Question

I am using the maths package SAGE, which is written in Python.

I am trying to manipulate some sets of vectors and I keep on getting the above error message. I have absolutely no idea what it is telling me, although I do understand what mutable means Can someone please explain in fairly simple conceptual terms what it is telling me is wrong?

Answer 1

It means that Python cannot use the content of ‘vectors’ to build a set, because the ‘vector object’ is just a ‘pointer’ to its content: the content is not ‘fixed’ and you can change it re-using the same vector vatiable (Python would have to rebuild the set to keep it working).

So, you can use ‘set’ only of immutable objects (like tuples, strings, numbers).

E.g.

>>> l1 = [1, 2, 3]
>>> l2 = [3, 4, 5]
>>> set([l1, l2])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unhashable type: 'list'

This is because you can change l1 and l2 (e.g. with .append).

>>> t1 = (1, 2, 3)
>>> t2 = (3, 4, 5)
>>> set([t1, t2])
set([(3, 4, 5), (1, 2, 3)])

In this case you cannot change the content of t1 and t2.