[New-bugs-announce] [issue29814] parsing f-strings -- opening brace of expression gets duplicated when preceeded by backslash

[Mateusz Bysiek]

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 <report at bugs.python.org>
<http://bugs.python.org/issue29814>
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