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py.ml: OCaml bindings for Python

py.ml provides OCaml bindings for Python 2 and Python 3. This library subsumes the pycaml library, which is no longer actively maintained.

OPAM: opam install pyml

The Python library is linked at runtime and the same executable can be run in a Python 2 or a Python 3 environment. py.ml does not require any Python library at compile time. The only compile time dependency is Stdcompat to ensure compatibility with all OCaml compiler versions from 3.12.

Bindings are split in three modules:

• Py provides the initialization functions and some high-level bindings, with error handling and naming conventions closer to OCaml usages.

• Pycaml provides a signature close to the old Pycaml module, so as to ease migration.

• Pywrappers provides low-level bindings, which follow closely the conventions of the C bindings for Python. Submodules Pywrappers.Python2 and Pywrappers.Python3 contain version-specific bindings.

Custom top-level

A custom top-level with the C bindings can be compiled by make pymltop.

If you have utop and ocamlfind, you can make pymlutop.

For OPAM users: pymltop is installed by default by opam install pyml. pymlutop is installed whenever utop is available.

Getting started

Py.initialize () loads the Python library.

Py.Run.simple_string "print('Hello, world!')" executes a Python phrase and returns true if the execution succeeded, false otherwise.

Py.Run.eval "18 + 42" evaluates a Python phrase and returns a value of type Py.Object.t. Such a value can then be converted to an OCaml value: assert (Py.Int.to_int (Py.Run.eval "18 + 42") = 60). In case of error (either the phrase is syntactically incorrect or the evaluation raises an exception), an OCaml exception Py.E (type, msg) is raised. By default, Py.Run.eval evaluates an expression; use Py.Run.eval ~start:Py.File to evaluate a Python phrase as a module/file (with import directives and so on).

To make an OCaml value accessible from Python, we create a module (called ocaml in this example, but it can be any valid Python module name):

let m = Py.Import.add_module "ocaml" in
Py.Module.set m "example_value"
  (Py.List.of_list_map Py.Int.of_int [1;2;3]);
Py.Run.eval ~start:Py.File "
from ocaml import example_value
print(example_value)"

OCaml functions can be passed in the same way.

let m = Py.Import.add_module "ocaml" in
let hello args =
  Printf.printf "Hello, %s!\n" (Py.String.to_string args.(0));
  Py.none in
Py.Module.set m "hello" (Py.Callable.of_function hello);
Py.Run.eval ~start:Py.File "
from ocaml import hello
hello('World')"

Py.Module.set m "hello" (Py.Callable.of_function hello) can be written Py.Module.set_function m "hello" hello.

Python functions can be called from OCaml too.

let builtins = Py.Eval.get_builtins () in
let sorted_python = Py.Dict.find_string builtins "sorted" in
let sorted = Py.Callable.to_function sorted_python in
let result =
  sorted [| Py.List.of_list_map Py.Float.of_float [3.0; 2.0] |] in
assert (Py.List.to_list_map Py.Float.to_float result = [2.0; 3.0])

Py.Run.interactive () runs the Python top-loop. It can be run after some OCaml values have been made accessible through a module as above. If IPython is available, the top-loop can be run with Py.Run.ipython ().

With OCaml 4.06 and greater, the module Pyops declares indexing operators.

The "hello world" example above can be written:

let m = Py.Import.add_module "ocaml" in
let open Pyops in
m.&("hello") <- (fun args ->
  Printf.printf "Hello, %s!\n" (Py.String.to_string args.(0));
  Py.none);
Py.Run.eval ~start:Py.File "
from ocaml import hello
hello('World')"

Error handling

All Python exceptions are caught as OCaml exceptions Py.E (type, msg), where type and msg are two Python objects. Typically, one can convert type and msg to strings with Py.Object.to_string to display or analyse them.

When an OCaml function f is called from Python (passed by Py.Callable.of_function), f can raise a Python exception by raising an OCaml exception of the form Py.E (type, msg). To raise standard errors more conveniently, f can raise an exception of the form Py.Err (type, msg) instead, where type belongs to the enumeration Py.Err.t and msg is an OCaml string. If f raises an exception that is neither of the form Py.E nor Py.Err, then this exception is encapsulated with its backtrace in a Python exception (of class ocaml exception derived from BaseException and not from Exception, so as not to be caught), leading the Python interpreter to be interrupted, and the exception is raised back in OCaml.

Py.Run.simple_string catches all Python exceptions (and OCaml exceptions as well) and returns a single Boolean to indicate success. One can prefer Py.Run.eval to get proper error handling.

Data types

Python values have type Py.Object.t.

Py.String.of_string and Py.String.to_string convert back and forth OCaml to Python strings: Unicode strings and legacy strings are handled uniformly.

Py.Int.of_int and Py.Int.to_int convert back and forth OCaml to Python integers. For big numbers, one should use an intermediate textual representation with Py.Int.of_string and Py.Int.to_string.

Py.Float.of_float and Py.Float.to_float convert back and forth OCaml to Python floating-point values.

