# Grouping¶

A common need is bound to arise where you will need to look at an aggregate view of a DataFrame by a certain value. This is where grouping comes in.

CashBox has asked that we produce a list of the top 10 users who have been on the receiving side of transactions the most. They would like to see the user's first and last name, their email, and the total number of transactions where the user was the receiver.

We can acheive this by grouping our data. We need to take a look at the transactions DataFrame and group it by the receiver field, which is the username.

Grouping on a value returns a new type of object called the GroupBy.

Let's first explore how to create one of these, and then how to wield it's power.

In [1]:
# Setup
import os

import pandas as pd

pd.options.display.max_rows = 10
# Sanity check
(len(users), len(transactions))

Out[1]:
(475, 998)

Let's remind ourselves about the types of data we have in the transactions DataFrame.

In [2]:
transactions.dtypes

Out[2]:
sender        object
amount       float64
sent_date     object
dtype: object

Grouping by a specific column is pretty straight forward. We want to group by the receiver, so we use the DataFrame.groupby method.

In [3]:
grouped_by_receiver = transactions.groupby('receiver')

# Let's see what type of object we got back

Out[3]:
pandas.core.groupby.groupby.DataFrameGroupBy

We received a DataFrameGroupBy object. There are quite a few methods here.

Let's take a look first at GroupBy.size. This will return a Series of how many members are in each of the groups. In our case this is the number of transactions that each user received.

In [4]:
# Returns a Series of total number of rows

Out[4]:
receiver
aaron            6
acook            1
..
wilson           2
wking            2
wright3590       4
young            2
zachary.neal     4
Length: 410, dtype: int64

Similarly, we can use the DataFrameGroupBy.count method to see counts of how many non missing data points we have across each column in our group across the columns of our DataFrame.

In [5]:
grouped_by_receiver.count()

Out[5]:
sender amount sent_date
aaron 6 6 6
acook 1 1 1
... ... ... ...
wilson 2 2 2
wking 2 2 2
wright3590 4 4 4
young 2 2 2
zachary.neal 4 4 4

410 rows × 3 columns

The GroupBy object provides aggregate functions that makes getting calculations quick and seamless. For instance, if we use the GroupBy.sum method we can see each numeric column summed up for each grouping. In our case there is only one numeric column amount.

In [6]:
grouped_by_receiver.sum()

Out[6]:
amount
aaron 366.15
acook 94.65
... ...
wilson 44.39
wking 74.07
wright3590 195.45
young 83.57
zachary.neal 186.01

410 rows × 1 columns

Now where were we? Oh right, we're trying to figure out the people who was received the most transactions. So why don't we use this group to create a new column on our users DataFrame.

In [7]:
# Create a new column in users called transaction count, and set the values to the size of the matching group

# Not every user has made a transaction, let's see what kind of missing data we are dealing with
len(users[users.transaction_count.isna()])

Out[7]:
65

Since we don't have a transaction record for everyone, not every user will be in our grouping. So when we created the new column, we ended up adding some np.nan. Let's fix that.

In [8]:
# Set all missing data to 0, since in reality, there have been 0 received transactions for this user
users.transaction_count.fillna(0, inplace=True)
users

Out[8]:
first_name last_name email email_verified signup_date referral_count balance transaction_count
aaron Aaron Davis [email protected] True 2018-08-31 6 18.14 6.0
acook Anthony Cook [email protected] True 2018-05-12 2 55.45 1.0
... ... ... ... ... ... ... ... ...
wilson Robert Wilson [email protected] False 2018-05-16 5 59.75 2.0
wking Wanda King [email protected] True 2018-06-01 2 67.08 2.0
wright3590 Jacqueline Wright [email protected] True 2018-02-08 6 18.48 4.0
young Jessica Young [email protected] True 2018-07-17 4 75.39 2.0
zachary.neal Zachary Neal [email protected] True 2018-07-26 1 39.90 4.0

475 rows × 8 columns

Check it out! There's our column, but it's a floating point number, we don't need that. Let's convert it!

In [9]:
# Convert from the default type of float64 to int64 (no precision needed)
users.transaction_count = users.transaction_count.astype('int64')


Finally we want to get the user with the highest transaction count, so let's sort by that descending.

In [10]:
# Sort our values by the new field descending (so the largest comes first), and then by first name ascending
users.sort_values(
['transaction_count', 'first_name'],
ascending=[False, True],
inplace=True
)
# Take a look at our top 10 receivers, showing only the columns we want

Out[10]:
first_name last_name email transaction_count
scott3928 Scott NaN [email protected] 9
sfinley Samuel Finley [email protected] 8