[issue31263] Assigning to subscript/slice of literal is permitted
New submission from Isaac Elliott: In Python 3.5 and 3.6 (at least), the language reference presents a grammar that disallows assignment to literals. For example, `(a for 1 in [1,2,3])` is a syntax error, as is `(1, a) = (2, 3)`. However the grammar doesn't prevent assignment to subscripted or sliced literals. For example neither `(a for [1,2,3][0] in [1,2,3])` nor `([1,2,3][0], a) = (2, 3)` are considered syntax errors. Similar behavior is exhibited for slices. The problem is that the `target_list` production (https://docs.python.org/3.5/reference/simple_stmts.html#grammar-token-target_list) reuses the `subscription` and `slicing` productions which both use the `primary` production, allowing literals on their left side. -- messages: 300740 nosy: Isaac Elliott priority: normal severity: normal status: open title: Assigning to subscript/slice of literal is permitted type: behavior versions: Python 3.5, Python 3.6 ___ Python tracker <http://bugs.python.org/issue31263> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue31263] Assigning to subscript/slice of literal is permitted
Isaac Elliott added the comment: Yes I would disallow a script such as `a = [0]; [5, a][1][:] = [3]` (note: your example of just `[5, a][1][:] = [3]` does not run, so I assumed it must be used in a situation like this) Evaluating the target of an assignment is unnecessary, we can syntactically determine whether some left hand side can be assigned to: * Identifiers are assignable (`a = 2`) * Attribute accesses are assignable, provided that the left of the dot is assignable (`a.foo = 2`, `a.b.c.foo`, etc) * Subscripts are assignable, provided that the outer expression is assignable (`a[1] = 2`, `a.foo[b] = 2`, `a[1][2][3] = 2`, `a.b[1].c[2] = 2`) * Lists are assignable, provided that all their elements are assignable (`[a,b,c] = [1,2,3]`) * Expression lists/tuples are assignable, provided that all their elements are assignable (`a, b = (1, 2)`, `(a,b,c) = (1,2,3)`) * Unpackings are assignable, provided that their argument is assignable (`*a, = [1,2,3]`, `a, *b = [1,2,3]`) * Slices are assignable, provided that the outer expression is assignable (`a[:] = [1,2,3]`, `a.foo[1:2] = [1]` * Everything else is not assignable (did I forget anything?) This can definitely be encoded as a context-free grammar, although I don't know if it will present conflicts in the parser generator. I do think it's worth it. Python is one of the most widely used programming languages, and it's our responsibility to ensure it behaves correctly. -- ___ Python tracker <http://bugs.python.org/issue31263> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue31263] Assigning to subscript/slice of literal is permitted
Isaac Elliott added the comment: Does backward compatibility take priority over correct behavior? What process is followed when fixing a bug causes a breaking change? -- ___ Python tracker <http://bugs.python.org/issue31263> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33149] Parser stack overflows
New submission from Isaac Elliott : python3's parser stack overflows on deeply-nested expressions, for example: [[]] or aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa(aa() These are both minimal examples, so if you remove one level of nesting from either then python3 will behave normally. -- messages: 314485 nosy: Isaac Elliott priority: normal severity: normal status: open title: Parser stack overflows versions: Python 3.6 ___ Python tracker <https://bugs.python.org/issue33149> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33149] Parser stack overflows
Isaac Elliott added the comment: Because of the way recursive descent parsing works, [[ is actually the minimal input required to reproduce this in python3. In python2, the bug is still present, but requires a slightly deeper nesting: -- ___ Python tracker <https://bugs.python.org/issue33149> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33766] Grammar Incongruence
New submission from Isaac Elliott :
echo 'print("a");print("b")' > test.py
This program is grammatically incorrect according to the specification
(https://docs.python.org/3.8/reference/grammar.html). But Python 3 runs it
without issue.
It's this production here
simple_stmt: small_stmt (';' small_stmt)* [';'] NEWLINE
which says 'simple_stmt's must be terminated by a newline. However, the program
I wrote doesn't contain any newlines.
I think the grammar spec is missing some information, but I'm not quite sure
what. Does anyone have an idea?
--
components: Interpreter Core
messages: 318617
nosy: Isaac Elliott
priority: normal
severity: normal
status: open
title: Grammar Incongruence
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7, Python 3.8
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[issue33766] Grammar Incongruence
Isaac Elliott added the comment: Thanks for the clarification. Is there a reference to this in the documentation? -- ___ Python tracker <https://bugs.python.org/issue33766> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33766] Grammar Incongruence
Isaac Elliott added the comment: I went through that document before I created this issue. I can't find anything which describes this behavior - could you be more specific please? -- ___ Python tracker <https://bugs.python.org/issue33766> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue33766] Grammar Incongruence
Isaac Elliott added the comment: Cool, thanks for the help. Should I submit a PR with the updated documentation? -- ___ Python tracker <https://bugs.python.org/issue33766> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
