There’s a common warning in pandas about a SettingWithCopyWarning. While the error message covers some of the possible reasons for the error, it doesn’t cover them all. In this post, I’ll show another source of the error and how to fix it.

import os
import pandas as pd
shakespeare_path = os.path.join(os.getenv('DATA'), 'shakespeare.csv')
df = pd.read_csv(shakespeare_path)
Name Year Category
0 Titus Andronicus 1592 Tragedy
1 The Comedy of Errors 1594 Comedy
2 Richard II 1595 History
3 Romeo and Juliet 1595 Tragedy
4 A Midsummer Night’s Dream 1595 Comedy
5 King John 1596 History
6 Julius Caesar 1599 Tragedy
7 Othello 1604 Tragedy
8 Macbeth 1606 Tragedy

Let’s say you want to make a subset of the data by copying a couple columns.

sdf = df[['Name', 'Year']]
Name Year
0 Titus Andronicus 1592
1 The Comedy of Errors 1594
2 Richard II 1595
3 Romeo and Juliet 1595
4 A Midsummer Night’s Dream 1595
5 King John 1596
6 Julius Caesar 1599
7 Othello 1604
8 Macbeth 1606

Then you want to continue cleaning it up.

sdf.loc[:, 'Year'] = pd.to_datetime(sdf['Year'])
C:\Users\Julius\anaconda3\lib\site-packages\pandas\core\ SettingWithCopyWarning: 
A value is trying to be set on a copy of a slice from a DataFrame.
Try using .loc[row_indexer,col_indexer] = value instead

See the caveats in the documentation:
  self._setitem_single_column(ilocs[0], value, pi)

The error message doesn’t make any sense. You’re already doing what they suggest trying. What you need to do to avoid this is to make a complete copy of the DataFrame using .copy().

sdf2 = df[['Name', 'Year']].copy()
sdf2.loc[:, 'Year'] = pd.to_datetime(sdf2['Year'])

No more error message!