{"record":{"id":"6c2dca949f5b2c64","repo":"apache/beam","slug":"cython-not-found-cython-extensions-will-not-be-generated-to","errorCode":null,"errorMessage":"Cython not found, cython extensions will not be generated. To use cythonized extensions, pip install cython and run python setup.py build_ext --inplace","messagePattern":"Cython not found, cython extensions will not be generated\\. To use cythonized extensions, pip install cython and run python setup\\.py build_ext --inplace","errorType":"console","errorClass":null,"httpStatus":null,"severity":"warning","filePath":"sdks/python/setup.py","lineNumber":141,"sourceCode":"            _CYTHON_VERSION, REQUIRED_CYTHON_VERSION\n        )\n    )\nexcept PackageNotFoundError:\n  # do nothing if Cython is not installed\n  pass\n\ntry:\n  # pylint: disable=wrong-import-position\n  from Cython.Build import cythonize as cythonize0\n\n  def cythonize(*args, **kwargs):\n    import numpy\n    extensions = cythonize0(*args, **kwargs)\n    for e in extensions:\n      e.include_dirs.append(numpy.get_include())\n    return extensions\nexcept ImportError:\n  warnings.warn(\n      \"Cython not found, cython extensions will not be generated. \" \\\n      \"To use cythonized extensions, pip install cython and run \" \\\n      \"python setup.py build_ext --inplace\"\n  )\n  cythonize = lambda *args, **kwargs: []\n\n# [BEAM-8181] pyarrow cannot be installed on 32-bit Windows platforms.\nif sys.platform == 'win32' and sys.maxsize <= 2**32:\n  pyarrow_dependency = ['']\nelse:\n  pyarrow_dependency = [\n    # Generally try to cover versions released in the last two years.\n    # Update python/sdks/tox.ini to cover the same pyarrow versions\n    # when updating the bounds here.\n    'pyarrow>=14.0.1,<26.0.0',\n  ]\n\n# Exclude pandas<=1.4.2 since it doesn't work with numpy 1.24.x.","sourceCodeStart":123,"sourceCodeEnd":159,"githubUrl":"https://github.com/apache/beam/blob/12126d8942aaf848030c478b4c6a28c6af861c66/sdks/python/setup.py#L123-L159","documentation":"When the Cython package is not importable, Beam's setup.py catches ImportError and warns that cythonized extensions will not be built. The build falls back to cythonize returning an empty list, so the install succeeds but without optimized native extensions (e.g. Cython-based coders/transforms run in slower pure-Python mode).","triggerScenarios":"Running 'python setup.py build_ext --inplace' or installing from source without the cython package installed in the build environment.","commonSituations":"Fresh virtualenv without dev dependencies; building Beam wheels in CI images that omit build extras; users installing from a git checkout rather than a wheel.","solutions":["Install cython: pip install cython, then run python setup.py build_ext --inplace.","Install numpy as well (the cythonize wrapper needs it for include dirs): pip install cython numpy.","Prefer installing an official apache-beam wheel (pip install apache-beam) which ships pre-built extensions and skips this path entirely."],"exampleFix":"// before\n$ python setup.py build_ext --inplace   # cython missing\n// after\n$ pip install cython numpy\n$ python setup.py build_ext --inplace","handlingStrategy":"fallback","validationCode":"import importlib.util\nif importlib.util.find_spec('cython') is None or importlib.util.find_spec('numpy') is None:\n    print('Warning: extensions will be skipped; pip install cython numpy')","typeGuard":null,"tryCatchPattern":"try:\n    run_build()\nexcept SystemExit:\n    print('Falling back to pure-Python install (slower): pip install apache-beam')","preventionTips":["Install build extras: pip install cython numpy before source builds","Prefer official apache-beam wheels with prebuilt extensions","Add a build-time assertion for cython in CI"],"tags":["python","cython","build","missing-dependency"],"backgroundTag":"missing-optional-dependency","analyzedSha":"12126d8942aaf848030c478b4c6a28c6af861c66","analyzedAt":"2026-09-13T01:50:10.254Z","contentChangedAt":"2026-09-13T01:50:10.254Z","schemaVersion":2},"datasetVersion":"2026-09-20T03:17:13.778Z"}