5 Best Ways to Get the Week of the Year with Python Pandas

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💡 Problem Formulation: In data analysis, it’s often necessary to categorize or filter data by time periods, and one such categorization involves determining the week of the year for given dates. For example, given a Pandas Series of dates, the goal is to derive a corresponding Series indicating the week number of the year for each date (with the first week as 1). Such functionality is crucial for time series analysis.

Method 1: Using dt.week

One of the straightforward methods to find the week of the year is to use the dt.week accessor on a Pandas Series of datetime objects. This method returns the week ordinal of the year, ranging from 1 to 52 or 53, depending on the year and the end of the week convention. It’s simple, but deprecated in later versions of Pandas in favor of dt.isocalendar().week.

Here’s an example:

import pandas as pd

# Create a Pandas Series with dates
dates = pd.Series(pd.date_range('2020-01-01', periods=3, freq='D'))

# Get the week of the year
weeks = dates.dt.week
print(weeks)

Output:

0    1
1    1
2    1
dtype: int64

This snippet creates a Pandas Series of dates and then uses dt.week to extract the week of the year for each date. Here, all dates belong to the first week of 2020.

Method 2: Utilizing dt.isocalendar().week

With later versions of Pandas, it’s recommended to use dt.isocalendar().week over the deprecated dt.week. The isocalendar() function returns a DataFrame with the year, week, and day according to the ISO 8601 standard. Selecting the ‘week’ column gives us the week of the year.

Here’s an example:

weeks_iso = dates.dt.isocalendar().week
print(weeks_iso)

Output:

0    1
1    1
2    1
Name: week, dtype: UInt32

This code achieves the same result as method 1 but adheres to the ISO standard and does not raise any deprecation warnings. It’s the more up-to-date way to retrieve the week of the year in Pandas.

Method 3: Custom Function with strftime('%U')

A custom function using Python’s strftime('%U') method can also be used to get the week of the year. This method formats the date into a string representing the week number where Sunday is considered the first day of the week.

Here’s an example:

weeks_custom = dates.apply(lambda x: x.strftime('%U'))
print(weeks_custom)

Output:

0     01
1     01
2     01
dtype: object

The apply function is used to apply a lambda function that converts each datetime object into a formatted string representing the week of the year.

Method 4: Applying strftime('%V') for ISO Week Number

For ISO 8601 compliance, where the first week of the year is defined as the one with the year’s first Thursday and weeks starting on Monday, the strftime('%V') method can be employed.

Here’s an example:

weeks_iso_strf = dates.apply(lambda x: x.strftime('%V'))
print(weeks_iso_strf)

Output:

0    01
1    01
2    01
dtype: object

Similar to method 3, this example uses a custom lambda function. However, it employs the strftime('%V') format code for an ISO 8601 week number.

Bonus One-Liner Method 5: Using dt.weekofyear

Although deprecated and not recommended for new code, the Pandas dt.weekofyear is a succinct one-liner to get the week of the year.

Here’s an example:

weeks_deprecated = dates.dt.weekofyear
print(weeks_deprecated)

Output:

0    1
1    1
2    1
dtype: int64

Even though it’s a simple method, it should be avoided in favor of the new dt.isocalendar().week to future-proof your code.

Summary/Discussion

  • Method 1: dt.week. Simple to use. Deprecated in later versions.
  • Method 2: dt.isocalendar().week. Conforms to ISO 8601. Recommended method in newer Pandas versions.
  • Method 3: strftime('%U'). Offers flexibility with custom week definitions. Requires use of apply, which might be slower.
  • Method 4: strftime('%V'). ISO 8601 compatible. Similar to Method 3 but with the ISO standard week definition.
  • Method 5: dt.weekofyear. A simple one-liner. Deprecated and should not be used in new code.