ModuleNotFoundError: No module named 'fastparquet.cencoding'

TL;DR: fastparquet's compiled speed-up module did not build during install. Reinstall fastparquet with a working C compiler (or grab a prebuilt wheel) and the import works.

The error

ModuleNotFoundError: No module named 'fastparquet.cencoding'

Fix it

1. Reinstall fastparquet, letting pip pick a prebuilt wheel:

   pip install --force-reinstall fastparquet

   Expected: installs a wheel that already contains cencoding.

2. If pip builds from source instead, install a C compiler first (Debian/Ubuntu: sudo apt install build-essential; then rerun the reinstall). Expected: the build compiles cencoding.

3. Verify:

   python -c "import fastparquet; print(fastparquet.__version__)"

   Expected: prints the version, no error.

When this applies

Use this when import fastparquet raises the cencoding error, typical on platforms without a matching wheel.

When it does not apply

If the error is plain No module named 'fastparquet', just install it. If cencoding imports but reads are wrong, that is a data issue, not this one.

Compatibility

fastparquet installs that compile the optional C extension; wheels for major platforms already include it.

Root cause

fastparquet ships an optional Cython extension (cencoding) for speed. Source installs without a compiler silently skip it, and the import of the extension module then fails.

Edge cases

fastparquet works in pure-python mode without cencoding, just slower; the import error only bites because the package tries the extension first. Pinning fastparquet in CI avoids surprise source builds.

GitHub issue dask/fastparquet#977 reports ModuleNotFoundError: No module named 'fastparquet.cencoding' when importing fastparquet, because the C speed-up extension failed to compile during install. The verified fix: reinstalling fastparquet with a working C compiler toolchain (or installing a prebuilt wheel that already contains cencoding) so the extension builds, after which the import succeeds. Full thread: https://github.com/dask/fastparquet/issues/977