Instantiate variables to None in Python

Question

Why do these two blocks of code not do the same thing?

category = None
prodid_brand = None
prod_type = None
prod_application = None
prod_handletype = None
prod_series = None

I wanted to "clean up" my code by doing the following, but it does not work the same as the code above.

column_list = [category, prodid_brand, prod_type, prod_application, 
               prod_handletype, prod_series]

for col in column_list:
    col = None

Also is there a "cleaner" way to instantiate all the variables than the top block of code.

I doubt it works at all, you've got a list full of NameErrors. You should read nedbatchelder.com/text/names.html; it's not clear why you thought they would do the same thing. — jonrsharpe
– jonrsharpe, Commented May 16, 2017 at 21:53
category = prodid_brand = prod_type = prod_application = prod_handletype = prod_series = None — RobertB
– RobertB, Commented May 16, 2017 at 21:54

Stephen Rauch · Accepted Answer · 2017-05-16 21:54:12Z

6

You can chain the assignments like:

category = prodid_brand = prod_type = None
prod_application = prod_handletype = prod_series = None

answered May 16, 2017 at 21:54

Stephen Rauch♦

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CaptainPlanet · Accepted Answer · 2017-05-16 22:38:45Z

3

The other answers are great ways to more cleanly/efficiently set all the variables to None.

However, to answer this question:

Why do these two blocks of code not do the same thing?

The reason is, with your first line

column_list = [category, prodid_brand, prod_type, prod_application, 
               prod_handletype, prod_series]

you're actually setting the variable at each index in the list equal to each of those variables. So, column_list[0] = category, column_list[1] = prodid_brand, etc.

Then with the next lines

for col in column_list:
    col = None

you just changing the variable at each of those list indexes, and setting them to None (equivalent to column_list[0] = None).

Hence why none of your initial variables (category, prodid_brand, etc) are getting set, and you're ending up with a list of six None values instead.

edited May 16, 2017 at 22:38

answered May 16, 2017 at 22:14

CaptainPlanet

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2 Comments

kindall Over a year ago

You're not changing the list indexes. The list is not changed. You're merely changing the variable col, which has no knowledge of any list elements.

CaptainPlanet Over a year ago

@kindall good point. I updated my answer to be more accurate.

Keifer Sebastian · Accepted Answer · 2017-05-16 21:55:14Z

2

You can assign multiple variables to the same object

var1 = var2 = None

answered May 16, 2017 at 21:55

Keifer Sebastian

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Comments

Shaun · Accepted Answer · 2017-05-16 22:18:53Z

Your question "Why do these two blocks of code not do the same thing?"

Your first block of code is assigning None to variables - aka initializing them

Your second block of code is generating a variable as it iterates through the list column_list - assigning a value to the variable col in your for loop does actually make col evaluate to None but only in that loop (aka it isn't assigning it back to the original list)

If you want to assign variables in the loop -

column_list = [category, prodid_brand, prod_type, prod_application, 
           prod_handletype, prod_series]

for index,value in enumerate(column_list):
    column_list[index] = None

but this will infact give you a list of None's

column_list = [None,None,None,None,None,None]
not
column_list = [category=None, prodid_brand=None, prod_type=None, prod_application=None, prod_handletype=None, prod_series=None]

Looks like maybe you would be better served by a dictionary than a list.

TrueY · Accepted Answer · 2022-02-16 12:59:28Z

0

I use this:

a, b, c, d = (None,) * 4
f, g, h, i = (False,) * 4
print(a, b, c, d, f, g, h, i)

output:

None None None None False False False False

answered Feb 16, 2022 at 12:59

TrueY

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