ImportError: DLL load failed: %1 is not a valid Win32 application

On Windows this means the lxml binary does not match your Python: a 32-bit wheel under 64-bit Python, or a corrupted download. Reinstall cleanly with pip uninstall lxml -y then pip install --only-binary :all: lxml, and make sure the Python bitness matches the wheel pip picked. The import works immediately after a matching wheel lands.

The error

ImportError: DLL load failed: %1 is not a valid Win32 application

Fix it

1. Confirm your Python's bitness

python -c "import struct; print(struct.calcsize('P') * 8)"

Expected: prints 64 on 64-bit Python or 32 on 32-bit Python. Write the number down.

1. Reinstall lxml from a binary wheel only

pip uninstall lxml -y
pip install --only-binary :all: lxml

Expected: pip downloads a wheel whose filename contains win_amd64 for 64-bit Python or win32 for 32-bit Python, matching step 1.

1. Confirm the import

python -c "from lxml import etree; print('ok')"

Expected: prints ok with no DLL error.

When this applies

• the full text is ImportError: DLL load failed: %1 is not a valid Win32 application on from lxml import etree
• you are on Windows and recently switched Python versions or bitness
• the error appeared right after a pip install or upgrade of lxml

When it does NOT apply

• ModuleNotFoundError means lxml is not installed at all
• DLL load failed: The specified module could not be found is a different message (missing VC++ redist or dependency DLLs)
• Linux/macOS Symbol not found errors are the Unix cousin, different fix

Compatibility

lxml 4.x/5.x on Windows, CPython 3.8+. The %1 phrasing is Windows-specific and comes from the OS loader.

Variant phrasings

ImportError: DLL load failed while importing etree: %1 is not a valid Win32 application

Newer Python versions name the module being imported (while importing etree). Same cause, same fix.

Why it happens

lxml ships compiled C extensions, and Windows is strict about binary compatibility. If pip installs a 32-bit (win32) wheel into a 64-bit Python, or a wheel file gets truncated mid-download, the OS loader refuses the DLL and Python reports %1 is not a valid Win32 application. The %1 is the loader's placeholder for the DLL path.

Edge cases

• If you must support 32-bit Python, keep it, but install the win32 wheel deliberately. Do not mix.
• A stale lxml directory left in site-packages by a manual copy can shadow the wheel. Uninstall first (step 2 does this).
• If the error persists after a clean wheel install, check for conflicting libxml2.dll copies on PATH from other software. The wheel's bundled DLLs normally win, but a PATH entry can interfere.