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>
a.add(l)
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.
This question is related to
python
list
python-2.7
set
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.
Hopefully this helps:
>>> seta = set('1234')
>>> listb = ['a','b','c']
>>> seta.union(listb)
set(['a', 'c', 'b', '1', '3', '2', '4'])
>>> seta
set(['1', '3', '2', '4'])
>>> seta = seta.union(listb)
>>> seta
set(['a', 'c', 'b', '1', '3', '2', '4'])
I found I needed to do something similar today. The algorithm knew when it was creating a new list that needed to added to the set, but not when it would have finished operating on the list.
Anyway, the behaviour I wanted was for set to use id
rather than hash
. As such I found mydict[id(mylist)] = mylist
instead of myset.add(mylist)
to offer the behaviour I wanted.
Please notice the function set.update()
. The documentation says:
Update a set with the union of itself and others.
You'll want to use tuples, which are hashable (you can't hash a mutable object like a list).
>>> a = set("abcde")
>>> a
set(['a', 'c', 'b', 'e', 'd'])
>>> t = ('f', 'g')
>>> a.add(t)
>>> a
set(['a', 'c', 'b', 'e', 'd', ('f', 'g')])
Sets can't have mutable (changeable) elements/members. A list, being mutable, cannot be a member of a set.
As sets are mutable, you cannot have a set of sets! You can have a set of frozensets though.
(The same kind of "mutability requirement" applies to the keys of a dict.)
Other answers have already given you code, I hope this gives a bit of insight. I'm hoping Alex Martelli will answer with even more details.
You want to add a tuple, not a list:
>>> a=set('abcde')
>>> a
set(['a', 'c', 'b', 'e', 'd'])
>>> l=['f','g']
>>> l
['f', 'g']
>>> t = tuple(l)
>>> t
('f', 'g')
>>> a.add(t)
>>> a
set(['a', 'c', 'b', 'e', 'd', ('f', 'g')])
If you have a list, you can convert to the tuple, as shown above. A tuple is immutable, so it can be added to the set.
Try using *
unpack, like below:
>>> a=set('abcde')
>>> a
{'a', 'd', 'e', 'b', 'c'}
>>> l=['f','g']
>>> l
['f', 'g']
>>> {*l, *a}
{'a', 'd', 'e', 'f', 'b', 'g', 'c'}
>>>
Non Editor version:
a=set('abcde')
l=['f', 'g']
print({*l, *a})
Output:
{'a', 'd', 'e', 'f', 'b', 'g', 'c'}
Here is how I usually do it:
def add_list_to_set(my_list, my_set):
[my_set.add(each) for each in my_list]
return my_set
To add the elements of a list to a set, use update
From https://docs.python.org/2/library/sets.html
s.update(t): return set s with elements added from t
E.g.
>>> s = set([1, 2])
>>> l = [3, 4]
>>> s.update(l)
>>> s
{1, 2, 3, 4}
If you instead want to add the entire list as a single element to the set, you can't because lists aren't hashable. You could instead add a tuple, e.g. s.add(tuple(l))
. See also TypeError: unhashable type: 'list' when using built-in set function for more information on that.
list objects are unhashable. you might want to turn them in to tuples though.
Source: Stackoverflow.com