# 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 used here is the same as in the preceding docs, and is described in the notation section, except for an extra complication:

- `~` (”cut”): commit to the current alternative; fail the rule if the alternative fails to parse

  Python mainly uses cuts for optimizations or improved error messages. They often appear to be useless in the listing below.

  Cuts currently don’t appear inside parentheses, brackets, lookaheads and similar. Their behavior in these contexts is deliberately left unspecified.

    # 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.
    # &&e
    #   Eager parse e. The parser will not backtrack and will immediately
    #   fail with SyntaxError if e cannot be parsed.
    #

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

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

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

    statements: statement+

    statement:
        | compound_stmt
        | simple_stmts

    single_compound_stmt:
        | compound_stmt

    statement_newline:
        | single_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
        | type_alias
        | star_expressions
        | return_stmt
        | import_stmt
        | raise_stmt
        | pass_stmt
        | del_stmt
        | yield_stmt
        | assert_stmt
        | break_stmt
        | continue_stmt
        | 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 '=' )+ annotated_rhs !'=' [TYPE_COMMENT]
        | single_target augassign ~ annotated_rhs

    annotated_rhs: yield_expr | star_expressions

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

    return_stmt:
        | 'return' [star_expressions]

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

    pass_stmt:
        | 'pass'

    break_stmt:
        | 'break'

    continue_stmt:
        | 'continue'

    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 [type_params] ['(' [arguments] ')' ] ':' block

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

    function_def:
        | decorators function_def_raw
        | function_def_raw

    function_def_raw:
        | 'def' NAME [type_params] '(' [params] ')' ['->' expression ] ':' [func_type_comment] block
        | 'async' 'def' NAME [type_params] '(' [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+ ','? ')' ':' [TYPE_COMMENT] 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 ':' block
        | 'except' expression 'as' NAME ':' block
        | 'except' expressions ':' block
        | 'except' ':' block
    except_star_block:
        | 'except' '*' expression ':' block
        | 'except' '*' expression 'as' NAME ':' block
        | 'except' '*' expressions ':' 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

    # Type statement
    # ---------------

    type_alias:
        | "type" NAME [type_params] '=' expression

    # Type parameter declaration
    # --------------------------

    type_params:
        | '[' type_param_seq ']'

    type_param_seq: ','.type_param+ [',']

    type_param:
        | NAME [type_param_bound] [type_param_default]
        | '*' NAME [type_param_starred_default]
        | '**' NAME [type_param_default]

    type_param_bound: ':' expression
    type_param_default: '=' expression
    type_param_starred_default: '=' star_expression

    # 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
    # ========

    fstring_middle:
        | fstring_replacement_field
        | FSTRING_MIDDLE
    fstring_replacement_field:
        | '{' annotated_rhs '='? [fstring_conversion] [fstring_full_format_spec] '}'
    fstring_conversion:
        | "!" NAME
    fstring_full_format_spec:
        | ':' fstring_format_spec*
    fstring_format_spec:
        | FSTRING_MIDDLE
        | fstring_replacement_field
    fstring:
        | FSTRING_START fstring_middle* FSTRING_END

    tstring_format_spec_replacement_field:
        | '{' annotated_rhs '='? [fstring_conversion] [tstring_full_format_spec] '}'
    tstring_format_spec:
        | TSTRING_MIDDLE
        | tstring_format_spec_replacement_field
    tstring_full_format_spec:
        | ':' tstring_format_spec*
    tstring_replacement_field:
        | '{' annotated_rhs '='? [fstring_conversion] [tstring_full_format_spec] '}'
    tstring_middle:
        | tstring_replacement_field
        | TSTRING_MIDDLE
    tstring:
        | TSTRING_START tstring_middle* TSTRING_END

    string: STRING
    strings:
        | (fstring|string)+
        | tstring+

    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 =======================