5 Row Index Values

In this chapter we will focus on index operations, including changing and resetting the index.

5.1 Setting the Index

Every DataFrame has a default index values, but we can set our own.

We might do this in practice when one of the columns in the dataset contains values which uniquely identify each row in the dataset.

For example, this products dataset has an “id” column that uniquely represents each row, so we could set that column as the index:

from pandas import read_csv

request_url = "https://raw.githubusercontent.com/prof-rossetti/intro-to-python/main/data/products.csv"
df = read_csv(request_url)
df.head()

	id	name	aisle	department	price
0	1	Chocolate Sandwich Cookies	cookies cakes	snacks	3.50
1	2	All-Seasons Salt	spices seasonings	pantry	4.99
2	3	Robust Golden Unsweetened Oolong Tea	tea	beverages	2.49
3	4	Smart Ones Classic Favorites Mini Rigatoni Wit...	frozen meals	frozen	6.99
4	5	Green Chile Anytime Sauce	marinades meat preparation	pantry	7.99

Setting the “id” column as the index:

# set the index and name it:
df.index = df["id"]
df.index.name = "id"

# drop old column, as necessary and desired:
df.drop(columns=["id"], inplace=True)

df.head()

	name	aisle	department	price
id
1	Chocolate Sandwich Cookies	cookies cakes	snacks	3.50
2	All-Seasons Salt	spices seasonings	pantry	4.99
3	Robust Golden Unsweetened Oolong Tea	tea	beverages	2.49
4	Smart Ones Classic Favorites Mini Rigatoni Wit...	frozen meals	frozen	6.99
5	Green Chile Anytime Sauce	marinades meat preparation	pantry	7.99

5.2 Resetting the Index

If you want to revert a DataFrame to use the default index values (auto-incrementing integers), we can use the reset method:

df.reset_index(inplace=True)
df.head()

	id	name	aisle	department	price
0	1	Chocolate Sandwich Cookies	cookies cakes	snacks	3.50
1	2	All-Seasons Salt	spices seasonings	pantry	4.99
2	3	Robust Golden Unsweetened Oolong Tea	tea	beverages	2.49
3	4	Smart Ones Classic Favorites Mini Rigatoni Wit...	frozen meals	frozen	6.99
4	5	Green Chile Anytime Sauce	marinades meat preparation	pantry	7.99

5.3 Resetting a Multi-Index

If our dataset has a multi-index (like a composite primary key comprised of multiple columns), we can convert the index components to their own columns, and reset the index to be simpler.

Consider this dataset of time-series stock prices from the yahooquery package, where we have a row per stock per date:

from yahooquery import Ticker

symbols = ["MSFT", "AAPL", "GOOGL"]
ticker = Ticker(symbols)
df = ticker.history(start="2024-01-01", end="2024-01-05", adj_ohlc=True)
df

		open	high	low	volume	close
symbol	date
MSFT	2024-01-02	371.085077	373.109944	364.047705	25258600	368.117279
	2024-01-03	366.271049	370.489503	365.774760	23083500	367.849243
	2024-01-04	367.918763	370.330719	364.444741	20901500	365.209015
AAPL	2024-01-02	186.237603	187.521323	182.993502	82488700	184.734970
	2024-01-03	183.321893	184.973804	182.535736	58414500	183.351746
	2024-01-04	181.261998	182.197418	179.998201	71983600	181.023178
GOOGL	2024-01-02	138.049561	138.946304	135.987031	23711200	137.670929
	2024-01-03	136.754262	139.125670	136.584878	24212100	138.418228
	2024-01-04	137.920020	138.657353	135.857505	27137700	135.897354

We see the row index is a gnarly MultiIndex consisting of two values, representing both the “symbol” and the “date”:

df.index

MultiIndex([( 'MSFT', 2024-01-02),
            ( 'MSFT', 2024-01-03),
            ( 'MSFT', 2024-01-04),
            ( 'AAPL', 2024-01-02),
            ( 'AAPL', 2024-01-03),
            ( 'AAPL', 2024-01-04),
            ('GOOGL', 2024-01-02),
            ('GOOGL', 2024-01-03),
            ('GOOGL', 2024-01-04)],
           names=['symbol', 'date'])

Resetting the index:

# moving multi-index to their own separate columns:
df["symbol"] = df.index.get_level_values(0)
df["date"] = df.index.get_level_values(1)

# resetting the index:
df.reset_index(drop=True, inplace=True)
df.head()

	open	high	low	volume	close	symbol	date
0	371.085077	373.109944	364.047705	25258600	368.117279	MSFT	2024-01-02
1	366.271049	370.489503	365.774760	23083500	367.849243	MSFT	2024-01-03
2	367.918763	370.330719	364.444741	20901500	365.209015	MSFT	2024-01-04
3	186.237603	187.521323	182.993502	82488700	184.734970	AAPL	2024-01-02
4	183.321893	184.973804	182.535736	58414500	183.351746	AAPL	2024-01-03

This gives us a flatter structure which may be easier to work with.