Common Beginner Workflow

Now that you have built a function and a module, use this loop for your own project: edit the source, review its Python interface, build, and test.

This layout continues with scale.f90 from First Wrapped Function:

my-project/
├── src/
│   └── scale.f90
├── build/              # ← Generated, do not commit
├── tests/
│   └── test_scale.py
└── contracts/          # Optional edited semantic contracts

Keep src/ and tests/ under version control. Never commit the build/ folder.

1. Edit and Review

Edit the Fortran source, then preview the generated Python interface:

python3 -m prik generate --pyi src/scale.f90

Check the function names, arguments, result types, and required NumPy dtypes. This review is especially useful after changing a public Fortran declaration.

2. Build the Extension

python3 -m prik src/scale.f90 --out-dir build/scale

Rerun the same command after source changes. Add --verbose only when you need the compiler and linker details.

3. Write a Small Test

Create tests/test_scale.py:

import sys

import numpy as np

sys.path.insert(0, "build/scale")
import scale

def test_scale_function():
    result = scale.scale(np.float64(3.0), np.float64(2.5))
    assert result == 7.5

Run it with:

python3 -m pytest tests/test_scale.py -q

4. Optionally Edit the Contract

Save a contract package when you want to change the Python interface:

python3 -m prik generate --pyi src/scale.f90 --out contracts/scale

Edit contracts/scale/scale.pyi, then build through its package entry:

python3 -m prik contracts/scale/__init__.pyi \
  --native-fortran-sources src/scale.f90 \
  --out-dir build/scale-edited

Use this form instead of the source build in step 2 when the edited contract should control the wrapper. The .pyi controls the Python surface; the Fortran source still supplies the native implementation. Keep its native symbol names, types, rank, and argument order accurate.

The User Guide introduces small edits next to the feature they affect, such as renaming a function, changing array layout, adding an overload, or exposing a module procedure as a method.

Use Editing .pyi Contracts to find every supported edit and its complete rules.

5. Diagnose a Failure

If a build fails, rerun it with --verbose. If a Python call fails, compare the arguments with the generated contract. Use a clean output directory only when you need to rule out stale build files.

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