f string with outer and inner double quotes produce no syntax issue in python 3.13, but it does on previous versions.

[orlando-fbn]

Bug report

Bug description:

Bug Report: f-string Regression in Python 3.13 Parser

Python Versions

• 3.11 – correctly rejects inner/outer matching‐quote f-strings
• 3.13 – silently accepts invalid f-string syntax

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Issue Type

Parser / grammar regression

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Description

In Python 3.11 and earlier, an f-string whose inner quotes match the outer quotes (for example f"…{x.get("inner")}…") raises a SyntaxError. In Python 3.13, however, the same invalid code is accepted at compile time, leading to silent misparsing.

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Steps to Reproduce

1. Create a file broken_fstring.py:

# broken_fstring.py

def foo():
    # This *should* be a SyntaxError:
    x = {}
    yield f"Hello, {x.get("inner")}!"

2. Under Python 3.11:

    python3.11 -m py_compile broken_fstring.py

Expected:

  File "reportlib/report_generator/python_bug.py", line 7
    yield f"Hello, {x.get("inner")}!"
                           ^^^^^
SyntaxError: f-string: unmatched '('

3. Under Python 3.13:

    python3.13 -m py_compile broken_fstring.py

Actual: no error, exit code 0.

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Expected Behavior

The compiler should reject the invalid f-string syntax in all supported versions, consistent with PEP 498.

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Minimal Test Case

# Create the test file
cat > broken_fstring.py << 'EOF'
def foo():
    x = {}
    yield f"Hello, {x.get("inner")}!"
EOF

Under 3.11:

python3.11 -m py_compile broken_fstring.py # raises SyntaxError

Under 3.13:

python3.13 -m py_compile broken_fstring.py # silently succeeds

CPython versions tested on:

3.13

Operating systems tested on:

macOS

[ericvsmith]

In general, PEPs are historical documents. They shouldn't be relied on for the description of Python's behavior. Even current PEPs! Instead, refer to the documentation of the feature in question.

That said, I don't know if the docs have been updated to reflect this change.

[ned-deily]

https://docs.python.org/3/reference/lexical_analysis.html#formatted-string-literals

Changed in version 3.12: Prior to Python 3.12, reuse of the same quoting type of the outer f-string inside a replacement field was not possible.