pandas merge columns to create new column with comma separated values

Question

My dataframe has four columns with colors. I want to combine them into one column called "Colors" and use commas to separate the values.

For example, I'm trying to combine into a Colors column like this :

ID  Black Red  Blue  Green  Colors   
120 NaN   red  NaN   green  red, green  
121 black Nan  blue  NaN    black, blue

My code is:

df['Colors'] = df[['Black, 'Red', 'Blue', 'Green']].apply(lambda x: ', '.join(x), axis=1)

But the output for ID 120 is:

, red, , green

And the output for ID 121 is:

black, , blue,

FOUND MY PROBLEM! Earlier in my code, I replaced "None" with " " instead of NaN. Upon making the change, plus incorporating feedback to insert [x.notnull()], it works!

df['Black'].replace('None', np.nan, inplace=True)
df['Colors'] = df[['Black, 'Red', 'Blue', 'Green']].apply(lambda x: ', '.join(x[x.notnull()]), axis=1)

Answer 1

You just need to handle NaNs

df['Colors'] = df[['Black', 'Red', 'Blue', 'Green']].apply(lambda x: ', '.join(x[x.notnull()]), axis = 1)

    ID      Black   Red Blue    Green   Colors
0   120     NaN     red NaN     green   red, green
1   121     black   NaN blue    NaN     black, blue

Answer 2

Using dot

s=df.iloc[:,1:]
s.notnull()
   Black   Red   Blue  Green
0  False  True  False   True
1   True  True   True  False
s.notnull().dot(s.columns+',').str[:-1]
0         Red,Green
1    Black,Red,Blue
dtype: object

df['color']=s.notnull().dot(s.columns+',').str[:-1]