different behaviour of unhashable typeError between cpp and python

May 5, 2022

It is ok to insert a vector to a set in cpp, which not works in python however. Codes shown below:

// OK in cpp
set<vector<int>> cpp_set;
cpp_set.insert(vector<int>{1,2,3});

// not OK in python
py_set = set()
py_set.add([1,2,3])  # TypeError: unhashable type: 'list'
py_set.add({1,2,3})  # TypeError: unhashable type: 'set'

Even adding a set to a set is not allowed, and I don’t understand why a set is unhashable.

Why does the difference happens? And is it a permitted good way to use in cpp? I mean adding a vector to a set.

>Solution :

Python’s set is a hash table, and has no hashing functions for set and list.

std::set is not a hash table but a sequence ordered by an ordering relation (std::less by default).
You can use std::set with any type where you can define a strict weak ordering.

Try std::unordered_set in C++ and you will encounter problems.