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