python - pandas combine two strings ignore nan values

Question

I have two columns with strings. I would like to combine them and ignore nan values. Such that:

ColA, Colb, ColA+ColB
str   str    strstr
str   nan    str
nan   str    str

I tried df['ColA+ColB'] = df['ColA'] + df['ColB'] but that creates a nan value if either column is nan. I've also thought about using concat.

I suppose I could just go with that, and then use some df.ColA+ColB[df[ColA] = nan] = df[ColA] but that seems like quite the workaround.

Answer 1

Call fillna and pass an empty str as the fill value and then sum with param axis=1:

In [3]:
df = pd.DataFrame({'a':['asd',np.NaN,'asdsa'], 'b':['asdas','asdas',np.NaN]})
df

Out[3]:
       a      b
0    asd  asdas
1    NaN  asdas
2  asdsa    NaN

In [7]:
df['a+b'] = df.fillna('').sum(axis=1)
df

Out[7]:
       a      b       a+b
0    asd  asdas  asdasdas
1    NaN  asdas     asdas
2  asdsa    NaN     asdsa