Using pandas.tseries.offsets.Week.is_year_end to Identify Year-End Weeks

Data Offsets in pandas

pandas provides powerful tools for working with time series data, including mechanisms for representing and manipulating dates and times. Data offsets are a crucial part of this functionality. They allow you to specify how to move a date or datetime object by a certain amount of time relative to its current position.

Week Offset

The Week class in pandas.tseries.offsets represents an offset of one or more weeks. It's used to add or subtract weeks from a date or datetime object.

is_year_end Method

The is_year_end method is specifically designed for the Week offset. It takes a timestamp (date or datetime object) as input and returns a boolean value:

• False
  If the week containing the timestamp does not occur on the last week of the year.
• True
  If the week containing the timestamp falls on the last week of the year (considering the week as defined by the Week offset's parameters, typically starting on Sunday).

Example

import pandas as pd

# Create a datetime object
dt = pd.to_datetime('2023-12-31')  # Last day of 2023 (assuming Sunday as start of week)

# Create a Week offset
week_offset = pd.offsets.Week()

# Check if the week containing dt is the year-end week
is_year_end_week = week_offset.is_year_end(dt)

print(is_year_end_week)  # Output: True (assuming Sunday as start of week)

Key Points

• The determination of whether a week is the year-end week depends on the Week offset's definition of a week (typically starting on Sunday). If you use a different week start day, the behavior might change.
• is_year_end only works with the Week offset, not other offset types like MonthEnd or YearBegin.

• If you need to check for year-end based on a different definition of a week (e.g., starting on Monday), you might need to create a custom offset or use alternative logic.
• pandas documentation for Week.is_year_end might not explicitly mention its dependence on the week start day, but it's an important factor to consider for accurate results.

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Example 1: Checking for Year-End Week with Monday Start

This code assumes a week starts on Monday:

import pandas as pd

# Create a datetime object
dt = pd.to_datetime('2023-12-25')  # Assuming Monday as start of week

# Create a Week offset with Monday start
week_offset_mon = pd.offsets.Week(weekday=calendar.MONDAY)

# Check if the week containing dt is the year-end week (Monday start)
is_year_end_week_mon = week_offset_mon.is_year_end(dt)

print(is_year_end_week_mon)  # Output: False (since 2023-12-25 is not the last Monday of 2023)

Example 2: Checking for Year-End Week with Custom Logic

This code defines a custom function to check if a week is the year-end week, regardless of the offset definition:

import pandas as pd
from datetime import datetime, timedelta

def is_year_end_week_custom(dt):
    """
    Checks if a datetime object falls in the last week of the year.

    Args:
        dt (datetime): The datetime object to check.

    Returns:
        bool: True if dt is in the last week of the year, False otherwise.
    """

    year_start = dt.replace(month=1, day=1)
    year_end = dt.replace(month=12, day=31)

    # Calculate the number of weeks between year start and dt
    weeks_since_year_start = (dt - year_start) // timedelta(days=7)

    # Check if dt falls in the last week of the year (considering 52 weeks in a year)
    return weeks_since_year_start == 52

# Example usage
dt = pd.to_datetime('2023-12-31')  # Any date
is_year_end_week_custom_result = is_year_end_week_custom(dt)

print(is_year_end_week_custom_result)  # Output: True

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1. • If you only care about checking for the exact year-end date (not necessarily within a week), you can use the YearEnd offset:

     import pandas as pd
     
     dt = pd.to_datetime('2023-12-31')
     year_end_offset = pd.offsets.YearEnd()
     
     # Check if dt is on the year-end date
     is_year_end = dt == dt + year_end_offset
     
     print(is_year_end)  # Output: True
     

2. Combining Week Offset with Comparison
   

   • For more flexibility, combine a Week offset with comparison operators:

     import pandas as pd
     
     dt = pd.to_datetime('2023-12-30')
     week_offset = pd.offsets.Week()
     year_end = pd.to_datetime('2023-12-31')  # Explicit year-end date
     
     # Check if dt is within the last week of the year (assuming Sunday start)
     is_last_week_of_year = (dt + week_offset) == year_end
     
     print(is_last_week_of_year)  # Output: True (assuming Sunday as start of week)
     

3. Custom Function with Date Arithmetic
   

   • Create a custom function using date arithmetic to handle specific week start days:

     import pandas as pd
     from datetime import datetime
     
     def is_year_end_week_custom(dt, week_start=calendar.SUNDAY):
         """
         Checks if a datetime object falls in the last week of the year.
     
         Args:
             dt (datetime): The datetime object to check.
             week_start (int, optional): The day of the week considered as the start
                                          of a week. Defaults to calendar.SUNDAY.
     
         Returns:
             bool: True if dt is in the last week of the year, False otherwise.
         """
     
         year_end = dt.replace(month=12, day=31)
     
         if dt.month == 12 and dt.day == year_end.day:
             return True  # Exact year-end date
     
         # Calculate days remaining in the current year
         days_left_in_year = (year_end - dt).days
     
         # Calculate remaining days until next week start (considering week_start)
         days_to_next_week_start = (week_start - dt.weekday()) % 7
     
         return days_left_in_year <= days_to_next_week_start  # Within last week
     
     # Example usage (assuming Monday start)
     dt = pd.to_datetime('2023-12-25')
     is_year_end_week_custom_result = is_year_end_week_custom(dt, week_start=calendar.MONDAY)
     
     print(is_year_end_week_custom_result)  # Output: False