Question

I’ve found some code that uses sets in Python. I tried to emulate them with lists, but I get different results when pop()ing from them!

I opened up Ipython to test how do these things work, and found something pretty strange:

In [16]: x
Out[16]: set([])

In [17]: x.add("a")

In [18]: x.add("b")

In [19]: x.add("c")

In [20]: x
Out[20]: set(['a', 'c', 'b'])

Shouldn’t 'b' come before c, because it was added before it? I don’t understand this.

Answer 1

http://docs.python.org/library/stdtypes.html#set

Being an unordered collection, sets do not record element position or order of insertion. Accordingly, sets do not support indexing, slicing, or other sequence-like behavior.

The underlying data structure of a set is a hash map, there is lots of information on those here.