Exploring Python Pandas: Applying Weekmask on CustomBusinessDay Offset

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πŸ’‘ Problem Formulation: In financial data analysis, it’s common to encounter situations where one has to adjust for custom business days that don’t conform to the typical Monday to Friday workweek. Python’s Pandas library accommodates this need with the CustomBusinessDay class, which allows setting a weekmask. This article illustrates how to get the weekmask applied on the CustomBusinessDay offset, defining custom business days and ensuring the calculations take into account these specified days of the week.

Method 1: Defining Weekmask on CustomBusinessDay Initialization

When initializing CustomBusinessDay, a weekmask can be supplied which represents the days of the week to be considered as valid business days. The weekmask is a string with weekdays abbreviated to Mon, Tue, Wed, Thu, Fri, Sat, Sun with a boolean flag (1 for business days, 0 for non-business days).

Here’s an example:

import pandas as pd

weekmask = 'Mon Wed Fri'
cday = pd.offsets.CustomBusinessDay(weekmask=weekmask)

'Mon Wed Fri'

This snippet demonstrates the creation of a CustomBusinessDay object while specifying Mon Wed Fri as the weekmask. When we print cday.weekmask, it gives us the weekmask string that was originally used during the initialization of the CustomBusinessDay.

Method 2: Using Weekmask with Holidays

One can combine a weekmask with a list of holidays to define even more customized business days. This way, we exclude specific dates along with the non-business days defined in the weekmask.

Here’s an example:

import pandas as pd
from pandas.tseries.holiday import USFederalHolidayCalendar

weekmask = 'Mon Tue Wed Thu Fri'
holidays = USFederalHolidayCalendar().holidays('2020-01-01', '2020-12-31')

cday = pd.offsets.CustomBusinessDay(holidays=holidays, weekmask=weekmask)


In this example, we define a custom business day that excludes weekends and US federal holidays in the year 2020. Importing a holiday calendar and combining it with a weekmask gives us a CustomBusinessDay object that can be used to advance dates in a series while skipping over both weekends and holidays.

Method 3: Modifying an Existing CustomBusinessDay Object

Sometimes you might need to change the weekmask of an already existing CustomBusinessDay object. You can achieve this by modifying its weekmask attribute directly.

Here’s an example:

import pandas as pd

cday = pd.offsets.CustomBusinessDay(weekmask='Mon Tue Wed Thu Fri')
cday.weekmask = 'Tue Wed Thu'

'Tue Wed Thu'

This code snipped modifies the weekmask of cday object from a full workweek to only Tuesday through Thursday. After the assignment, the printed weekmask reflects the updated business days.

Method 4: Creating CustomBusinessDay with Dynamic Weekmask

Sometimes weekmasks may need to be dynamically generated based on certain conditions, for example, based on data retrieved from a database or an API. Once the weekmask is generated, it can be applied to the CustomBusinessDay initializer.

Here’s an example:

import pandas as pd

# Imagine retrieving this dynamically from somewhere
dynamic_weekmask = ' '.join(['Mon', 'Tue', 'Thu', 'Fri'])

cday = pd.offsets.CustomBusinessDay(weekmask=dynamic_weekmask)

'Mon Tue Thu Fri'

This code demonstrates the flexibility of setting a weekmask based on external conditions. The weekmask is generated dynamically in the example and then passed to CustomBusinessDay when creating the object.

Bonus One-Liner Method 5: Lambda Functions for Weekmask Customization

Lambda functions in Python can be a powerful tool to generate a weekmask on-the-fly. This one-liner method can create a CustomBusinessDay object with a concise and dynamic approach.

Here’s an example:

import pandas as pd

cday = pd.offsets.CustomBusinessDay(weekmask=''.join(map(lambda x: x if x in ['Mon', 'Wed', 'Fri'] else '', ['Mon', 'Tue', 'Wed', 'Thu', 'Fri', 'Sat', 'Sun'])))

'Mon Wed Fri'

This clever use of a lambda function maps over a list of all weekdays, adding a day to the weekmask string only if it’s a Monday, Wednesday, or Friday. It’s a succinct and flexible way to specify business days when creating the CustomBusinessDay object.


  • Method 1: Initialization. Straightforward definition during object creation. Limited to static definitions.
  • Method 2: Weekmask with Holidays. Allows for precise control over business days by excluding holidays. Requires holiday data and can be more complex to set up.
  • Method 3: Modify Existing Object. Provides the ability to change the business days of an object post-creation. Only feasible if the object is mutable and can interrupt the flow of logic if used improperly.
  • Method 4: Dynamic Weekmask. Offers flexibility to adapt to external variables or conditions. Adds complexity by requiring additional logic to generate the weekmask.
  • Bonus Method 5: Lambda Functions. A concise, powerful one-liner capable of dynamic weekmask generation. May not be as readable for those less familiar with functional programming concepts.