New submission from Mateusz Bysiek:
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
https://github.com/python/typed_ast/issues/34 but the behaviour of ast is
identical and since developers of typed_ast aim at compatibility with ast, I
bring this issue here.
----------
components: Library (Lib)
messages: 289642
nosy: mbdevpl
priority: normal
severity: normal
status: open
title: parsing f-strings -- opening brace of expression gets duplicated when
preceeded by backslash
type: behavior
versions: Python 3.6
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Python tracker <[email protected]>
<http://bugs.python.org/issue29814>
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