Python: Create list from variables as long as they are not None

Question

How do I add multiple variables to a list as long as they are not None?

If either one of them is None, then only the other one should be added to the list.

a = "A"
b = None

list_items = [a + b]

Gives:

TypeError: (“cannot concatenate ‘str’ and ‘NoneType’ objects”,
u’occurred at index 0′)

In the above example, the components of the list will always be a and b. I suspect a solution that can handle an arbitrary number of variables would make use of .extend() to empty list [] as long as the next added variable is not None. That could be useful, but what is the simplest solution?

Asked By: P A N

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Source

Answer 1

You could create a function that takes arbitrary number of arguments and filters out the ones which are None:

def create_list(*args):
    return [a for a in args if a is not None]

print create_list(1, 4, None, 'a', None, 'b')

Output:

[1, 4, 'a', 'b']

Answered By: niemmi

Answer 2

You can query the variable directly for None:

 a = "A"
 b = None

 if b != None:
     list_items = [a + b]

Answered By: armatita

Answer 3

Another method would be a list comprehension:

a = "A"
b = None

lst = [e for e in [a, b] if e]

It’s a little arcane, but this should neatly return you a list of only elements that are "truthy" – i.e. not None, non-empty string, integer greater than 0, True boolean etc.

Answered By: Lou

Python: Create list from variables as long as they are not None

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