Wrapping Derived Types

A supported Fortran type becomes a generated Python extension class. Constructors and ordinary function results create independent Fortran instances that are released with their Python objects. A nested component belongs to its parent, and a module object belongs to the Fortran module. Python accesses fields through generated getters and setters. Methods call wrapped Fortran procedures. Python never reads the native memory layout directly.

Complete Example

Create points.f90:

module points
  implicit none

  type :: point
    real(8) :: x = 0.0_8
    real(8) :: y = 0.0_8
  end type point

  type :: holder
    type(point) :: origin
  end type holder

contains

  subroutine move(item, dx, dy)
    type(point), intent(inout) :: item
    real(8), intent(in) :: dx, dy
    item%x = item%x + dx
    item%y = item%y + dy
  end subroutine move

  function make_point(x, y) result(item)
    real(8), intent(in) :: x, y
    type(point) :: item
    item%x = x
    item%y = y
  end function make_point

  subroutine set_origin(container, item)
    type(holder), intent(inout) :: container
    type(point), intent(in) :: item
    container%origin = item
  end subroutine set_origin

end module points

Build it:

python3 -m prik points.f90 --out geometry --out-dir build/geometry

Usage in Python

import sys

import numpy as np

sys.path.insert(0, "build/geometry")
import geometry.points as points

# Create new object
item = points.point(x=np.float64(1.0), y=np.float64(2.0))

# Call method (inout mutation)
points.move(item, np.float64(3.0), np.float64(4.0))
print(item.x, item.y)  # 4.0 6.0

# Function returning derived type
made = points.make_point(np.float64(8.0), np.float64(9.0))

# Nested component
container = points.holder()
points.set_origin(container, made)
container.origin.x = np.float64(12.0)
print(container.origin.x)  # 12.0

Inspect the Class

The class docstring gives a short index:

print(points.point.__doc__)

point

Opaque wrapper for native type point.

Constructor
-----------
point(*, x=0.0, y=0.0) -> point

Fields
------
x : float64
y : float64

The constructor has its own detailed docstring:

print(points.point.__init__.__doc__)

Key Concepts

  • Lifetime: A constructed or returned object is released when its Python object is no longer used. A nested component stays tied to its parent.
  • Mutation: intent(out) and intent(inout) modify a caller-provided instance and do not return it again.
  • Missing intent: A dummy without intent follows the same conservative in-place rule as intent(inout).
  • Fields: Public scalar numeric/logical/complex fields become Python attributes.
  • Nested types: Appear as generated objects tied to their parent.
  • Results: Derived-type function results create new independent objects.
  • Default constructor: Automatically generated from public, writable primitive scalar fields.
  • Constructor fields: Passed by keyword (logical, integer, real, and complex).

Custom Constructor

The default constructor assigns public fields directly. If the native module already provides initialize_point(item, x, y), an edited contract can use it as the constructor.

In this mapping, @bind selects the native initializer, @native_call(...) gives its argument order, Pass() inserts the new point, and Addr(Arg(i)) passes Python argument i by address:

from prik.contracts import Addr, Arg, Float64, Pass, bind, native_call

class point:
    x: Float64
    y: Float64

    @bind("initialize_point")
    @native_call([Pass(), Addr(Arg(0)), Addr(Arg(1))])
    def __init__(self, x: Float64, y: Float64) -> None: ...

Replace the generated field-keyword __init__ declaration with this one. The edit changes construction only; it does not create initialize_point in the native module.

After rebuilding, points.point.__init__.__doc__ starts with point(x, y) -> point and lists both parameters.

For the complete replacement rules, see Replace the Constructor.

Type-Bound Methods

A public type-bound procedure becomes a method on the generated class. The passed object becomes self and is not repeated in the Python call.

type :: counter
  integer(4) :: value = 0
contains
  procedure :: increment
end type counter

contains

subroutine increment(self, amount)
  class(counter), intent(inout) :: self
  integer(4), intent(in) :: amount
  self%value = self%value + amount
end subroutine increment

item = counters.counter(value=np.int32(4))
item.increment(np.int32(3))
print(item.value)  # 7

The method mutates the existing counter; it does not replace the Python object.

