First Wrapped Function

Choose the Fortran or C path below. Both define the same operation, build an extension named scale, and expose scale.scale to Python. After building, continue with Call the Function.

Fortran Path

Create scale.f90:

real(8) function scale(value, factor) result(output)
  real(8), intent(in) :: value
  real(8), intent(in) :: factor
  output = value * factor
end function scale

Preview the Python interface before building:

python3 -m prik generate --pyi scale.f90

The generated semantic .pyi contains:

from prik.contracts import Addr, Arg, Float64, native_call, standalone

@standalone
@native_call([Addr(Arg(0)), Addr(Arg(1))])
def scale(
    value: Float64,
    factor: Float64
) -> Float64: ...

Build the extension:

python3 -m prik scale.f90 \
  --out scale \
  --out-dir build/first-function

@standalone records that the procedure is outside a Fortran module, and @native_call(...) records Fortran's by-address scalar arguments.

Continue with Call the Function, or read the C path to compare the native source and generated contract.

C Path

Create scale.c:

double scale(double value, double factor) {
    return value * factor;
}

Preview the Python interface:

python3 -m prik generate --pyi --language c scale.c

The generated semantic .pyi contains:

from prik.contracts import Float64

def scale(value: Float64, factor: Float64) -> Float64: ...

Build the extension:

python3 -m prik --language c scale.c \
  --compiler cc \
  --out scale \
  --out-dir build/first-function

Call the Function

Whichever source path you chose, import and call the extension in the same way:

import sys

import numpy as np

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

result = scale.scale(np.float64(3.0), np.float64(2.5))
print(result)          # 7.5
assert result == 7.5

Common Pitfall: Wrong Scalar Type

Float64 requires numpy.float64 scalar arguments and returns a numpy.float64 result. Pass the exact NumPy scalar types:

# This will raise TypeError
scale.scale(3.0, 2.5)

# Correct way
scale.scale(np.float64(3.0), np.float64(2.5))

Always convert at the call site for scalar arguments. If the build fails, rerun the command for your selected path with --verbose.

The semantic .pyi is an editable description of the Python interface. The Common Beginner Workflow shows how to save and edit it.

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