10. Full Grammar specification — Python 3.8.2 documentation

# 10. Full Grammar specification

This is the full Python grammar, as it is read by the parser generator and used to parse Python source files:

    # Grammar for Python

    # NOTE WELL: You should also follow all the steps listed at
    # https://devguide.python.org/grammar/

    # Start symbols for the grammar:
    #       single_input is a single interactive statement;
    #       file_input is a module or sequence of commands read from an input file;
    #       eval_input is the input for the eval() functions.
    #       func_type_input is a PEP 484 Python 2 function type comment
    # NB: compound_stmt in single_input is followed by extra NEWLINE!
    # NB: due to the way TYPE_COMMENT is tokenized it will always be followed by a NEWLINE
    single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE
    file_input: (NEWLINE | stmt)* ENDMARKER
    eval_input: testlist NEWLINE* ENDMARKER

    decorator: '@' dotted_name [ '(' [arglist] ')' ] NEWLINE
    decorators: decorator+
    decorated: decorators (classdef | funcdef | async_funcdef)

    async_funcdef: ASYNC funcdef
    funcdef: 'def' NAME parameters ['->' test] ':' [TYPE_COMMENT] func_body_suite

    parameters: '(' [typedargslist] ')'

    # The following definition for typedarglist is equivalent to this set of rules:
    #
    #     arguments = argument (',' [TYPE_COMMENT] argument)*
    #     argument = tfpdef ['=' test]
    #     kwargs = '**' tfpdef [','] [TYPE_COMMENT]
    #     args = '*' [tfpdef]
    #     kwonly_kwargs = (',' [TYPE_COMMENT] argument)* (TYPE_COMMENT | [',' [TYPE_COMMENT] [kwargs]])
    #     args_kwonly_kwargs = args kwonly_kwargs | kwargs
    #     poskeyword_args_kwonly_kwargs = arguments ( TYPE_COMMENT | [',' [TYPE_COMMENT] [args_kwonly_kwargs]])
    #     typedargslist_no_posonly  = poskeyword_args_kwonly_kwargs | args_kwonly_kwargs
    #     typedarglist = (arguments ',' [TYPE_COMMENT] '/' [',' [[TYPE_COMMENT] typedargslist_no_posonly]])|(typedargslist_no_posonly)"
    #
    # It needs to be fully expanded to allow our LL(1) parser to work on it.

    typedargslist: (
      (tfpdef ['=' test] (',' [TYPE_COMMENT] tfpdef ['=' test])* ',' [TYPE_COMMENT] '/' [',' [ [TYPE_COMMENT] tfpdef ['=' test] (
            ',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] [
            '*' [tfpdef] (',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] ['**' tfpdef [','] [TYPE_COMMENT]]])
          | '**' tfpdef [','] [TYPE_COMMENT]]])
      | '*' [tfpdef] (',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] ['**' tfpdef [','] [TYPE_COMMENT]]])
      | '**' tfpdef [','] [TYPE_COMMENT]]] )
    |  (tfpdef ['=' test] (',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] [
       '*' [tfpdef] (',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] ['**' tfpdef [','] [TYPE_COMMENT]]])
      | '**' tfpdef [','] [TYPE_COMMENT]]])
      | '*' [tfpdef] (',' [TYPE_COMMENT] tfpdef ['=' test])* (TYPE_COMMENT | [',' [TYPE_COMMENT] ['**' tfpdef [','] [TYPE_COMMENT]]])
      | '**' tfpdef [','] [TYPE_COMMENT])
    )
    tfpdef: NAME [':' test]

