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casr-python analyzes Python crash output — including unhandled exceptions and tracebacks — and produces structured .casrep reports. It integrates directly with Atheris, Google’s coverage-guided Python fuzzer built on libFuzzer, making it a natural fit for Python fuzzing pipelines. When the target program produces sanitizer output (AddressSanitizer, MemorySanitizer, or libFuzzer markers) alongside a Python exception, casr-python automatically delegates to casr-san for ASAN-level analysis. Otherwise it extracts the Python Traceback from stderr, parses each File "…", line N, in fn frame, resolves the crash line, and records the source snippet.

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Synopsis

casr-python [OPTIONS] <--stdout|--output <REPORT>> -- <ARGS>...
The -- separator is required. Everything after it is the command CASR will execute and capture output from.

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Options

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-o, --output
path
Path to save the .casrep report file. If a directory is provided, the report filename is generated automatically from the crash context.
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--stdout
flag
Print the CASR report as JSON to stdout instead of writing it to a file. Mutually exclusive with --output; exactly one of the two is required.
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--stdin
path
Path to a file that will be fed to the target program’s standard input. Useful when the fuzz target reads a corpus entry from stdin.
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-t, --timeout
integer
default:"0"
Maximum number of seconds to wait for the target process to finish. A value of 0 disables the timeout entirely.
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--ignore
path
Path to a file containing regular expressions (one per section) for function names and file paths that should be excluded from stack trace analysis. See the ignore-file format below.
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--strip-path
string
A path prefix that will be removed from every file path in the stack trace and crash line. Keeps reports portable when shared across machines. Also settable via the CASR_STRIP_PATH environment variable.
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--ld-preload
string
One or more shared library paths to inject via LD_PRELOAD into the target process only. Both space ( ) and colon (:) are accepted as delimiters. CASR’s own process is not affected. Also settable via CASR_PRELOAD.

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Ignore-file format

FUNCTIONS
^atheris\.
^_?fuzz

FILES
/usr/lib/python
site-packages/
Headers FUNCTIONS and FILES may appear in either order; either section may be omitted. Frames whose function name or source file matches any listed regex are skipped during analysis.

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Example

Run a plain Python script that raises an unhandled exception: 
casr-python -o python.casrep -- casr/tests/casr_tests/python/test_casr_python.py
Print the report to stdout (useful for piping into jq or other tools): 
casr-python --stdout -- casr/tests/casr_tests/python/test_casr_python.py
Pass a corpus file on stdin and limit execution to 10 seconds: 
casr-python -o python.casrep -t 10 --stdin crash_input -- python3 fuzz_target.py

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Fuzzer integration

casr-python handles single-crash analysis. For bulk triage of Atheris crash directories, use casr-libfuzzer — it auto-detects Python targets and invokes casr-python for each input file. 
# Triage a directory of Atheris crashes
casr-libfuzzer \
  -i casr/tests/casr_tests/casrep/atheris_crashes_ruamel_yaml \
  -o out/ \
  -- casr/tests/casr_tests/python/yaml_fuzzer.py
When Atheris runs with AddressSanitizer enabled, crash output contains both a Python traceback and ASAN diagnostics. casr-python detects this automatically and forwards to casr-san so the full ASAN severity analysis is applied.
If your Atheris fuzz target wraps the fuzzed library in a try/except block that re-raises, ensure the final exception propagates unhandled — casr-python looks for the Traceback (most recent call last): marker in stderr to locate the report.

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