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parsing f-strings -- opening brace gets duplicated when backslash is followed by an expression #34
Description
with Python 3.6.0 and typed-ast 1.0.2, I get the following:
#!/usr/bin/env python3.6
import typed_ast.ast3 as ast
code1 = '''"\\{x}"'''
code2 = '''f"\\{x}"'''
tree1 = ast.parse(code1, mode='eval')
print(ast.dump(tree1))
tree2 = ast.parse(code2, mode='eval')
print(ast.dump(tree2))
output:
Expression(body=Str(s='\\{x}'))
Expression(body=JoinedStr(values=[Str(s='\\{'), FormattedValue(value=Name(id='x', ctx=Load()), conversion=-1, format_spec=None)]))
Therefore, the normal string is '\\{x}'.
But the f-string has two parts: '\\{' and an expression Name(id='x', ctx=Load()).
Where does the { in the string part of f-string come from? This happens also with built-in ast. I can't believe this is the intended behavior... Is it?
Activity
[-]parsing f-string -- opening brace gets duplicated when escaped backslash is followed by an expression[/-][+]f-strings -- opening brace gets duplicated when escaped backslash is followed by an expression[/+][-]f-strings -- opening brace gets duplicated when escaped backslash is followed by an expression[/-][+]parsing f-strings -- opening brace gets duplicated when escaped backslash is followed by an expression[/+][-]parsing f-strings -- opening brace gets duplicated when escaped backslash is followed by an expression[/-][+]parsing f-strings -- opening brace gets duplicated when backslash is followed by an expression[/+]gvanrossum commentedon Mar 14, 2017
What does the stdlib ast module print for the same inputs?
refi64 commentedon Mar 14, 2017
Sounds like you might have hit a CPython bug...
mbdevpl commentedon Mar 14, 2017
When parsing using built-in ast, the exact same thing happens:
Also, I just noticed that when I escape the backslash once, what gets parsed is actually unescaped backslash. So this might just boil down to inconsistency in parsing
\{in normal vs. f-strings.gvanrossum commentedon Mar 14, 2017
Check the PEP, there's a rule against backslashes before curly braces.
ddfisher commentedon Mar 14, 2017
It looks like the PEP only disallows backslashes within fstrings. @mbdevpl: this seems like a CPython bug -- could you file it in the Python bug tracker?
typed_ast's is intended to match the implicit spec that is CPython, so until this behavior is fixed upstream, typed_ast is behaving correctly.
mbdevpl commentedon Mar 15, 2017
Sure, thanks for taking a look! I had yet another careful read of the PEP, and I also cannot find anything about backslashes being disallowed outside of expressions. The only mention of
\{seems to be in discussion about possible alternate expression opening/closing delimiters.I reported this issue here: https://bugs.python.org/issue29814
gvanrossum commentedon Mar 15, 2017
Where it was deemed a dupe of https://bugs.python.org/issue29104.
Merge pull request python#34 from python-lz4/development