[issue42284] The grammar specification is inconsistent with the implementation of Python parser.
New submission from Xinmeng Xia <xiaxm@smail.nju.edu.cn>: In full grammar specification of Python 3.6 official documentation (Python 3.6 official documentation: https://docs.python.org/3.6/reference/grammar.html ), we can find a very clear definition on the grammar about the usage of 'break'. According to the definition, we can find the following derivation, which indicates the keyword 'break' can appear in the block of if statement without being nested into a loop block: %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% # Start symbols for the grammar: # single_input is a single interactive statement; single_input: NEWLINE | simple_stmt | compound_stmt NEWLINE compound_stmt: if_stmt | while_stmt | for_stmt | try_stmt | with_stmt | funcdef | classdef | decorated | async_stmt if_stmt: 'if' test ':' suite ('elif' test ':' suite)* ['else' ':' suite] suite: simple_stmt | NEWLINE INDENT stmt+ DEDENT simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE small_stmt: (expr_stmt | del_stmt | pass_stmt | flow_stmt | import_stmt | global_stmt | nonlocal_stmt | assert_stmt) flow_stmt: break_stmt | continue_stmt | return_stmt | raise_stmt | yield_stmt break_stmt: 'break' %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% However, the implementation of the Python parser requires the 'break' can only be embedded into a loop statement. See the following example: Example A(without loop):
compile("if True:break",'','exec') Traceback (most recent call last): File "<stdin>", line 1, in <module> File "", line 1 SyntaxError: 'break' outside loop
Example B(with a loop):
compile("while True:\n\tif True:break",'','exec') <code object <module> at 0x7f5f4de90b70, file "", line 1>
Similar problems exist between if-statement and keywords: 'continue', 'yield', 'return', 'nonlocal' in Python 3.6 and later versions. ---------- assignee: docs@python components: Documentation messages: 380502 nosy: docs@python, xxm priority: normal severity: normal status: open title: The grammar specification is inconsistent with the implementation of Python parser. type: compile error versions: Python 3.6 _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue42284> _______________________________________
Georg Brandl <georg@python.org> added the comment: This grammar specification doesn't contain a full specification of code that won't raise SyntaxError. There are several conditions that aren't checked by the generated parser, but at a later stage in the compilation process. While probably possible to express in general, this would make the grammar much more complex. For this example, it would require different definitions of `suite`, `stmt`, `simple_stmt`, `compound_stmt` and so on, to track where control-flow statements are allowed. Other definitions need to track `nonlocal` and you'd get a combinatorial explosion of productions. You could propose a PR to add a note somewhere on that page (but on the master branch, not 3.6 which is unmaintained). ---------- nosy: +georg.brandl _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue42284> _______________________________________
Raymond Hettinger <raymond.hettinger@gmail.com> added the comment: I concur with Georg. In writing compilers, it is often convenient to allow the grammar to be permissive and save the enforcement of additional syntax restrictions for the downstream semantic analysis phase. For example, the C language grammar specifies the "break" statement in much the same way as Python does: http://www.quut.com/c/ANSI-C-grammar-y.html Thank you for the suggestion, but I'm going to mark this as declined. ---------- nosy: +rhettinger resolution: -> not a bug stage: -> resolved status: open -> closed _______________________________________ Python tracker <report@bugs.python.org> <https://bugs.python.org/issue42284> _______________________________________
participants (3)
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Georg Brandl -
Raymond Hettinger
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Xinmeng Xia