Question

class A:
  def f(self):
    print('f')
  def g(self):
    print('g')
  def h(self):
    print('h')

x = A()
y = A()
x.f = x.g # creates a new attribute 'f' for x
x.f() # 'g'; resolves at the instance attribute level to call instance method 'g'
y.f() # 'f'; instance methods are unaffected
A.f = A.h # redefines instance method 'f' to print 'h'
x.f() # 'g'; still resolves at the attribute level to call instance method 'g'
y.f() # 'h'; instance method 'f' now prints 'h'
A.g = A.h # redefines instance method 'g' to print 'h'
x.f() # 'g'; still calls the old instance method 'g' because it kept the link to it
y.f() # 'h'

Is my understanding correct?

I’m trying to use this in the following way:

  class Attributes:
    def __init__(self, params, cache_field = None):
      # ...
      self.cache_field = cache_field
      if cache_field is None:
        # I hope I'm setting instance attribute only
        self.check_cache = self.check_external_cache
      else:
        self.check_cache = self.check_internal_cache
        self.internal_cache = {}

    def check_internal_cache(self, record):
      return self.internal_cache[record.id]

    def check_external_cache(self, record):
      return record[self.cache_field]

    def calculate_attributes(self, record):
      try:
        return self.check_cache(record) # I hope it will resolve to instance attribute
      except KeyError:
        # calculate and cache the value here
        # ...

Would this work correctly? Is it ok to do this? Originally I was hoping to save time compared to checking self.cache_field in every call to calculate_attributes; but I’m no longer sure it would save any time.

Answer 1

I think the basic idea here is correct, with a couple of minor corrections. First,

A.f = A.h # redefines instance method 'f' to print 'h'

That should read class method, not instance method. You’re changing the class here. And second, this doesn’t correspond to any defined variable here:

    if cache is None:

I guess maybe you mean cache_field?

In general, setting instance attributes in __init__ is perfectly normal and acceptable. It doesn’t matter that this is a method rather than some other kind of object — it’s not any different from saying self.foo = 'bar'.

Also, sometimes this depends, but in general, it is indeed faster to set the method in init than to test cache_field every time check_cache is called.