Build and Run

Python Version#

Fable targets Python 3.12 or higher.

Python 3.10 and 3.11 are deprecated.

Installing fable-library#

Fable Python requires the fable-library package to be installed from PyPI. This package contains the core runtime library that Fable-generated Python code depends on.

Version Pinning#

It is important to pin fable-library to a version compatible with your Fable compiler version. If you install an incompatible version, your generated code may not work correctly.

For stable releases (e.g., Fable 5.x):

pip install "fable-library>=5.0.0,<6.0.0"

or with Poetry (in pyproject.toml):

fable-library = ">=5.0.0,<6.0.0"

or with uv:

uv add "fable-library>=5.0.0,<6.0.0"

For alpha/beta releases: Pin to the exact version matching your Fable compiler. Note that PyPI uses a different naming convention (e.g., 5.0.0-alpha.17 becomes 5.0.0a17):

pip install "fable-library==5.0.0a17"

Running Python Code#

When targeting Python, you can use the output of Fable directly by running it with the Python interpreter.

For example:

python3 Program.py

Custom fable-library Path#

If you need to use a custom version of fable-library (e.g., for development or testing), you can use the --fableLib option:

dotnet fable --lang python --fableLib /path/to/custom/fable-library

Publishing to PyPI#

When publishing a library to PyPI that uses Fable Python, your package should declare fable-library as a dependency with appropriate version constraints so users get a compatible version automatically.