Can't append Column Names of Dataframe To A List Using For Loop

Question

I have a Dataframe like this:-

data = [['a', 'b', 'c', 'd'],['q', 'r', 's', 't'],['n'],['w', 'x', 'y', 'z']]
df = pd.DataFrame(data, columns = ['Full_1', 'Full_2', 'Full_3', 'Full_4'])

Now I want to append the columns of a dataframe which contains 'None' value using for loop inside a function

lst=[]
def lister(df):
    for c in df.columns:
        if (df[c].isna().max())==True:
            lst.append(c)
            return lst
        else:
            nope = 'None'
            return nope

It returns me 'None' intsead of lst

Now If I print c Inside of for loop ie

lst=[]
def lister(df):
    for c in df.columns:
        if (df[c].isna().max())==True:

            print(c)
            #return lst
        else:
            nope = 'None'
            #return nope

Output of c inside for loop:-

Full_2
Full_3
Full_4

So Why these values not appending in list named lst?

Expected output of lst :-

['Full_2','Full_3','Full_4']

Answer 1

>>> df.columns[df.isna().any()].to_list()
['Full_2', 'Full_3', 'Full_4']

Edit : update your function like this.

def lister(df):
    lst = []
    for c in df.columns:
        if (df[c].isna().max()) == True:
            lst.append(c)
    return lst

>>> lister(df)
['Full_2', 'Full_3', 'Full_4']

Answer 2

You're initialising the list every time.

So it resets to an empty list inside the if statement.

Move your lst=[] line outside of the for loop.