Accessors

Accessors#

We do our best to expose coherent C++ and Python interfaces. Yet, the languages have some peculiarities that may present some pitfalls, one of which are accessors to mutable fields.

C++#

Given a C++ class A with two private attributes (one mutable, one immutable) and public accessors to them

#include <string>

class MutableInt {
 public:
  MutableInt(int value) : _value(value) {}
  void set(int value) {_value = value};
  int get() const {return _value};
  std::string repr() const {
    return "MutableInt(" + std::to_string(_value) +  ")";
  }
 private:
  int _value;
}

class A {
  A() : _i(0), _mi(0) {}
  // returns a copy;
  int get_i() const { return _i; }
  void set_i(int value) { _i = value; }
  // returns a copy;
  MutableInt get_mi() const { return _mi; }
  // returns a reference;
  MutableInt & get_mi_ref() { return _mi; }
  void set_mi(const MutableInt & value) { _mi = value; }

 private:
  // immutable type;
  int _i;
  // mutable type;
  MutableInt _mi;
};

A a;

there are two ways to modify the attributes:

  1. setting new values

    a.set_i(2);
a.set_mi(MutableInt{2});

  2. call a modifier on the mutable attribute,

    a.get_mi_ref().set(2);

    provided that the class does expose an accessors that returns a reference.

Note that accessors that returns a reference have the further effect to track changes:

auto mi = a.get_mi();
auto & mi_ref = a.get_mi_ref();
a.set_mi(MutableInt{3});

now mi_ref is equal to Mutable(3), while mi is still equal to Mutable(2) as it has been copied.

Pybind11#

When these accessors are exposed as Python properties (or as Python methods), e.g. through

#include <pybind11/pybind11.h>
#include <string>

namespace py = pybind11;

PYBIND11_MODULE(my_module, m) {
  py::class_<MutableInt>(m, "MutableInt")
     .def(py::init<int>())
     .def("set", &MutableInt::get)
     .def("get", &MutableInt::set)
     .def("__repr__", &MutableInt::repr);
  py::class_<A>(m, "A")
     .def(py::init<int>())
     .def_property("i", &A::get_i, &A::set_i)
     .def_property("mi", &A::get_mi, &A::set_mi)
     .def_property("mi_ref", &A::get_mi_ref, nullptr);

some methods do not modify A as a Python user may instead expect (see below)

from my_module import A, MutableInt

a = A(1)

  • do modify a:

    >>> a.i = 2
>>> a.mi = MutableInt(2)
>>> a.i, a.mi
2, MutableInt(2)
>>> a.mi_ref.set(3)
>>> a.mi
MutableInt(3)

  • does not modify a as a.mi returns a copy:

    >>> a.mi.set(4)
>>> a.mi
MutableInt(3)

Python#

The equivalent Python class, implemented like

class PyA:

    def __init__(self):
        self.i = 0
        self.mi = MutableInt(0)

py_a = PyA()

uses references to holds/pass objects, copying only if asked explicitly, for instances using a property like

@property
def mi_copy(self):
    return MutableInt(self.mi.get())

Therefore both methods to modify the (mutable) attribute are available:

  1. setting a value

    >>> py_a.i = 2
>>> py_a.mi = MutableInt(2)
>>> i, py_a.mi
2, MutableInt(2)

  2. modifying the existing value

    >>> py_a.mi.set(3)
MutableInt(3)

This difference may confuse users, as they may consider the classes A and PyA to be equivalent, as they expose the same interface (ignoring A.mi_ref).

py_a = PyA()
# this has an effect
py_a.mi.set(2)
a = A()
# while this does not
a.mi.set(2)

Note that this difference does not apply to immutable attributes.

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