    # The following definition for varargslist is equivalent to this set of rules:
    #
    #     arguments = argument (',' argument )*
    #     argument = vfpdef ['=' test]
    #     kwargs = '**' vfpdef [',']
    #     args = '*' [vfpdef]
    #     kwonly_kwargs = (',' argument )* [',' [kwargs]]
    #     args_kwonly_kwargs = args kwonly_kwargs | kwargs
    #     poskeyword_args_kwonly_kwargs = arguments [',' [args_kwonly_kwargs]]
    #     vararglist_no_posonly = poskeyword_args_kwonly_kwargs | args_kwonly_kwargs
    #     varargslist = arguments ',' '/' [','[(vararglist_no_posonly)]] | (vararglist_no_posonly)
    #
    # It needs to be fully expanded to allow our LL(1) parser to work on it.

    varargslist: vfpdef ['=' test ](',' vfpdef ['=' test])* ',' '/' [',' [ (vfpdef ['=' test] (',' vfpdef ['=' test])* [',' [
            '*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
          | '**' vfpdef [',']]]
      | '*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
      | '**' vfpdef [',']) ]] | (vfpdef ['=' test] (',' vfpdef ['=' test])* [',' [
            '*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
          | '**' vfpdef [',']]]
      | '*' [vfpdef] (',' vfpdef ['=' test])* [',' ['**' vfpdef [',']]]
      | '**' vfpdef [',']
    )
    vfpdef: NAME

    stmt: simple_stmt | compound_stmt
    simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
    small_stmt: (expr_stmt | del_stmt | pass_stmt | flow_stmt |
                 import_stmt | global_stmt | nonlocal_stmt | assert_stmt)
    expr_stmt: testlist_star_expr (annassign | augassign (yield_expr|testlist) |
                         [('=' (yield_expr|testlist_star_expr))+ [TYPE_COMMENT]] )
    annassign: ':' test ['=' (yield_expr|testlist_star_expr)]
    testlist_star_expr: (test|star_expr) (',' (test|star_expr))* [',']
    augassign: ('+=' | '-=' | '*=' | '@=' | '/=' | '%=' | '&=' | '|=' | '^=' |
                '<<=' | '>>=' | '**=' | '//=')
    # For normal and annotated assignments, additional restrictions enforced by the interpreter
    del_stmt: 'del' exprlist
    pass_stmt: 'pass'
    flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt
    break_stmt: 'break'
    continue_stmt: 'continue'
    return_stmt: 'return' [testlist_star_expr]
    yield_stmt: yield_expr
    raise_stmt: 'raise' [test ['from' test]]
    import_stmt: import_name | import_from
    import_name: 'import' dotted_as_names
    # note below: the ('.' | '...') is necessary because '...' is tokenized as ELLIPSIS
    import_from: ('from' (('.' | '...')* dotted_name | ('.' | '...')+)
                  'import' ('*' | '(' import_as_names ')' | import_as_names))
    import_as_name: NAME ['as' NAME]
    dotted_as_name: dotted_name ['as' NAME]
    import_as_names: import_as_name (',' import_as_name)* [',']
    dotted_as_names: dotted_as_name (',' dotted_as_name)*
    dotted_name: NAME ('.' NAME)*
    global_stmt: 'global' NAME (',' NAME)*
    nonlocal_stmt: 'nonlocal' NAME (',' NAME)*
    assert_stmt: 'assert' test [',' test]

    compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt
    async_stmt: ASYNC (funcdef | with_stmt | for_stmt)
    if_stmt: 'if' namedexpr_test ':' suite ('elif' namedexpr_test ':' suite)* ['else' ':' suite]
    while_stmt: 'while' namedexpr_test ':' suite ['else' ':' suite]
    for_stmt: 'for' exprlist 'in' testlist ':' [TYPE_COMMENT] suite ['else' ':' suite]
    try_stmt: ('try' ':' suite
               ((except_clause ':' suite)+
                ['else' ':' suite]
                ['finally' ':' suite] |
               'finally' ':' suite))
    with_stmt: 'with' with_item (',' with_item)*  ':' [TYPE_COMMENT] suite
    with_item: test ['as' expr]
    # NB compile.c makes sure that the default except clause is last
    except_clause: 'except' [test ['as' NAME]]
    suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT

