Python:Pandas size()

The size() method in pandas returns the number of rows or elements in each group created by the groupby() function. It provides a quick way to determine group sizes without applying an aggregation function.

Syntax

DataFrameGroupBy.size()

Parameters:

The size() method doesn’t take any parameters.

Return value:

The size() method returns a Series containing the size (row count) of each group created by groupby().

Example 1: Counting Rows by Group

In this example, a DataFrame of employees is grouped by their department, and size() counts how many employees belong to each department:

import pandas as pd

data = {
    'Department': ['HR', 'IT', 'HR', 'Finance', 'IT', 'Finance'],
    'Employee': ['John', 'Sara', 'Mike', 'Anna', 'Tom', 'Chris']
}
df = pd.DataFrame(data)

group_sizes = df.groupby('Department').size()
print(group_sizes)

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The output of this code is:

Department
Finance    2
HR         2
IT         2
dtype: int64

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Example 2: Using Multiple Grouping Columns

In this example, size() counts the number of members in each combination of team and shift within a dataset:

import pandas as pd

data = {
    'Team': ['A', 'A', 'B', 'B', 'B', 'C'],
    'Shift': ['Day', 'Night', 'Day', 'Night', 'Day', 'Day'],
    'Name': ['John', 'Sara', 'Mike', 'Anna', 'Tom', 'Chris']
}
df = pd.DataFrame(data)

group_sizes = df.groupby(['Team', 'Shift']).size()
print(group_sizes)

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The output of this code is:

Team  Shift
A     Day      1
      Night    1
B     Day      2
      Night    1
C     Day      1
dtype: int64

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Codebyte Example: Counting Transactions Per Product

In this example, size() is used to count how many sales transactions occurred for each product in a store dataset:

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Frequently Asked Questions

1. What is the pandas groupby().size() method?

groupby().size() returns the number of rows in each group created by groupby().

2. What is the purpose of groupby() in pandas?

groupby() splits data into groups based on selected column values to enable aggregation and summarization.

3. What does NaN stand for in pandas?

NaN stands for Not a Number and indicates missing or undefined data.