__set_name__ (PEP-487)

Published: 21 July 2020, 18:00

If you’re going to store data in the descriptor, the reasonable question is “where”.

If data stored in the descriptor’s attribute, it will be shared between all instances of the class where the descriptor is assigned.
If data is stored in a dict inside of the descriptor, where the key is hash of class and value is data, it will lead to a memory leak.

So, the best solution is to store data in the class itself. But how to name the attribute?

@cached_property that we implemented above, relies on the passed function name and it is wrong:

class C:
    @cached_property
    def a(self):
        print('called')
        return 1
    b = a
c = C()

# `a` is cached:
c.a
# called
# 1
c.a
# 1

# but `b` isn't:
c.b
# called
# 1
c.b
# called
# 1

PEP-487 introduced __set_name__ hook. It is called on descriptor assignment to a class attribute and accepts the class itself at the name of the attribute. Let’s use it and fix the implementation:

class cached_property:
  def __init__(self, func):
    self.func = func

  def __set_name__(self, owner, name):
    self.name = name

  def __get__(self, obj, cls):
    if obj is None:
      return self
    # we've replaced `self.func.__name__` by `self.name` here
    value = obj.__dict__[self.name] = self.func(obj)
    return value

Python etc / __set_name__ (PEP-487)