Update only some values in a dataframe using another dataframe

Question

How to update certain values in one column of a dataframe with values from another dataframe:

# Main dataframe
df1 = pd.DataFrame({'a':[1,2,3,4,5], 'b':['a2', 'a1', '?', '?', 'b2'], 'c':[100,101, 102, 103, 104]})

# Dataframe with new values for column 'b'
df2 = pd.DataFrame({'a':[3,4], 'b':['b5', 'c5']})

# Result: Main dataframe with updated values for 'b', using column 'a' as index/key
pd.DataFrame({'a':[1,2,3,4,5], 'b':['a2', 'a1', 'b5', 'c5', 'b2'], 'c':[100,101, 102, 103, 104]})

`df1 = df1.replace('?', np.nan); df1.set_index('a').combine_first(df2.set_index('a')).reset_index()`? — It_is_Chris, Mar 09 '21 at 20:32
@It_is_Chris worked! Thanks very much. If you care to post as answer, I can accept right away. — user2696565, Mar 09 '21 at 20:35

score 1 · Accepted Answer · answered Mar 09 '21 at 20:38

1

You can replace the ? with numpy.nan and then use pandas' combine_first

df1 = df1.replace('?', np.nan)
df1.set_index('a').combine_first(df2.set_index('a')).reset_index()

answered Mar 09 '21 at 20:38

It_is_Chris

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Update only some values in a dataframe using another dataframe

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