Puzzle 13 A 10% Discount

Pandas Brain Teasers — by Miki Tebeka (21 / 34)

👈 Multiplying | TOC | A Tale of One City 👉

discount.py

​ ​import​ ​pandas​ ​as​ ​pd​
​ 
​ df = pd.DataFrame([
​     [​'Bugs'​, True, 72.3],
​     [​'Daffy'​, False, 30.7],
​     [​'Tweety'​, True, 23.5],
​     [​'Elmer'​, False, 103.9],
​ ], columns=[​'Customer'​, ​'Member'​, ​'Amount'​])
​ 
​ df[df[​'Member'​]][​'Amount'​] *= 0.9
​ ​print​(df)

Guess the Output

IMPORTANT

Try to guess what the output is before moving to the next page.

This code will print a warning and then

​ Customer  Member  Amount
​ 0     Bugs    True    72.3
​ 1    Daffy   False    30.7
​ 2   Tweety    True    23.5
​ 3    Elmer   False   103.9</code></pre></td>

The change is not reflected in df. The reason is that Pandas does a lot of work under the hood to avoid copying data. However, in some cases it can’t, and then you’ll get a copy of the data.

The warning is very helpful; sadly, a lot of developers ignore it.