Fix: prevent PositionalArgumentsFormatter from crashing on TypeError (#258) - #832
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When PositionalArgumentsFormatter encounters a mismatch between the
event string and the positional args (e.g. 'Info message' with args=('x',)),
it raises an uncaught TypeError, crashing the application.
This behavior differs from the Python stdlib logging module, which
gracefully catches formatting errors to prevent logging from taking
down the application.
This patch wraps the formatting operation in a try/except block
catching TypeError and ValueError. If formatting fails, the event
string is left unformatted and the application continues running.
Added a regression test to ensure this behavior is maintained.
Closes hynek#258
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Fixes #258.
Problem
When using
structlogwithPositionalArgumentsFormatter, passing a positional argument that does not match a formatting placeholder in the event string (e.g.,log.warning('Info message', 'x')) raises an uncaughtTypeError: not all arguments converted during string formatting. This crashes the entire application.Root Cause
In
src/structlog/stdlib.py, the formatter directly executesevent_dict["event"] %= argswithout catching formatting exceptions. This differs from the Python standard libraryloggingmodule, which gracefully catchesTypeErrorandValueErrorduring formatting to ensure logging never crashes the host application.Solution
This PR wraps the
%=formatting operation in atry...except (TypeError, ValueError)block. If formatting fails due to a user error (mismatched placeholders), the exception is caught and the event string is simply left unformatted. This mimics the fail-safe behavior of stdlib logging and ensures that bad log statements do not result in fatal runtime errors.A regression test has been added to
tests/test_stdlib.pyto verify that the processor gracefully handles mismatched placeholders without raising an exception.