9. Full Grammar specification — Python 2.7.18 documentation

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# 9. 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:  Changing the grammar specified in this file will most likely
    #        require corresponding changes in the parser module
    #        (../Modules/parsermodule.c).  If you can't make the changes to
    #        that module yourself, please co-ordinate the required changes
    #        with someone who can; ask around on python-dev for help.  Fred
    #        Drake <fdrake@acm.org> will probably be listening there.

    # NOTE WELL: You should also follow all the steps listed in PEP 306,
    # "How to Change Python's 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() and input() 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)
    funcdef: 'def' NAME parameters ':' suite
    parameters: '(' [varargslist] ')'
    varargslist: ((fpdef ['=' test] ',')*
                  ('*' NAME [',' '**' NAME] | '**' NAME) |
                  fpdef ['=' test] (',' fpdef ['=' test])* [','])
    fpdef: NAME | '(' fplist ')'
    fplist: fpdef (',' fpdef)* [',']

    stmt: simple_stmt | compound_stmt
    simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
    small_stmt: (expr_stmt | print_stmt  | del_stmt | pass_stmt | flow_stmt |
                 import_stmt | global_stmt | exec_stmt | assert_stmt)
    expr_stmt: testlist (augassign (yield_expr|testlist) |
                         ('=' (yield_expr|testlist))*)
    augassign: ('+=' | '-=' | '*=' | '/=' | '%=' | '&=' | '|=' | '^=' |
                '<<=' | '>>=' | '**=' | '//=')
    # For normal assignments, additional restrictions enforced by the interpreter
    print_stmt: 'print' ( [ test (',' test)* [','] ] |
                          '>>' test [ (',' test)+ [','] ] )
    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 [',' test [',' test]]]
    import_stmt: import_name | import_from
    import_name: 'import' dotted_as_names
    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)*
    exec_stmt: 'exec' expr ['in' test [',' test]]
    assert_stmt: 'assert' test [',' test]

    compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated
    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' | ',') test]]
    suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT

    # Backward compatibility cruft to support:
    # [ x for x in lambda: True, lambda: False if x() ]
    # even while also allowing:
    # lambda x: 5 if x else 2
    # (But not a mix of the two)
    testlist_safe: old_test [(',' old_test)+ [',']]
    old_test: or_test | old_lambdef
    old_lambdef: 'lambda' [varargslist] ':' old_test

    test: or_test ['if' or_test 'else' test] | lambdef
    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)*
    comp_op: '<'|'>'|'=='|'>='|'<='|'<>'|'!='|'in'|'not' 'in'|'is'|'is' 'not'
    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 trailer* ['**' factor]
    atom: ('(' [yield_expr|testlist_comp] ')' |
           '[' [listmaker] ']' |
           '{' [dictorsetmaker] '}' |
           '`' testlist1 '`' |
           NAME | NUMBER | STRING+)
    listmaker: test ( list_for | (',' test)* [','] )
    testlist_comp: test ( comp_for | (',' test)* [','] )
    lambdef: 'lambda' [varargslist] ':' test
    trailer: '(' [arglist] ')' | '[' subscriptlist ']' | '.' NAME
    subscriptlist: subscript (',' subscript)* [',']
    subscript: '.' '.' '.' | test | [test] ':' [test] [sliceop]
    sliceop: ':' [test]
    exprlist: expr (',' expr)* [',']
    testlist: test (',' test)* [',']
    dictorsetmaker: ( (test ':' test (comp_for | (',' test ':' test)* [','])) |
                      (test (comp_for | (',' test)* [','])) )

    classdef: 'class' NAME ['(' [testlist] ')'] ':' suite

    arglist: (argument ',')* (argument [',']
                             |'*' test (',' argument)* [',' '**' test]
                             |'**' test)
    # 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.
    argument: test [comp_for] | test '=' test

    list_iter: list_for | list_if
    list_for: 'for' exprlist 'in' testlist_safe [list_iter]
    list_if: 'if' old_test [list_iter]

    comp_iter: comp_for | comp_if
    comp_for: 'for' exprlist 'in' or_test [comp_iter]
    comp_if: 'if' old_test [comp_iter]

    testlist1: test (',' test)*

    # not used in grammar, but may appear in "node" passed from Parser to Compiler
    encoding_decl: NAME

    yield_expr: 'yield' [testlist]