Most pythonic way to construct an object not in a container?

Question

Consider a list, or more generally a container C, that contains different hashable objects of different types.

I would like to construct an hashable object that is not in C in a efficient and pythonic way.

Currently, I do the following:

i = 0

while i in C:           # while i is in the container C
    i += 1

# at the end of the loop, i is not in C

However, it feels very inefficient and not very pythonic...

So my question is: given a container that contains different hashable objects, what is the best way to construct an hashable object that is not in this container?

(For context, I am doing some graph theory and I need to add to my graph a temporary and artificial node that I shall remove later.)

Answer 1

Your approach can be simplified by using Graph.__len__:

G.add_node(len(G))

Note however that this approach works only if you know a priori that your graph only contains nodes 0 .. len(G) - 1.

To solve the general problem of finding an object that is not in the container, I would create a new class from scratch. Here is an example:

container = ['A', 3, None, 3.0, True]
class MyClass:
    _hash_val = max([hash(x) for x in container]) + 1
    def __hash__(self):
        return MyClass._hash_val
    def __eq__(self, other):
        return isinstance(other, MyClass)
container.append(MyClass())