10. Full Grammar specification — Python 3.7.4 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.
    # NB: compound_stmt in single_input is followed by extra 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] ':' suite

    parameters: '(' [typedargslist] ')'
    typedargslist: (tfpdef ['=' test] (',' tfpdef ['=' test])* [',' [
            '*' [tfpdef] (',' tfpdef ['=' test])* [',' ['**' tfpdef [',']]]
          | '**' tfpdef [',']]]
      | '*' [tfpdef] (',' tfpdef ['=' test])* [',' ['**' tfpdef [',']]]
      | '**' tfpdef [','])
    tfpdef: NAME [':' test]
    varargslist: (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))*)
    annassign: ':' test ['=' test]
    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]
    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' test ':' suite ('elif' test ':' suite)* ['else' ':' suite]
    while_stmt: 'while' test ':' suite ['else' ':' suite]
    for_stmt: 'for' exprlist 'in' testlist ':' suite ['else' ':' suite]
    try_stmt: ('try' ':' suite
               ((except_clause ':' suite)+
                ['else' ':' suite]
                ['finally' ':' suite] |
               'finally' ':' suite))
    with_stmt: 'with' with_item (',' with_item)*  ':' 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

    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: (test|star_expr) ( comp_for | (',' (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 )

    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