# 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). See PEP 617 for more details on the grammar’s syntax.

    # PEG grammar for Python

    # ========================= START OF THE GRAMMAR =========================

    # General grammatical elements and rules:
    #
    # * Strings with double quotes (") denote SOFT KEYWORDS
    # * Strings with single quotes (') denote KEYWORDS
    # * Upper case names (NAME) denote tokens in the Grammar/Tokens file
    # * Rule names starting with "invalid_" are used for specialized syntax errors
    #     - These rules are NOT used in the first pass of the parser.
    #     - Only if the first pass fails to parse, a second pass including the invalid
    #       rules will be executed.
    #     - If the parser fails in the second phase with a generic syntax error, the
    #       location of the generic failure of the first pass will be used (this avoids
    #       reporting incorrect locations due to the invalid rules).
    #     - The order of the alternatives involving invalid rules matter
    #       (like any rule in PEG).
    #
    # Grammar Syntax (see PEP 617 for more information):
    #
    # rule_name: expression
    #   Optionally, a type can be included right after the rule name, which
    #   specifies the return type of the C or Python function corresponding to the
    #   rule:
    # rule_name[return_type]: expression
    #   If the return type is omitted, then a void * is returned in C and an Any in
    #   Python.
    # e1 e2
    #   Match e1, then match e2.
    # e1 | e2
    #   Match e1 or e2.
    #   The first alternative can also appear on the line after the rule name for
    #   formatting purposes. In that case, a | must be used before the first
    #   alternative, like so:
    #       rule_name[return_type]:
    #            | first_alt
    #            | second_alt
    # ( e )
    #   Match e (allows also to use other operators in the group like '(e)*')
    # [ e ] or e?
    #   Optionally match e.
    # e*
    #   Match zero or more occurrences of e.
    # e+
    #   Match one or more occurrences of e.
    # s.e+
    #   Match one or more occurrences of e, separated by s. The generated parse tree
    #   does not include the separator. This is otherwise identical to (e (s e)*).
    # &e
    #   Succeed if e can be parsed, without consuming any input.
    # !e
    #   Fail if e can be parsed, without consuming any input.
    # ~
    #   Commit to the current alternative, even if it fails to parse.
    #

    # STARTING RULES
    # ==============

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

    # GENERAL STATEMENTS
    # ==================

    statements: statement+

    statement: compound_stmt  | simple_stmts

    statement_newline:
        | compound_stmt NEWLINE
        | simple_stmts
        | NEWLINE
        | ENDMARKER

    simple_stmts:
        | simple_stmt !';' NEWLINE  # Not needed, there for speedup
        | ';'.simple_stmt+ [';'] NEWLINE

    # NOTE: assignment MUST precede expression, else parsing a simple assignment
    # will throw a SyntaxError.
    simple_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
        | match_stmt

    # SIMPLE STATEMENTS
    # =================

    # 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)

    annotated_rhs: yield_expr | star_expressions

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

    return_stmt:
        | 'return' [star_expressions]

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

    global_stmt: 'global' ','.NAME+

    nonlocal_stmt: 'nonlocal' ','.NAME+

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

    yield_stmt: yield_expr

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

    import_stmt: import_name | import_from

    # Import statements
    # -----------------

    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

    # COMPOUND STATEMENTS
    # ===================

    # Common elements
    # ---------------

    block:
        | NEWLINE INDENT statements DEDENT
        | simple_stmts

    decorators: ('@' named_expression NEWLINE )+

    # Class definitions
    # -----------------

    class_def:
        | decorators class_def_raw
        | class_def_raw

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

    # Function definitions
    # --------------------

    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

    # Function parameters
    # -------------------

    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_no_default_star_annotation 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_no_default_star_annotation:
        | param_star_annotation ',' TYPE_COMMENT?
        | param_star_annotation 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?
    param_star_annotation: NAME star_annotation
    annotation: ':' expression
    star_annotation: ':' star_expression
    default: '=' expression  | invalid_default

    # If statement
    # ------------

    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 statement
    # ---------------

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

    # For statement
    # -------------

    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 statement
    # --------------

    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 statement
    # -------------

    try_stmt:
        | 'try' ':' block finally_block
        | 'try' ':' block except_block+ [else_block] [finally_block]
        | 'try' ':' block except_star_block+ [else_block] [finally_block]

    # Except statement
    # ----------------

    except_block:
        | 'except' expression ['as' NAME ] ':' block
        | 'except' ':' block
    except_star_block:
        | 'except' '*' expression ['as' NAME ] ':' block
    finally_block:
        | 'finally' ':' block

    # Match statement
    # ---------------

    match_stmt:
        | "match" subject_expr ':' NEWLINE INDENT case_block+ DEDENT

    subject_expr:
        | star_named_expression ',' star_named_expressions?
        | named_expression

    case_block:
        | "case" patterns guard? ':' block

    guard: 'if' named_expression

    patterns:
        | open_sequence_pattern
        | pattern

    pattern:
        | as_pattern
        | or_pattern

    as_pattern:
        | or_pattern 'as' pattern_capture_target

    or_pattern:
        | '|'.closed_pattern+

    closed_pattern:
        | literal_pattern
        | capture_pattern
        | wildcard_pattern
        | value_pattern
        | group_pattern
        | sequence_pattern
        | mapping_pattern
        | class_pattern

    # Literal patterns are used for equality and identity constraints
    literal_pattern:
        | signed_number !('+' | '-')
        | complex_number
        | strings
        | 'None'
        | 'True'
        | 'False'