Expose a Module Procedure as a Method

The move(item, dx, dy) procedure from this page's example can remain a module-level function and also become point.move(dx, dy).

Pass() supplies self to the native call. Arg(i) refers to a visible Python argument. Add the method to the existing point class while keeping the module declaration:

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

class point:
    @native_call([Pass(), Addr(Arg(0)), Addr(Arg(1))])
    def move(self, dx: Float64, dy: Float64) -> None: ...

@native_call([Arg(0), Addr(Arg(1)), Addr(Arg(2))])
def move(item: point, dx: Float64, dy: Float64) -> None: ...

Both declarations call the existing native move procedure:

points.move(item, np.float64(2.0), np.float64(3.0))
item.move(np.float64(2.0), np.float64(3.0))

To expose only the method, import private and add @private to the module-level declaration.

The class docstring now lists move(dx, dy) -> None under Methods. points.point.move.__doc__ contains its complete parameter and return details.

For the complete mapping rules, see Expose a Module Procedure as a Method.

Type-Bound Generics

A type-bound generic groups several concrete methods under one Python method. For example, add can accept exact integer or real amounts:

type :: counter
  integer(4) :: value = 0
contains
  procedure :: add_integer
  procedure :: add_real
  generic :: add => add_integer, add_real
end type counter

The generated contract uses the same explicit overload links as a module-level generic:

from prik.contracts import Float64, Int32, bind, overload, private

class counter:
    @private
    def add_integer(self, amount: Int32) -> Int32: ...

    @private
    def add_real(self, amount: Float64) -> Float64: ...

    @bind("add")
    @overload("add_integer")
    def add(self, amount: Int32) -> Int32: ...

    @bind("add")
    @overload("add_real")
    def add(self, amount: Float64) -> Float64: ...

print(item.add(np.int32(2)))       # exact Int32 candidate
print(item.add(np.float64(0.5)))   # exact Float64 candidate

The passed object participates in native dispatch but is already fixed by the generated class. The remaining arguments must still match one candidate exactly. Each @overload retains a concrete contract. @bind("add") routes the native call through the public type-bound generic because its specifics are private.

Defined Operators

A defined operator with a wrapped derived-type operand becomes a Python magic method. Its overload candidates are attached to the generated class.

interface operator(+)
  module procedure add_points
end interface operator(+)

contains

function add_points(left, right) result(output)
  type(point), intent(in) :: left, right
  type(point) :: output
  output%x = left%x + right%x
  output%y = left%y + right%y
end function add_points

The generated contract exposes operator(+) as __add__:

from prik.contracts import overload, private

class point:
    @overload("add_points")
    def __add__(self, right: point) -> point: ...

@private
def add_points(left: point, right: point) -> point: ...

Python uses the normal operator:

left = points.point(x=np.float64(1.0), y=np.float64(2.0))
right = points.point(x=np.float64(3.0), y=np.float64(4.0))
total = left + right
print(total.x, total.y)  # 4.0 6.0

The magic method docstring shows the accepted operator signatures:

print(points.point.__add__.__doc__)

The relevant part is:

__add__(*args, **kwargs)

Supported Signatures
--------------------
__add__(right: point) -> point
Fortran generic Python method Python syntax
Binary +, -, *, /, ** Direct and reflected magic methods left + right
Unary +, - __pos__, __neg__ +value, -value
Relational operators __eq__, __lt__, and related methods left == right
.and., .or., .not. __and__, __or__, __invert__ left & right, ~value
Named operator .name. operator_name or r_operator_name Explicit method call
assignment(=) assign target.assign(value)

Python and, or, and not cannot be overloaded, so logical operator generics use &, |, and ~. Python assignment only rebinds a name, so defined assignment uses .assign(...).

At least one operand must be a wrapped derived type. Other operands can be supported primitive scalars, arrays, or generated classes. Their dispatch is exact.

For the overload rules shared by type-bound generics and operators, see Edit an Overload Set.

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