
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# not used in grammar, but may appear in "node" passed from Parser to Compiler

‎Grammar/Tokens‎

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@@ -55,6 +55,8 @@ ELLIPSIS '...'
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COLONEQUAL ':='
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OP
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AWAIT
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ASYNC
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TYPE_IGNORE
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TYPE_COMMENT
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ERRORTOKEN

‎Include/Python-ast.h‎

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‎Include/compile.h‎

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@@ -27,6 +27,7 @@ PyAPI_FUNC(PyCodeObject *) PyNode_Compile(struct _node *, const char *);
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#ifndef Py_LIMITED_API
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typedef struct {
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int cf_flags; /* bitmask of CO_xxx flags relevant to future */
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int cf_feature_version; /* minor Python version (PyCF_ONLY_AST) */
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} PyCompilerFlags;
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#endif
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‎Include/parsetok.h‎

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@@ -35,6 +35,7 @@ typedef struct {
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#define PyPARSE_IGNORE_COOKIE 0x0010
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#define PyPARSE_BARRY_AS_BDFL 0x0020
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#define PyPARSE_TYPE_COMMENTS 0x0040
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#define PyPARSE_ASYNC_HACKS 0x0080
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PyAPI_FUNC(node *) PyParser_ParseString(const char *, grammar *, int,
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perrdetail *);

‎Include/token.h‎

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‎Lib/ast.py‎

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@@ -27,7 +27,8 @@
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from _ast import *
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def parse(source, filename='<unknown>', mode='exec', *, type_comments=False):
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def parse(source, filename='<unknown>', mode='exec', *,
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type_comments=False, feature_version=-1):
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"""
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Parse the source into an AST node.
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Equivalent to compile(source, filename, mode, PyCF_ONLY_AST).
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flags = PyCF_ONLY_AST
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if type_comments:
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flags |= PyCF_TYPE_COMMENTS
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return compile(source, filename, mode, flags)
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return compile(source, filename, mode, flags,
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feature_version=feature_version)
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def literal_eval(node_or_string):

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