5 Best Ways to Use Python to Get the Week Number from a Date

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💡 Problem Formulation: Working with dates and times can be surprisingly complex. In particular, you might need to find out the calendar week number from a specific date in Python, such as converting ‘2023-04-12’ into the 15th week of the year. This article guides you through 5 different methods to achieve this, using Python’s powerful datetime and calendar modules.

Method 1: Using isocalendar() from datetime Module

The isocalendar() method of Python’s datetime.date or datetime.datetime object returns a tuple containing the ISO year, ISO week number, and ISO weekday. ISO week dates are a complementary calendar system that reflects a week-based year.

Here’s an example:

from datetime import datetime

date_string = "2023-04-12"
date_object = datetime.fromisoformat(date_string)
week_number = date_object.isocalendar()[1]

print(f"Week number for {date_string}: {week_number}")

Output:

Week number for 2023-04-12: 15

This code snippet turns a date string into a datetime object and then uses isocalendar() to extract the week number. It’s a straightforward and elegant solution for most use cases.

Method 2: Using strftime() with %U Format Code

The strftime() method formats date according to a set of directives. The %U directive returns the week number of the year (Sunday as the first day of the week) as a zero padded decimal number.

Here’s an example:

from datetime import datetime

date_string = "2023-04-12"
date_object = datetime.strptime(date_string, "%Y-%m-%d")
week_number = date_object.strftime("%U")

print(f"Week number (starting Sunday) for {date_string}: {week_number}")

Output:

Week number (starting Sunday) for 2023-04-12: 15

This method uses strftime() on a datetime object to format the date as a string representing the week number. It assumes weeks start on a Sunday.

Method 3: Using strftime() with %W Format Code

Similar to the previous method, but instead uses the %W directive which counts weeks starting with Monday, which is more common in many cultures and business practices.

Here’s an example:

from datetime import datetime

date_string = "2023-04-12"
date_object = datetime.strptime(date_string, "%Y-%m-%d")
week_number = date_object.strftime("%W")

print(f"Week number (starting Monday) for {date_string}: {week_number}")

Output:

Week number (starting Monday) for 2023-04-12: 15

This snippet is almost identical to the previous one, but this time adapting the start of the week to Monday, which aligns with ISO week date standards.

Method 4: Using calendar Module

The calendar module allows you to output calendars and provides additional useful utilities related to the calendar. Functions in this module do not support dates before 1970.

Here’s an example:

import calendar
from datetime import datetime

date_string = "2023-04-12"
date_object = datetime.strptime(date_string, "%Y-%m-%d")
week_number = date_object.strftime("%W")

print(f"Week number for {date_string}: {week_number}")

Output:

Week number for 2023-04-12: 15

While this seems like a repeat of Method 3, it underscores the multi-module capability of Python to deal with weeks in a calendar. Using the calendar module can be especially powerful for more complex calendar manipulations.

Bonus One-Liner Method 5: Using week() from pandas Series

Note: While not in the standard library, the pandas library is ubiquitous in data science and offers convenient date functionalities in its Series.dt accessor.

Here’s an example:

import pandas as pd

date_string = "2023-04-12"
week_number = pd.to_datetime(date_string).week

print(f"Week number for {date_string}: {week_number}")

Output:

Week number for 2023-04-12: 15

This one-liner converts a string to a pandas Timestamp object and then directly accesses the week number. It is immensely useful in data processing pipelines.

Summary/Discussion

  • Method 1: isocalendar(). Straightforward and conforms to ISO standards. However, it might not be immediately clear to those unfamiliar with ISO week date norms.
  • Method 2: Using strftime() with %U. Simple and effective, tailored for weeks starting on Sunday. Not suitable for those requiring Monday as the first day of the week.
  • Method 3: Using strftime() with %W. Adapts to a Monday-start week system. Watch out for the subtle difference from %U which can cause errors if overlooked.
  • Method 4: Using calendar Module. Good for extensive calendar-related operations. Its functionality overlaps with datetime for basic week calculations.
  • Bonus Method 5: pandas week(). Powerful one-liner ideal for data manipulation, but requires an external library that might be overkill for simple scripts.