Most Pythonic way to convert None to an empty list in case a default argument is not passed in a Function?

Question

I want to write a general function that takes two input variables var1, var2 and returns the concatenation of both.

Each variable has the default value None, and can be either a single element or a list.

The expected output should be a list (even if both var1 and var2 are None, it should return an empty list []).

Below is my function:

def my_func(var1=None, var2=None):
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]
    return var1 + var2

When I only input one variable, I get the following:

>>> lst = my_func(var2=[1, 2, 3])
>>> print(lst)
[None, 1, 2, 3]

I want to get

[1, 2, 3]

Is there any way to convert None to [] in the function, without changing the default None values?

solid.py · Answer 1 · 2020-12-28T14:51:45.317

Instead of None you could use [] as a default argument for each var.

def my_func(var1 = [], var2 = []):
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]
    return var1 + var2 
        
lst = my_func(var2=[1, 2, 3])
print(lst)

However as user mkrieger1 suggested in the comments it might be a bad idea as seen in this question "Least Astonishment" and the Mutable Default Argument so instead you can do this:

def my_func(var1 = None, var2 = None):
    var1 = [] if var1 is None else var1
    var2 = [] if var2 is None else var2

    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]
    return var1 + var2 

    
lst = my_func(var2=[1, 2, 3])
print(lst)

Output:

[1, 2, 3]

Cid · Accepted Answer · 2020-12-28T15:19:42.193

You can check first if var (1 and 2) are Not None

def my_func(var1=None, var2=None):
    var1 = var1 if var1 is not None else []
    var2 = var2 if var2 is not None else []
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]

    return var1 + var2

This will cover many cases, such as :

print(my_func(var2=[1, 2, 3]))
print(my_func(var1=None,var2=[1, 2, 3]))
print(my_func(var1=0,var2=[1, 2, 3]))
print(my_func(var1=False,var2=[1, 2, 3]))
print(my_func(var1='',var2=[1, 2, 3]))

[1, 2, 3]                                                                                                                                                                                                                                       
[1, 2, 3]                                                                                                                                                                                                                                       
[0, 1, 2, 3]                                                                                                                                                                                                                                    
[False, 1, 2, 3]                                                                                                                                                                                                                                
['', 1, 2, 3]

Federico Baù · Answer 3 · 2020-12-28T14:53:49.703

Adding to @solif.py answer in case you want to keep the None value (hence the default value is not a mutable object) then run a simple test as so:

def my_func(var1=None, var2=None):
    if not var1:
        var1 = []
    if not var2:
        var2 = []
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]

    return var1 + var2


lst = my_func(var2=[1, 2, 3])
print(lst)

Regarding function's parameters when are Mutable or not mutable check “Least Astonishment” and the Mutable Default Argument

EDIT:

After further review your code, you can actually just change the if not isinstance(var1, list) line as so:

def my_func(var1=None, var2=None):

    if not isinstance(var1, list):
        var1 = []
    if not isinstance(var2, list):
        var2 = []
    

    return var1 + var2


lst = my_func(var2=[1, 2, 3])
print(lst)

Other solution

NOTE @Cid answer basically changes the argument var1 or var2 only if is equal to None hence the following can be done:

def my_func(var1=None, var2=None):

    if var1 == None:
        var1 = []
    if var2 == None:
        var2 = []
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]
    return var1 + var2


lst = my_func(var1=False, var2=[1, 2, 3])
print(my_func(var2=[1, 2, 3]))
print(my_func(var1=None,var2=[1, 2, 3]))
print(my_func(var1=0,var2=[1, 2, 3]))
print(my_func(var1=False,var2=[1, 2, 3]))
print(my_func(var1='',var2=[1, 2, 3]))

output

[1, 2, 3]                                                                                                                                                                                                                                       
[1, 2, 3]                                                                                                                                                                                                                                       
[0, 1, 2, 3]                                                                                                                                                                                                                                    
[False, 1, 2, 3]                                                                                                                                                                                                                                
['', 1, 2, 3]

score -2 · Answer 4 · edited Jan 26 '21 at 21:16

-2

This is how I would remove all None from a list:

def my_func(var1=None, var2=None):
    lst = []
    c = 0
    
    if not isinstance(var1, list):
        var1 = [var1]
    if not isinstance(var2, list):
        var2 = [var2]
    lst =  var1 + var2

    for i in lst:
        if i == None:
            lst.pop(c)
        c += 1

    return lst
    

lst_ = my_func(var2=[1, 2, 3])
print(lst_)

I use a for loop to iterate each item in a list. If the item == None, the code removes it.

edited Jan 26 '21 at 21:16

halfer

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answered Dec 28 '20 at 14:42

The Pilot Dude

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I don't think is this... – Federico Baù Dec 28 '20 at 14:46
I mean it does work and give the desired output, doesn't it? – The Pilot Dude Dec 28 '20 at 15:20
it does, but is not what is asking, check the other answer and you should understand what he asking about – Federico Baù Dec 28 '20 at 15:25

Most Pythonic way to convert None to an empty list in case a default argument is not passed in a Function?

4 Answers4

Other solution