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Parsing the (invalid) expression `f"{\t"i}"` caused a panic because the
`TStringMiddle` character was "unreachable" due the way the parser
recovered from the line continuation (it ate the t-string start).
The cause of the issue is as follows:
The parser begins parsing the f-string and expects to see a list of
objects, essentially alternating between _interpolated elements_ and
ordinary strings. It is happy to see the first left brace, but then
there is a lexical error caused by the line-continuation character. So
instead of the parser seeing a list of elements with just one member, it
sees a list that starts like this:
- Interpolated element with an invalid token, stored as a `Name`
- Something else built from tokens beginning with `TStringStart` and
`TStringMiddle`
When it sees the `TStringStart` error recovery says "that's a list
element I don't know what to do with, let's skip it". When it sees
`TStringMiddle` it says "oh, that looks like the middle of _some
interpolated string_ so let's try to parse it as one of the literal
elements of my `FString`". Unfortunately, the function being used to
parse individual list elements thinks (arguably correctly) that it's not
possible to have a `TStringMiddle` sitting in your `FString`, and hits
`unreachable`.
Two potential ways (among many) to solve this issue are:
1. Allow a `TStringMiddle` as a valid "literal" part of an f-string
during parsing (with the hope/understanding that this would only occur
in an invalid context)
2. Skip the `TStringMiddle` as an "unexpected/invalid list item" in the
same way that we skipped `TStringStart`.
I have opted for the second approach since it seems somehow more morally
correct, even though it loses more information. To implement this, the
recovery context needs to know whether we are in an f-string or t-string
- hence the changes to that enum. As a bonus we get slightly more
specific error messages in some cases.
Closes #18860
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Ruff Python Parser
Ruff's Python parser is a hand-written recursive descent parser which can parse Python source code into an Abstract Syntax Tree (AST). It also utilizes the Pratt parsing technique to parse expressions with different precedence.
Try out the parser in the playground.
Python version support
The parser supports the latest Python syntax, which is currently Python 3.12.
It does not throw syntax errors if it encounters a syntax feature that is not
supported by the target-version.
This will be fixed in a future release (see https://github.com/astral-sh/ruff/issues/6591).
Contributing
Refer to the contributing guidelines to get started and GitHub issues with the parser label for issues that need help.