Understanding the behavior of Python's set

Question

The documentation for the built-in type set says:

class set([iterable])

Return a new set or frozenset object whose elements are taken from iterable. The elements of a set must be hashable.

That is all right but why does this work:

>>> l = range(10)
>>> s = set(l)
>>> s
set([0, 1, 2, 3, 4, 5, 6, 7, 8, 9])

And this doesn't:

 >>> s.add([10])

    Traceback (most recent call last):
      File "<pyshell#7>", line 1, in <module>
        s.add([10])
    TypeError: unhashable type: 'list'

Both are lists. Is some magic happening during the initialization?

Answer 1

When you initialize a set, you provide a list of values that must each be hashable.

s = set()
s.add([10])

is the same as

s = set([[10]])

which throws the same error that you're seeing right now.

Answer 2

Think of the constructor being something like:

class Set:
    def __init__(self,l):
        for elem in l:
            self.add(elem)

Nothing too interesting to be concerned about why it takes lists but on the other hand add(element) does not.

Answer 3

In [13]: (2).__hash__
Out[13]: <method-wrapper '__hash__' of int object at 0x9f61d84>

In [14]: ([2]).__hash__ # nothing.

The thing is that set needs its items to be hashable, i.e. implement the __hash__ magic method (this is used for ordering in the tree as far as I know). list does not implement that magic method, hence it cannot be added in a set.

Answer 4

In this line:

s.add([10])

You are trying to add a list to the set, rather than the elements of the list. If you want ot add the elements of the list, use the update method.