10. Full Grammar specification — Python 3.9.21 documentation

# 10. Full Grammar specification

This is the full Python grammar, derived directly from the grammar used to generate the CPython parser (see Grammar/python.gram). The version here omits details related to code generation and error recovery.

The notation is a mixture of EBNF and PEG. 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).

    # PEG grammar for Python

    file: [statements] ENDMARKER
    interactive: statement_newline
    eval: expressions NEWLINE* ENDMARKER
    func_type: '(' [type_expressions] ')' '->' expression NEWLINE* ENDMARKER
    fstring: star_expressions

    # type_expressions allow */** but ignore them
    type_expressions:
        | ','.expression+ ',' '*' expression ',' '**' expression
        | ','.expression+ ',' '*' expression
        | ','.expression+ ',' '**' expression
        | '*' expression ',' '**' expression
        | '*' expression
        | '**' expression
        | ','.expression+

    statements: statement+
    statement: compound_stmt  | simple_stmt
    statement_newline:
        | compound_stmt NEWLINE
        | simple_stmt
        | NEWLINE
        | ENDMARKER
    simple_stmt:
        | small_stmt !';' NEWLINE  # Not needed, there for speedup
        | ';'.small_stmt+ [';'] NEWLINE
    # NOTE: assignment MUST precede expression, else parsing a simple assignment
    # will throw a SyntaxError.
    small_stmt:
        | assignment
        | star_expressions
        | return_stmt
        | import_stmt
        | raise_stmt
        | 'pass'
        | del_stmt
        | yield_stmt
        | assert_stmt
        | 'break'
        | 'continue'
        | global_stmt
        | nonlocal_stmt
    compound_stmt:
        | function_def
        | if_stmt
        | class_def
        | with_stmt
        | for_stmt
        | try_stmt
        | while_stmt

    # NOTE: annotated_rhs may start with 'yield'; yield_expr must start with 'yield'
    assignment:
        | NAME ':' expression ['=' annotated_rhs ]
        | ('(' single_target ')'
             | single_subscript_attribute_target) ':' expression ['=' annotated_rhs ]
        | (star_targets '=' )+ (yield_expr | star_expressions) !'=' [TYPE_COMMENT]
        | single_target augassign ~ (yield_expr | star_expressions)

    augassign:
        | '+='
        | '-='
        | '*='
        | '@='
        | '/='
        | '%='
        | '&='
        | '|='
        | '^='
        | '<<='
        | '>>='
        | '**='
        | '//='

    global_stmt: 'global' ','.NAME+
    nonlocal_stmt: 'nonlocal' ','.NAME+

    yield_stmt: yield_expr

    assert_stmt: 'assert' expression [',' expression ]

    del_stmt:
        | 'del' del_targets &(';' | NEWLINE)

    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_from_targets
        | 'from' ('.' | '...')+ 'import' import_from_targets
    import_from_targets:
        | '(' import_from_as_names [','] ')'
        | import_from_as_names !','
        | '*'
    import_from_as_names:
        | ','.import_from_as_name+
    import_from_as_name:
        | NAME ['as' NAME ]
    dotted_as_names:
        | ','.dotted_as_name+
    dotted_as_name:
        | dotted_name ['as' NAME ]
    dotted_name:
        | dotted_name '.' NAME
        | NAME

    if_stmt:
        | 'if' named_expression ':' block elif_stmt
        | 'if' named_expression ':' block [else_block]
    elif_stmt:
        | 'elif' named_expression ':' block elif_stmt
        | 'elif' named_expression ':' block [else_block]
    else_block: 'else' ':' block

    while_stmt:
        | 'while' named_expression ':' block [else_block]

    for_stmt:
        | 'for' star_targets 'in' ~ star_expressions ':' [TYPE_COMMENT] block [else_block]
        | ASYNC 'for' star_targets 'in' ~ star_expressions ':' [TYPE_COMMENT] block [else_block]

    with_stmt:
        | 'with' '(' ','.with_item+ ','? ')' ':' block
        | 'with' ','.with_item+ ':' [TYPE_COMMENT] block
        | ASYNC 'with' '(' ','.with_item+ ','? ')' ':' block
        | ASYNC 'with' ','.with_item+ ':' [TYPE_COMMENT] block
    with_item:
        | expression 'as' star_target &(',' | ')' | ':')
        | expression

    try_stmt:
        | 'try' ':' block finally_block
        | 'try' ':' block except_block+ [else_block] [finally_block]
    except_block:
        | 'except' expression ['as' NAME ] ':' block
        | 'except' ':' block
    finally_block: 'finally' ':' block

    return_stmt:
        | 'return' [star_expressions]

    raise_stmt:
        | 'raise' expression ['from' expression ]
        | 'raise'

    function_def:
        | decorators function_def_raw
        | function_def_raw

    function_def_raw:
        | 'def' NAME '(' [params] ')' ['->' expression ] ':' [func_type_comment] block
        | ASYNC 'def' NAME '(' [params] ')' ['->' expression ] ':' [func_type_comment] block
    func_type_comment:
        | NEWLINE TYPE_COMMENT &(NEWLINE INDENT)   # Must be followed by indented block
        | TYPE_COMMENT

    params:
        | parameters

    parameters:
        | slash_no_default param_no_default* param_with_default* [star_etc]
        | slash_with_default param_with_default* [star_etc]
        | param_no_default+ param_with_default* [star_etc]
        | param_with_default+ [star_etc]
        | star_etc

