Pythonic way of ignoring the last element when doing set difference

Question

Let's say I have two set()s:

a = {('1', '2', '3', 'a'), ('1', '2', '4', 'a'), ('1', '2', '5', 'b')}
b = {('1', '2', '3', 'b'), ('1', '2', '4', 'b'), ('1', '2', '6', 'b')}

Now, what I want to do is to find the set difference b \ a but ignoring the last element from every tuple. So it's just like doing something like this:

a = {('1', '2', '3'), ('1', '2', '4'), ('1', '2', '5')}
b = {('1', '2', '3'), ('1', '2', '4'), ('1', '2', '6')}

In[1]: b - a
Out[1]: {('1', '2', '6')}

Expected output:

b \ a = {('1', '2', '6', 'b')}

Is there any obvious / pythonic way of achieving this without having to manually iterate over each set and check against each tuple[:3]?

Answer 1

Here's how you might write your own class to override a tuple's normal hashing behaviour:

a_data = [('1', '2', '3', 'a'), ('1', '2', '4', 'a'), ('1', '2', '5', 'b')]
b_data = [('1', '2', '3', 'b'), ('1', '2', '4', 'b'), ('1', '2', '6', 'b')]

class HashableIgnoresLastElement(tuple):
    def __eq__(self, other):
        return self[:-1] == other[:-1]

    def __hash__(self):
        return hash(self[:-1])

a = set(map(HashableIgnoresLastElement, a_data))
b = set(map(HashableIgnoresLastElement, b_data))

print(b - a)

with output

{('1', '2', '6', 'b')}

To modify the way sets of tuples behave, we have to modify the way tuples are hashed.

From here,

An object is hashable if it has a hash value which never changes during its lifetime (it needs a __hash__() method), and can be compared to other objects (it needs an __eq__() method). Hashable objects which compare equal must have the same hash value.

Hashability makes an object usable as a dictionary key and a set member, because these data structures use the hash value internally.

So in order to make the hashing ignore the last element, we have to overload the dunder methods __eq__ and __hash__ appropriately. This doesn't end up being so hard because all we have to do is slice off the last element and then delegate to the appropriate methods of a normal tuple.

Further reading:

• How to make an object properly hashable?
• https://docs.python.org/3/reference/datamodel.html