5 Best Ways to Create a PeriodIndex and Get the Day of the Month in Python Pandas

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πŸ’‘ Problem Formulation: When working with time series data in Pandas, it’s common to handle periods and extract specific components of dates, such as the day of the month. For example, given a series of periods, our aim is to create a PeriodIndex and retrieve the day component, turning the input '2023-04' into the output 30, representing the last day of April 2023.

Method 1: Using PeriodIndex with ‘D’ Frequency

PeriodIndex in pandas can be used to represent periods of time. By specifying the frequency as ‘D’ for days, you create a range of periods. The .day attribute can then be used to extract the day component from these periods.

Here’s an example:

import pandas as pd

# Create PeriodIndex
period_index = pd.period_range(start='2023-04', periods=1, freq='D')

# Access the day of the month
day_of_month = period_index.day[0]

print(day_of_month)

Output:

1

This snippet creates a PeriodIndex starting from the first day of April 2023, and the day attribute is accessed to get the day of the month. As it starts at day 1, the output is 1.

Method 2: PeriodIndex with ‘M’ Frequency and Day Property

Setting the frequency to ‘M’ in PeriodIndex will generate periods for each month end. Accessing the day property of this PeriodIndex will yield the last day of the month in question.

Here’s an example:

import pandas as pd

# Create PeriodIndex for monthly frequency
monthly_period = pd.period_range(start='2023-04', periods=1, freq='M')

# Get the last day of the month
last_day_of_month = monthly_period.day[0]

print(last_day_of_month)

Output:

30

In this example, a PeriodIndex is created that represents the last day of April 2023. The day property is used to get the last day, returning 30, as April has 30 days.

Method 3: PeriodIndex with ‘M’ Frequency and to_timestamp()

Another way to find the last day of a month is by converting the PeriodIndex with month frequency to timestamps with to_timestamp(). This conversion gives you the exact timestamp which can be further manipulated to get the day of the month.

Here’s an example:

import pandas as pd

# Create PeriodIndex with 'M' frequency
period_idx = pd.period_range(start='2023-04', periods=1, freq='M')

# Convert to timestamp and get the day
day_of_month = period_idx.to_timestamp().day[0]

print(day_of_month)

Output:

30

This code transforms a PeriodIndex into a DateTimeIndex using to_timestamp(), and the day attribute is accessed to yield the day of the timestamp, which is the last day of April 2023.

Method 4: Using Period and its day Attribute

If you’re working with a single period, you can directly create a Period object and access its day attribute to get the day of the month.

Here’s an example:

import pandas as pd

# Create a single Period object
period = pd.Period('2023-04', freq='M')

# Get the last day of the month
last_day = period.day

print(last_day)

Output:

30

This code creates a Period object representing the month of April 2023. The day attribute provides the day of the month which is the last day of April, hence 30.

Bonus One-Liner Method 5: Inline Period Creation and Day Extraction

For a concise one-liner, create a Period object and immediately access the day attribute in one go.

Here’s an example:

import pandas as pd

# Create Period object and get the last day in one line
last_day = pd.Period('2023-04', freq='M').day

print(last_day)

Output:

30

This single line of code performs inline creation of a Period object for April 2023 and extracts the last day of the month, which outputs 30.

Summary/Discussion

  • Method 1: Using PeriodIndex with daily frequency. Simple and able to select beginning dates. Less efficient for end-of-month dates.
  • Method 2: PeriodIndex with monthly frequency. Efficient for end-of-month dates. Doesn’t directly specify the start date within the month.
  • Method 3: Converts PeriodIndex to timestamps. Versatile for other time manipulations. A bit more complex for this specific task.
  • Method 4: Directly using a single Period object. Optimized for individual periods. Not as convenient for a range of dates.
  • Method 5: Inline period creation and extraction. Very concise. May sacrifice readability for compactness.