    # Some duplication here because we can't write (',' | &')'),
    # which is because we don't support empty alternatives (yet).
    #
    slash_no_default:
        | param_no_default+ '/' ','
        | param_no_default+ '/' &')'
    slash_with_default:
        | param_no_default* param_with_default+ '/' ','
        | param_no_default* param_with_default+ '/' &')'

    star_etc:
        | '*' param_no_default param_maybe_default* [kwds]
        | '*' ',' param_maybe_default+ [kwds]
        | kwds

    kwds: '**' param_no_default

    # One parameter.  This *includes* a following comma and type comment.
    #
    # There are three styles:
    # - No default
    # - With default
    # - Maybe with default
    #
    # There are two alternative forms of each, to deal with type comments:
    # - Ends in a comma followed by an optional type comment
    # - No comma, optional type comment, must be followed by close paren
    # The latter form is for a final parameter without trailing comma.
    #
    param_no_default:
        | param ',' TYPE_COMMENT?
        | param TYPE_COMMENT? &')'
    param_with_default:
        | param default ',' TYPE_COMMENT?
        | param default TYPE_COMMENT? &')'
    param_maybe_default:
        | param default? ',' TYPE_COMMENT?
        | param default? TYPE_COMMENT? &')'
    param: NAME annotation?

    annotation: ':' expression
    default: '=' expression

    decorators: ('@' named_expression NEWLINE )+

    class_def:
        | decorators class_def_raw
        | class_def_raw
    class_def_raw:
        | 'class' NAME ['(' [arguments] ')' ] ':' block

    block:
        | NEWLINE INDENT statements DEDENT
        | simple_stmt

    star_expressions:
        | star_expression (',' star_expression )+ [',']
        | star_expression ','
        | star_expression
    star_expression:
        | '*' bitwise_or
        | expression

    star_named_expressions: ','.star_named_expression+ [',']
    star_named_expression:
        | '*' bitwise_or
        | named_expression
    named_expression:
        | NAME ':=' ~ expression
        | expression !':='

    annotated_rhs: yield_expr | star_expressions

    expressions:
        | expression (',' expression )+ [',']
        | expression ','
        | expression
    expression:
        | disjunction 'if' disjunction 'else' expression
        | disjunction
        | lambdef

    lambdef:
        | 'lambda' [lambda_params] ':' expression

    lambda_params:
        | lambda_parameters

    # lambda_parameters etc. duplicates parameters but without annotations
    # or type comments, and if there's no comma after a parameter, we expect
    # a colon, not a close parenthesis.  (For more, see parameters above.)
    #
    lambda_parameters:
        | lambda_slash_no_default lambda_param_no_default* lambda_param_with_default* [lambda_star_etc]
        | lambda_slash_with_default lambda_param_with_default* [lambda_star_etc]
        | lambda_param_no_default+ lambda_param_with_default* [lambda_star_etc]
        | lambda_param_with_default+ [lambda_star_etc]
        | lambda_star_etc

    lambda_slash_no_default:
        | lambda_param_no_default+ '/' ','
        | lambda_param_no_default+ '/' &':'
    lambda_slash_with_default:
        | lambda_param_no_default* lambda_param_with_default+ '/' ','
        | lambda_param_no_default* lambda_param_with_default+ '/' &':'

    lambda_star_etc:
        | '*' lambda_param_no_default lambda_param_maybe_default* [lambda_kwds]
        | '*' ',' lambda_param_maybe_default+ [lambda_kwds]
        | lambda_kwds

    lambda_kwds: '**' lambda_param_no_default

    lambda_param_no_default:
        | lambda_param ','
        | lambda_param &':'
    lambda_param_with_default:
        | lambda_param default ','
        | lambda_param default &':'
    lambda_param_maybe_default:
        | lambda_param default? ','
        | lambda_param default? &':'
    lambda_param: NAME

    disjunction:
        | conjunction ('or' conjunction )+
        | conjunction
    conjunction:
        | inversion ('and' inversion )+
        | inversion
    inversion:
        | 'not' inversion
        | comparison
    comparison:
        | bitwise_or compare_op_bitwise_or_pair+
        | bitwise_or
    compare_op_bitwise_or_pair:
        | eq_bitwise_or
        | noteq_bitwise_or
        | lte_bitwise_or
        | lt_bitwise_or
        | gte_bitwise_or
        | gt_bitwise_or
        | notin_bitwise_or
        | in_bitwise_or
        | isnot_bitwise_or
        | is_bitwise_or
    eq_bitwise_or: '==' bitwise_or
    noteq_bitwise_or:
        | ('!=' ) bitwise_or
    lte_bitwise_or: '<=' bitwise_or
    lt_bitwise_or: '<' bitwise_or
    gte_bitwise_or: '>=' bitwise_or
    gt_bitwise_or: '>' bitwise_or
    notin_bitwise_or: 'not' 'in' bitwise_or
    in_bitwise_or: 'in' bitwise_or
    isnot_bitwise_or: 'is' 'not' bitwise_or
    is_bitwise_or: 'is' bitwise_or

