Mastering Pandas DataFrame Last Method

Introduction

In this lab, we will walk through an example of using the Pandas DataFrame last() method. The last() method allows us to select the last few rows of time series data based on a date offset. This can be useful when working with dataframes that have dates as the index. The method returns the dataframe with the selected rows, and it raises a TypeError if the index is not a DatetimeIndex.

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Create a DataFrame with dates as the index

First, let's create a DataFrame with dates as the index. This will allow us to use the last() method to select rows based on dates. We will use the date_range() function from Pandas to create a range of dates.

import pandas as pd

## Create a range of dates
dates = pd.date_range(start='2021-01-01', end='2021-01-10')

## Create a DataFrame with dates as the index
df = pd.DataFrame({'A': [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]}, index=dates)

## Display the DataFrame
print(df)

Select the last few rows based on a date offset

Now that we have our DataFrame with dates as the index, let's use the last() method to select the last few rows based on a date offset. The offset can be specified in various ways, such as a string or a DateOffset object. For example, we can select the last 3 days by passing '3D' as the offset.

## Select the last 3 days
last_3_days = df.last('3D')

## Display the selected rows
print(last_3_days)

Handle a non-DatetimeIndex

If the index of the DataFrame is not a DatetimeIndex, the last() method will raise a TypeError. To handle this, we can check the type of the index before calling the method.

import pandas as pd

## Create a DataFrame with a non-DatetimeIndex
df = pd.DataFrame({'A': [1, 2, 3, 4, 5]}, index=[1, 2, 3, 4, 5])

## Check if the index is a DatetimeIndex
if isinstance(df.index, pd.DatetimeIndex):
    ## Select the last 3 rows
    last_3_rows = df.last(3)
    print(last_3_rows)
else:
    print("The index is not a DatetimeIndex.")

Summary

In this lab, we learned how to use the Pandas DataFrame last() method to select the last few rows of time series data based on a date offset. We saw how to create a DataFrame with dates as the index, and how to use the last() method to select rows based on a date offset. We also learned how to handle a non-DatetimeIndex when using the last() method. This method can be useful when working with time series data and needing to select specific periods of time.

Pandas DataFrame Last Method