Append an entry of dataframe to a list, if the entry of another column is nan in pandas?

Question

first question here so it might be a little bit messy.

So I have a data frame like this:

           A          B
1:        'a'       'aa'
2:        'b'       NaN
3:        'c'       NaN
4:        'd'       'dd'

And I have a list already created:

lst=[]

I want to append the value in column A to this list if the value of Column B is NaN , aka to get ['b','c'] in this case.

Loops definitely work but is there an elegant way (by using lambdas, for example) to do so?

Thanks!

Answer 1

Use boolean indexing for filtering and str.strip for remove ' :

lst = df.loc[df['B'].isnull(), 'A'].tolist()
print (lst)
["'b'", "'c'"]

---

lst = df.loc[df['B'].isnull(), 'A'].str.strip("'").tolist()
print (lst)
['b', 'c']

Detail:

print (df['B'].isnull())
1:    False
2:     True
3:     True
4:    False
Name: B, dtype: bool

print (df.loc[df['B'].isnull(), 'A'])
2:    'b'
3:    'c'
Name: A, dtype: object

print (df.loc[df['B'].isnull(), 'A'].str.strip("'"))
2:    b
3:    c
Name: A, dtype: object