    namedexpr_test: test [':=' test]
    test: or_test ['if' or_test 'else' test] | lambdef
    test_nocond: or_test | lambdef_nocond
    lambdef: 'lambda' [varargslist] ':' test
    lambdef_nocond: 'lambda' [varargslist] ':' test_nocond
    or_test: and_test ('or' and_test)*
    and_test: not_test ('and' not_test)*
    not_test: 'not' not_test | comparison
    comparison: expr (comp_op expr)*
    # <> isn't actually a valid comparison operator in Python. It's here for the
    # sake of a __future__ import described in PEP 401 (which really works :-)
    comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'
    star_expr: '*' expr
    expr: xor_expr ('|' xor_expr)*
    xor_expr: and_expr ('^' and_expr)*
    and_expr: shift_expr ('&' shift_expr)*
    shift_expr: arith_expr (('<<'|'>>') arith_expr)*
    arith_expr: term (('+'|'-') term)*
    term: factor (('*'|'@'|'/'|'%'|'//') factor)*
    factor: ('+'|'-'|'~') factor | power
    power: atom_expr ['**' factor]
    atom_expr: [AWAIT] atom trailer*
    atom: ('(' [yield_expr|testlist_comp] ')' |
           '[' [testlist_comp] ']' |
           '{' [dictorsetmaker] '}' |
           NAME | NUMBER | STRING+ | '...' | 'None' | 'True' | 'False')
    testlist_comp: (namedexpr_test|star_expr) ( comp_for | (',' (namedexpr_test|star_expr))* [','] )
    trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
    subscriptlist: subscript (',' subscript)* [',']
    subscript: test | [test] ':' [test] [sliceop]
    sliceop: ':' [test]
    exprlist: (expr|star_expr) (',' (expr|star_expr))* [',']
    testlist: test (',' test)* [',']
    dictorsetmaker: ( ((test ':' test | '**' expr)
                       (comp_for | (',' (test ':' test | '**' expr))* [','])) |
                      ((test | star_expr)
                       (comp_for | (',' (test | star_expr))* [','])) )

    classdef: 'class' NAME ['(' [arglist] ')'] ':' suite

    arglist: argument (',' argument)*  [',']

    # The reason that keywords are test nodes instead of NAME is that using NAME
    # results in an ambiguity. ast.c makes sure it's a NAME.
    # "test '=' test" is really "keyword '=' test", but we have no such token.
    # These need to be in a single rule to avoid grammar that is ambiguous
    # to our LL(1) parser. Even though 'test' includes '*expr' in star_expr,
    # we explicitly match '*' here, too, to give it proper precedence.
    # Illegal combinations and orderings are blocked in ast.c:
    # multiple (test comp_for) arguments are blocked; keyword unpackings
    # that precede iterable unpackings are blocked; etc.
    argument: ( test [comp_for] |
                test ':=' test |
                test '=' test |
                '**' test |
                '*' test )

    comp_iter: comp_for | comp_if
    sync_comp_for: 'for' exprlist 'in' or_test [comp_iter]
    comp_for: [ASYNC] sync_comp_for
    comp_if: 'if' test_nocond [comp_iter]

    # not used in grammar, but may appear in "node" passed from Parser to Compiler
    encoding_decl: NAME

    yield_expr: 'yield' [yield_arg]
    yield_arg: 'from' test | testlist_star_expr

    # the TYPE_COMMENT in suites is only parsed for funcdefs,
    # but can't go elsewhere due to ambiguity
    func_body_suite: simple_stmt | NEWLINE [TYPE_COMMENT NEWLINE] INDENT stmt+ DEDENT

    func_type_input: func_type NEWLINE* ENDMARKER
    func_type: '(' [typelist] ')' '->' test
    # typelist is a modified typedargslist (see above)
    typelist: (test (',' test)* [','
           ['*' [test] (',' test)* [',' '**' test] | '**' test]]
         |  '*' [test] (',' test)* [',' '**' test] | '**' test)