I have a list of sets. I want to add an element to each of these sets, and I want to do this with list comprehension. This is what I have tried:
In [1]: sets1 = [set()]
In [2]: sets2 = [{1,2}, {1,2,3}]
In [3]: [e.add(0) for e in sets1]
Out[3]: [None]
In [4]: [e.add(0) for e in sets2]
Out[4]: [None, None]
My desired output is:
[{0}]
[{1,2,0}, {1,2,3,0}]
Why does the above code return None instead of an addition of elements to the list, and how I can make this work?
I would suggest:
[e | {0} for e in sets1]
or:
[e.union({0}) for e in sets1]
I wouldn't use a list comprehension in this case, a plain for loop would be simpler:
for subset in sets1:
subset.add(0)
print(sets1)
should give you the desired output.
I already pointed it out in the comments why your approach seemingly did not work:
set.addworks in place and does not return anything (thus yourNones). If you want your desired output then run the list-comprehension but don't save its result. Check your set1 and set2 after the list-comprehension to get the desired output.
So you could just check sets1 and sets2 after the list comprehension. It should return: [{0}] and [{1,2,0}, {1,2,3,0}] (order may vary because sets are unordered).
Actually your sets1 and sets2 variables have become the results that you want, because the add statement operates the sets1 but not generate a new list.
You can print(sets1) and print(sets2) to testify.
Let's first regenerate your problem.
>>> test_set = set()
>>> test_set
set()
>>> print(test_set.add(0))
None
>>> test_set
{0}
>>>
As you can see, test_set.add(0) returns None. But this is an in place operation, so the item did get added., which is evident from the above snippet.
How to solve the problem:
You can union after making the element a set rather than using the add method.
>>> [i.union({0}) for i in sets2]
[{0, 1, 2}, {0, 1, 2, 3}]
If you have a list/set of element to add to the exiting list of sets, you can do the following:
elements_to_add = [3,4,5]
>>> [i.union(set(elements_to_add)) for i in sets2]
[{1, 2, 3, 4, 5}, {1, 2, 3, 4, 5}]
However, this is not an in-place operation. sets2 would be exactly same before and after running the above list comprehension.
set.addworks in place and does not return anything (thus yourNones). If you want your desired output then run the list-comprehension but don't save its result. Check yourset1andset2after the list-comprehension to get the desired output.