The module Py.Number define common arithmetic and bitwise operations on Python numbers. It can be open locally to take benefit from operator overloading. E.g.:

let m = Py.Import.add_module "ocaml" in
let square args = Py.Number.(args.(0) ** of_int 2) in
Py.Module.set_function m "square" square;
ignore (Py.Run.eval ~start:Py.File "
from ocaml import square
print(square(3))")

Py.Tuple.of_array and Py.Tuple.to_array can be used to construct and destruct Python tuples. Py.Tuple.of_list and Py.Tuple.to_list are available as well. The empty tuple is Py.Tuple.empty. Converters can be composed with the _map suffix: for example, Py.Tuple.of_list_map Py.Int.of_int converts a list of integers to a Python value. Short tuples can be constructed and destructed with Py.Tuple.of_tuple1, ..., Py.Tuple.of_tuple5 and Py.Tuple.to_tuple1, ..., Py.Tuple.to_tuple5.

For Python lists, Py.List.of_array, Py.List.to_array, Py.List.of_list and Py.List.to_list are available (along with their _map variant). Py.List and Py.Tuple include the Py.Sequence module: along with to_array and to_list, iterators like fold_left fold_right, for_all, etc. are available.

Python iterators can be iterated with Py.Iter.next which returns an option value, where None represents the end of the iteration. Py.Iter.iter, Py.Iter.fold_left,Py.Iter.fold_right and Py.Iter.to_list can be used as well. A Python iterator can be created with Py.Iter.create from an OCaml function that returns an option.

Python dictionaries can be constructed and destructed with Py.Dict.of_bindings_string and Py.Dict.to_bindings_string from and to associative lists between string keys and Python values. Py.Dict.find_string can be used to find a single value (the exception Not_found is raised if the key is not in the dictionary; Py.Dict.get_item_string provides the option variant).

Python closures can be called from OCaml with Py.Callable.to_function, or Py.Callable.to_function_with_keywords to pass keywords as an associative list. Symmetrically, OCaml functions can be turned into Python closures with Py.Callable.of_function and Py.Callable.of_function_with_keywords (the latter function passes keywords as a dictionary to the OCaml callback: values can be retrieved efficiently with Py.Dict.find_string).

let m = Py.Import.add_module "ocaml" in
Py.Module.set_function_with_keywords m "length"
  (fun args kw ->
    let x = Py.Dict.find_string kw "x" in
    let y = Py.Dict.find_string kw "y" in
    Py.Number.((x ** of_int 2 + y ** of_int 2) ** of_float 0.5));
ignore (Py.Run.eval ~start:Py.File "
from ocaml import length
print(length(x=3, y=4))")

Modules

New modules can be defined with Py.Import.add_module and existing modules can be imported with Py.Import.import_module (or the shorter Py.import alias). Trying to import a module that does not exist leads to a Python exception: use Py.Import.import_module_opt to get an option result instead (or the shorter Py.import_opt alias).

Module.get and Module.set allow to retrieve and define module members. For function members, there are shortcuts to do the conversion with Py.Callable: Module.get_function and Module.set_function (and Module.get_function_with_keywords and Module.set_function_with_keywords).

If we consider the following Python code taken from matplotlib documentation:

import numpy as np
import matplotlib.pyplot as plt

x = np.arange(0, 5, 0.1);
y = np.sin(x)
plt.plot(x, y)
plt.show()

The code can be written directly in OCaml as such:

let np = Py.import "numpy" in
let plt = Py.import "matplotlib.pyplot" in
let x = Py.Module.get_function np "arange"
  (Array.map Py.Float.of_float [| 0.; 5.; 0.1 |]) in
let y = Py.Module.get_function np "sin" [| x |] in
ignore (Py.Module.get_function plt "plot" [| x; y |]);
assert (Py.Module.get_function plt "show" [| |] = Py.none)

or, using indexing operators (OCaml 4.06):

let np = Py.import "numpy" in
let plt = Py.import "matplotlib.pyplot" in
let open Pyops in
let x = np.&("arange")(Array.map Py.Float.of_float [| 0.; 5.; 0.1 |]) in
let y = np.&("sin")[| x |] in
ignore (plt.&("plot")[| x; y |]);
assert (plt.&("show")[| |] = Py.none)

NumPy

If the NumPy library is installed, then OCaml float arrays and bigarrays can be shared in place with Python code as NumPy arrays (without copy). Python code can then directly read and write from and to the OCaml arrays and changes are readable from OCaml.

let array = [| 1.; 2. ; 3. |] in
let m = Py.Import.add_module "ocaml" in
Py.Module.set m "array" (Py.Array.numpy array);
ignore (Py.Run.eval ~start:Py.File "
from ocaml import array
array *= 2");
assert (array = [| 2.; 4.; 6. |])

Bigarrays are handled by the Numpy module that is shipped in numpy.cma/cmxa and requires bigarray.cma/cmxa. Numpy arrays can be obtained from bigarrays with Numpy.of_bigarray and bigarrays can be obtained from Numpy arrays with Numpy.to_bigarray (the provided kind and layout should match the format of the Numpy array).

let m = Py.Import.add_module "test" in
let callback arg =
  let bigarray =
    Numpy.to_bigarray Bigarray.nativeint Bigarray.c_layout arg.(0) in
  let array1 = Bigarray.array1_of_genarray bigarray in
  assert (Bigarray.Array1.get array1 0 = 0n);
  assert (Bigarray.Array1.get array1 1 = 1n);
  assert (Bigarray.Array1.get array1 2 = 2n);
  assert (Bigarray.Array1.get array1 3 = 3n);
  Py.none in
Py.Module.set m "callback" (Py.Callable.of_function callback);
assert (Py.Run.simple_string "
from test import callback
import numpy
callback(numpy.array([0,1,2,3]))
")