    bitwise_or:
        | bitwise_or '|' bitwise_xor
        | bitwise_xor
    bitwise_xor:
        | bitwise_xor '^' bitwise_and
        | bitwise_and
    bitwise_and:
        | bitwise_and '&' shift_expr
        | shift_expr
    shift_expr:
        | shift_expr '<<' sum
        | shift_expr '>>' sum
        | sum

    sum:
        | sum '+' term
        | sum '-' term
        | term
    term:
        | term '*' factor
        | term '/' factor
        | term '//' factor
        | term '%' factor
        | term '@' factor
        | factor
    factor:
        | '+' factor
        | '-' factor
        | '~' factor
        | power
    power:
        | await_primary '**' factor
        | await_primary
    await_primary:
        | AWAIT primary
        | primary
    primary:
        | primary '.' NAME
        | primary genexp
        | primary '(' [arguments] ')'
        | primary '[' slices ']'
        | atom

    slices:
        | slice !','
        | ','.slice+ [',']
    slice:
        | [expression] ':' [expression] [':' [expression] ]
        | expression
    atom:
        | NAME
        | 'True'
        | 'False'
        | 'None'
        | '__peg_parser__'
        | strings
        | NUMBER
        | (tuple | group | genexp)
        | (list | listcomp)
        | (dict | set | dictcomp | setcomp)
        | '...'

    strings: STRING+
    list:
        | '[' [star_named_expressions] ']'
    listcomp:
        | '[' named_expression ~ for_if_clauses ']'
    tuple:
        | '(' [star_named_expression ',' [star_named_expressions]  ] ')'
    group:
        | '(' (yield_expr | named_expression) ')'
    genexp:
        | '(' named_expression ~ for_if_clauses ')'
    set: '{' star_named_expressions '}'
    setcomp:
        | '{' named_expression ~ for_if_clauses '}'
    dict:
        | '{' [double_starred_kvpairs] '}'
    dictcomp:
        | '{' kvpair for_if_clauses '}'
    double_starred_kvpairs: ','.double_starred_kvpair+ [',']
    double_starred_kvpair:
        | '**' bitwise_or
        | kvpair
    kvpair: expression ':' expression
    for_if_clauses:
        | for_if_clause+
    for_if_clause:
        | ASYNC 'for' star_targets 'in' ~ disjunction ('if' disjunction )*
        | 'for' star_targets 'in' ~ disjunction ('if' disjunction )*

    yield_expr:
        | 'yield' 'from' expression
        | 'yield' [star_expressions]

    arguments:
        | args [','] &')'
    args:
        | ','.(starred_expression | named_expression !'=')+ [',' kwargs ]
        | kwargs
    kwargs:
        | ','.kwarg_or_starred+ ',' ','.kwarg_or_double_starred+
        | ','.kwarg_or_starred+
        | ','.kwarg_or_double_starred+
    starred_expression:
        | '*' expression
    kwarg_or_starred:
        | NAME '=' expression
        | starred_expression
    kwarg_or_double_starred:
        | NAME '=' expression
        | '**' expression

    # NOTE: star_targets may contain *bitwise_or, targets may not.
    star_targets:
        | star_target !','
        | star_target (',' star_target )* [',']
    star_targets_list_seq: ','.star_target+ [',']
    star_targets_tuple_seq:
        | star_target (',' star_target )+ [',']
        | star_target ','
    star_target:
        | '*' (!'*' star_target)
        | target_with_star_atom
    target_with_star_atom:
        | t_primary '.' NAME !t_lookahead
        | t_primary '[' slices ']' !t_lookahead
        | star_atom
    star_atom:
        | NAME
        | '(' target_with_star_atom ')'
        | '(' [star_targets_tuple_seq] ')'
        | '[' [star_targets_list_seq] ']'

    single_target:
        | single_subscript_attribute_target
        | NAME
        | '(' single_target ')'
    single_subscript_attribute_target:
        | t_primary '.' NAME !t_lookahead
        | t_primary '[' slices ']' !t_lookahead

    del_targets: ','.del_target+ [',']
    del_target:
        | t_primary '.' NAME !t_lookahead
        | t_primary '[' slices ']' !t_lookahead
        | del_t_atom
    del_t_atom:
        | NAME
        | '(' del_target ')'
        | '(' [del_targets] ')'
        | '[' [del_targets] ']'

    t_primary:
        | t_primary '.' NAME &t_lookahead
        | t_primary '[' slices ']' &t_lookahead
        | t_primary genexp &t_lookahead
        | t_primary '(' [arguments] ')' &t_lookahead
        | atom &t_lookahead
    t_lookahead: '(' | '[' | '.'