Difference between df.reindex() and df.set_index() methods in pandas

You can see the difference on a simple example. Let's consider this dataframe:

df = pd.DataFrame({'a': [1, 2],'b': [3, 4]})print (df)   a  b0  1  31  2  4

Indexes are then 0 and 1

If you use set_index with the column 'a' then the indexes are 1 and 2. If you do df.set_index('a').loc[1,'b'], you will get 3.

Now if you want to use reindex with the same indexes 1 and 2 such as df.reindex([1,2]), you will get 4.0 when you do df.reindex([1,2]).loc[1,'b']

What happend is that set_index has replaced the previous indexes (0,1) with (1,2) (values from column 'a') without touching the order of values in the column 'b'

df.set_index('a')   ba   1  32  4

while reindex change the indexes but keeps the values in column 'b' associated to the indexes in the original df

df.reindex(df.a.values).drop('a',1) # equivalent to df.reindex(df.a.values).drop('a',1)     b1  4.02  NaN# drop('a',1) is just to not care about column a in my example

Finally, reindex change the order of indexes without changing the values of the row associated to each index, while set_index will change the indexes with the values of a column, without touching the order of the other values in the dataframe

Just to add, the undo to set_index would be reset_index method (more or less):

df = pd.DataFrame({'a': [1, 2],'b': [3, 4]})print (df)df.set_index('a', inplace=True)print(df)df.reset_index(inplace=True, drop=False)print(df)

   a  b0  1  31  2  4   ba   1  32  4   a  b0  1  31  2  4

Besides great answer from Ben. T, I would like to give one more example of how they are different when you use reindex and set_index to an index column

import pandas as pdimport numpy as nptestdf = pd.DataFrame({'a': [1, 3, 2],'b': [3, 5, 4],'c': [5, 7, 6]})print(testdf)print(testdf.set_index(np.random.permutation(testdf.index)))print(testdf.reindex(np.random.permutation(testdf.index)))

Output:

• With set_index, when index column (the first column) is shuffled, the order of other columns are kept intact
• With reindex, the order of rows are changed accordingly to the shuffle of index column.

   a  b  c0  1  3  51  3  5  72  2  4  6   a  b  c1  1  3  52  3  5  70  2  4  6   a  b  c2  2  4  61  3  5  70  1  3  5