pandas: populate an empty column in a dataframe with values from multiple dataframes based on similar values in one column

Question

I have a large dataframe that has two columns but with many rows, so this is just an example:

df1 = {"text":["see you in five minutes.", "she is my friend.", "she goes to school in five minutes.","he is my friend.","that is right.","sky is blue.","sky is yellow."],
       "goal":[" "," "," "," "," "," "," "]}

and I also have three other dataframes that are of different sizes but they all have some rows from the text column in df1:

 df2= {"text":["see you in five minutes.", "he is my friend."],
       "second":["num","friend"]}

 df3 = {"text":["she goes to school in five minutes.","she is my friend.","that is right."],
       "third":["num","friend","correct"]}

 df4 = {"text":["sky is blue.","sky is yellow."],
       "fourth":["color","color"]}

what I would like to do is to merge the 'second','third' and 'fourth' column to df1 in a way that they would populate the empty column 'goal' in df1,

 desired output:
 df1 = {"text":["see you in five minutes.", "she is my friend.", "she goes to school in five minutes.","he is my friend.","that is right.","sky is blue.","sky is yellow."],
       "goal":["num","friend","num","friend","correct","color","color"]}

I tried left merge multiple times for each dataframe but the output will appear in a different column. Is there a way of doing it at once and add them to the goal column?

thank you

Answer 1

Create a mapping series m by concatenating the dataframes df2 , df3 , and df4 using pd.concat , then use this mapping series along with Series.map to map the text column in df1 :

m = pd.concat([df.set_index('text').iloc[:, 0] for df in (df2, df3, df4)])
df1['goal'] = df1['text'].map(m)

Result:

# print(df1)
                                  text     goal
0             see you in five minutes.      num
1                    she is my friend.   friend
2  she goes to school in five minutes.      num 
3                     he is my friend.   friend
4                       that is right.  correct
5                         sky is blue.    color
6                       sky is yellow.    color