Python: Add list to set?


Tested on Python 2.6 interpreter:

>>> a=set('abcde')
>>> a
set(['a', 'c', 'b', 'e', 'd'])
>>> l=['f','g']
>>> l
['f', 'g']
>>> a.add(l)
Traceback (most recent call last):
  File "<pyshell#35>", line 1, in <module>
TypeError: list objects are unhashable

I think that I can't add the list to the set because there's no way Python can tell If I have added the same list twice. Is there a workaround?

EDIT: I want to add the list itself, not its elements.

2/25/2011 7:25:32 PM

Accepted Answer

You can't add a list to a set because lists are mutable, meaning that you can change the contents of the list after adding it to the set.

You can however add tuples to the set, because you cannot change the contents of a tuple:

>>> a.add(('f', 'g'))
>>> print a
set(['a', 'c', 'b', 'e', 'd', ('f', 'g')])

Edit: some explanation: The documentation defines a set as an unordered collection of distinct hashable objects. The objects have to be hashable so that finding, adding and removing elements can be done faster than looking at each individual element every time you perform these operations. The specific algorithms used are explained in the Wikipedia article. Pythons hashing algorithms are explained on and pythons __hash__ function in the python reference.

Some facts:

  • Set elements as well as dictionary keys have to be hashable
  • Some unhashable datatypes:
    • list: use tuple instead
    • set: use frozenset instead
    • dict: has no official counterpart, but there are some recipes
  • Object instances are hashable by default with each instance having a unique hash. You can override this behavior as explained in the python reference.
5/23/2017 12:18:24 PM

Use set.update() or |=

>>> a = set('abc')
>>> l = ['d', 'e']
>>> a.update(l)
>>> a
{'e', 'b', 'c', 'd', 'a'}

>>> l = ['f', 'g']
>>> a |= set(l)
>>> a
{'e', 'b', 'f', 'c', 'd', 'g', 'a'}

edit: If you want to add the list itself and not its members, then you must use a tuple, unfortunately. Set members must be hashable.

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