[Python] how to pandas fast nested for loop for "non numeric" columns?

how to pandas fast nested for loop for "non numeric" columns? because this for loop is way to slow:

for i in range(len(df1[column_A]):
for j in range(len(df2[column_A]):
if df1[column_A] == df2[column_A][j]:
df1[column_B] = df2[column_B][j]
else:
pass


so any other way to do it by pandas itself or other libraries?

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