parsing f-strings -- opening brace of expression gets duplicated when preceeded by backslash #74000

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1. mbdevpl commented on Mar 15, 2017

   

   mbdevplmannequin

   on Mar 15, 2017

   MannequinAuthor

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   with Python 3.6.0 and the following script:
   

   #!/usr/bin/env python3.6
   
   import 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))
   

   I get the following 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? I can't believe this is the intended behavior... Or, is it?

   When I escape the backslash once like above, what gets parsed is actually unescaped backslash. So this might just boil down to inconsistency in parsing \{ in normal vs. f-strings.

   I originally discovered this in typed_ast python/typed_ast#34 but the behaviour of ast is identical and since developers of typed_ast aim at compatibility with ast, I bring this issue here.
2. mbdevpl

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Mar 15, 2017
3. serhiy-storchaka commented on Mar 15, 2017

   

   serhiy-storchaka

   on Mar 15, 2017

   Member

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   This is a duplicate of bpo-29104.
4. serhiy-storchaka

   closed this as completedon Mar 15, 2017
5. ezio-melotti

   transferred this issue fromon Apr 10, 2022