Set Intersection with None

Question

I am having trouble with set operation intersection.

Assume I have a list A = [0,1,2,3], and an integer B = 0. When I check if B in A:, I am getting of course True.

But when A and B are as default equal to None, than I can't do the intersection operation of A and B.

I am looking for a way to do following without getting an error:

A = None
B = None

if B in A:
    raise KeyError('B in A')

Normally A is a python List and B is a string. But I need to set them to None as default, while they are arguments in my function; where they have to have a value of None.

PS: Getting True or False with the search algo. is not important. I just need to get True or False, so that I can arrange my error-raising.

Can't you set `A = set() if not A else A` at the start of your function? — Arne, Aug 29 '18 at 14:21
Because the point is checking if B in A, and they have to be None as default in my functions argument. — oakca, Aug 29 '18 at 14:23
Use a default value for A I would recommend, as you usually don't want mutable in your arguments set: def fun(A=None, B=None): if A is None: A=[] return B in A — E.Serra, Aug 29 '18 at 14:24

score 1 · Answer 1 · answered Aug 29 '18 at 14:21

1

Why not try to check whether A or B is None itself?

A = None
B = None

if None not in [A, B] and B in A:
    print('B in A')
else:
    print('B or A is None, or B is not in A')

answered Aug 29 '18 at 14:21

SampleText2k77

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blue_note · Accepted Answer · 2018-08-29T14:39:33.223

1

The easiest would be to set A = [] instead of A = None (it's a reasonable value for something that will be a list eventually).

If A is argument to your function, be careful you don't write

def f(A=[])

but, instead

def f(A=None):
    if A is None:
       A = []

edited Aug 29 '18 at 14:39

answered Aug 29 '18 at 14:23

blue_note

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This looks nice, can I do also `A.isdisjoint(B)`? – oakca Aug 29 '18 at 14:25
@Icedkk no that's a set method. You said `A` is a list. If you want a set, why not use a set object. `A = set()`? – FHTMitchell Aug 29 '18 at 14:25
1

be careful: OP said he gets `None` from parameters. Don't use mutable default values in parameters. – Jean-François Fabre Aug 29 '18 at 14:26
@FHTMitchell you are right, well it creates a set afterwards... Anyway something similar like isdisjoint() to a list? – oakca Aug 29 '18 at 14:28
But why even use a `list` if you're going to turn it into a set, use a set from the beginning. `set(A).isdisjoint(...)` would work but isn't very efficient. – FHTMitchell Aug 29 '18 at 14:29
I just wanted to know if there is a function like isdisjoint also for lists, but ty. – oakca Aug 29 '18 at 14:30

score 1 · Answer 3 · answered Aug 29 '18 at 14:25

you could try:

if B in (A or []):

so if A is None it tests against empty list and yields False

if A is an empty list, it will also test against the rightmost operand of or but the result will be the same.

Larger scope: if you have default values as None in your function:

def foo(A=None,B=None):

leave them as None, and change them in the function code so the default argument isn't mutable ("Least Astonishment" and the Mutable Default Argument).

if A is None:
   A = []

score 1 · Answer 4 · answered Aug 29 '18 at 14:31

Continue using None as a default argument and then just set a to be the empty set set if it is None (I'm using lowercase argument names as per python convention):

def f(a=None, b=None):
   if a is None:
       a = set()

    if b in a:
        raise KeyError('b in a')

    if a.isdisjoint(other_set):
       ...

score 0 · Answer 5 · answered Aug 29 '18 at 14:22

0

A = [None, 1, 2]
B = None

if B in A: # where A should be a list, it should be iterable
    print('done')

output

done

answered Aug 29 '18 at 14:22

Nihal

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score 0 · Answer 6 · answered Aug 29 '18 at 14:31

The usual way one would go about None-initialized arguments in a function is to assign them to a sane default value at the start of said function. I use type annotation just to show which kind of values can be passed:

from typing import List, Optional

def check_intersect(A: Optional[List[str]] = None, B: Optional[str] = None):
    if A is None:
        A = []
    if B in A:  # this can't fail any more 
        raise KeyError('B in A')

The in keyword (if it is not part of a for-loop of course..) requires a python object as its right-side argument that offers a __contains__() function, which None doesn't.

Set Intersection with None

6 Answers6