    # Literal expressions are used to restrict permitted mapping pattern keys
    literal_expr:
        | signed_number !('+' | '-')
        | complex_number
        | strings
        | 'None'
        | 'True'
        | 'False'

    complex_number:
        | signed_real_number '+' imaginary_number
        | signed_real_number '-' imaginary_number

    signed_number:
        | NUMBER
        | '-' NUMBER

    signed_real_number:
        | real_number
        | '-' real_number

    real_number:
        | NUMBER

    imaginary_number:
        | NUMBER

    capture_pattern:
        | pattern_capture_target

    pattern_capture_target:
        | !"_" NAME !('.' | '(' | '=')

    wildcard_pattern:
        | "_"

    value_pattern:
        | attr !('.' | '(' | '=')

    attr:
        | name_or_attr '.' NAME

    name_or_attr:
        | attr
        | NAME

    group_pattern:
        | '(' pattern ')'

    sequence_pattern:
        | '[' maybe_sequence_pattern? ']'
        | '(' open_sequence_pattern? ')'

    open_sequence_pattern:
        | maybe_star_pattern ',' maybe_sequence_pattern?

    maybe_sequence_pattern:
        | ','.maybe_star_pattern+ ','?

    maybe_star_pattern:
        | star_pattern
        | pattern

    star_pattern:
        | '*' pattern_capture_target
        | '*' wildcard_pattern

    mapping_pattern:
        | '{' '}'
        | '{' double_star_pattern ','? '}'
        | '{' items_pattern ',' double_star_pattern ','? '}'
        | '{' items_pattern ','? '}'

    items_pattern:
        | ','.key_value_pattern+

    key_value_pattern:
        | (literal_expr | attr) ':' pattern

    double_star_pattern:
        | '**' pattern_capture_target

    class_pattern:
        | name_or_attr '(' ')'
        | name_or_attr '(' positional_patterns ','? ')'
        | name_or_attr '(' keyword_patterns ','? ')'
        | name_or_attr '(' positional_patterns ',' keyword_patterns ','? ')'

    positional_patterns:
        | ','.pattern+

    keyword_patterns:
        | ','.keyword_pattern+

    keyword_pattern:
        | NAME '=' pattern

    # EXPRESSIONS
    # -----------

    expressions:
        | expression (',' expression )+ [',']
        | expression ','
        | expression

    expression:
        | disjunction 'if' disjunction 'else' expression
        | disjunction
        | lambdef

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

    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

    assignment_expression:
        | NAME ':=' ~ expression

    named_expression:
        | assignment_expression
        | expression !':='

    disjunction:
        | conjunction ('or' conjunction )+
        | conjunction

    conjunction:
        | inversion ('and' inversion )+
        | inversion

    inversion:
        | 'not' inversion
        | comparison

    # Comparison operators
    # --------------------

    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 operators
    # -----------------

    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

    # Arithmetic operators
    # --------------------

    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

    # Primary elements
    # ----------------

    # Primary elements are things like "obj.something.something", "obj[something]", "obj(something)", "obj" ...

    await_primary:
        | AWAIT primary
        | primary

    primary:
        | primary '.' NAME
        | primary genexp
        | primary '(' [arguments] ')'
        | primary '[' slices ']'
        | atom

    slices:
        | slice !','
        | ','.(slice | starred_expression)+ [',']

    slice:
        | [expression] ':' [expression] [':' [expression] ]
        | named_expression

    atom:
        | NAME
        | 'True'
        | 'False'
        | 'None'
        | strings
        | NUMBER
        | (tuple | group | genexp)
        | (list | listcomp)
        | (dict | set | dictcomp | setcomp)
        | '...'

    group:
        | '(' (yield_expr | named_expression) ')'

    # Lambda functions
    # ----------------

    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

    # LITERALS
    # ========

    strings: STRING+

    list:
        | '[' [star_named_expressions] ']'

    tuple:
        | '(' [star_named_expression ',' [star_named_expressions]  ] ')'

    set: '{' star_named_expressions '}'

    # Dicts
    # -----

    dict:
        | '{' [double_starred_kvpairs] '}'

    double_starred_kvpairs: ','.double_starred_kvpair+ [',']

    double_starred_kvpair:
        | '**' bitwise_or
        | kvpair

    kvpair: expression ':' expression

    # Comprehensions & Generators
    # ---------------------------

    for_if_clauses:
        | for_if_clause+

    for_if_clause:
        | ASYNC 'for' star_targets 'in' ~ disjunction ('if' disjunction )*
        | 'for' star_targets 'in' ~ disjunction ('if' disjunction )*

    listcomp:
        | '[' named_expression for_if_clauses ']'

    setcomp:
        | '{' named_expression for_if_clauses '}'

    genexp:
        | '(' ( assignment_expression | expression !':=') for_if_clauses ')'

    dictcomp:
        | '{' kvpair for_if_clauses '}'

    # FUNCTION CALL ARGUMENTS
    # =======================

    arguments:
        | args [','] &')'

    args:
        | ','.(starred_expression | ( assignment_expression | 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

    # ASSIGNMENT TARGETS
    # ==================

    # Generic targets
    # ---------------

    # 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

    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: '(' | '[' | '.'

    # Targets for del statements
    # --------------------------

    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] ']'

    # TYPING ELEMENTS
    # ---------------

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

    func_type_comment:
        | NEWLINE TYPE_COMMENT &(NEWLINE INDENT)   # Must be followed by indented block
        | TYPE_COMMENT

    # ========================= END OF THE GRAMMAR ===========================

    # ========================= START OF INVALID RULES =======================