Question

In Python, how is it possible to reuse existing equal immutable objects (like is done for str)? Can this be done just by defining a __hash__ method, or does it require more complicated measures?

Answer 1

If you want to create via the class constructor and have it return a previously created object then you will need to provide a __new__ method (because by the time you get to __init__ the object has already been created).

Here is a simple example – if the value used to initialise has been seen before then a previously created object is returned rather than a new one created:

class Cached(object):
    """Simple example of immutable object reuse."""

    def __init__(self, i):
        self.i = i

    def __new__(cls, i, _cache={}):
        try:
            return _cache[i]
        except KeyError:
            # you must call __new__ on the base class
            x = super(Cached, cls).__new__(cls)
            x.__init__(i)
            _cache[i] = x
            return x

Note that for this example you can use anything to initialise as long as it’s hashable. And just to show that objects really are being reused:

>>> a = Cached(100)
>>> b = Cached(200)
>>> c = Cached(100)
>>> a is b
False
>>> a is c
True