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parsing f-strings -- opening brace gets duplicated when backslash is followed by an expression #34

@mbdevpl

Description

@mbdevpl

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

changed the title [-]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[/+] on Mar 14, 2017
changed the title [-]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[/+] on Mar 14, 2017
changed the title [-]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[/+] on Mar 14, 2017
gvanrossum

gvanrossum commented on Mar 14, 2017

@gvanrossum
Member

What does the stdlib ast module print for the same inputs?

refi64

refi64 commented on Mar 14, 2017

@refi64

This happens also with built-in ast.

Sounds like you might have hit a CPython bug...

mbdevpl

mbdevpl commented on Mar 14, 2017

@mbdevpl
Author

When parsing using built-in ast, the exact same thing happens:

Expression(body=Str(s='\\{x}'))
Expression(body=JoinedStr(values=[Str(s='\\{'), FormattedValue(value=Name(id='x', ctx=Load()), conversion=-1, format_spec=None)]))

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

gvanrossum commented on Mar 14, 2017

@gvanrossum
Member

Check the PEP, there's a rule against backslashes before curly braces.

ddfisher

ddfisher commented on Mar 14, 2017

@ddfisher
Collaborator

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

mbdevpl commented on Mar 15, 2017

@mbdevpl
Author

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

gvanrossum commented on Mar 15, 2017

@gvanrossum
Member

Where it was deemed a dupe of https://bugs.python.org/issue29104.

added a commit that references this issue on Dec 6, 2021
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          parsing f-strings -- opening brace gets duplicated when backslash is followed by an expression · Issue #34 · python/typed_ast