5 Best Ways to Write a Python Program to Print the Day of the Year in a Given Date Series

💡 Problem Formulation: Developers often need to calculate the position of a specific date within the year, commonly known as the “day of the year.” For example, January 1st is always day 1, and December 31st is day 365 (or 366 in a leap year). This article provides solutions for generating the day of the year when given a series of dates in Python. The input example could be a list of date strings such as ["2023-01-01", "2023-12-31"] and the desired output would be the corresponding day of the year for each date, such as [1, 365].

Method 1: Using the datetime Module

The datetime module in Python provides a timedelta class, which can be used to calculate the difference between a given date and the beginning of the year. By extracting the .timetuple().tm_yday attribute, it returns the day of the year for any date object.

Here’s an example:

from datetime import datetime

date_series = ["2023-01-01", "2023-12-31"]
for date_str in date_series:
    date_obj = datetime.strptime(date_str, '%Y-%m-%d')
    print(date_obj.timetuple().tm_yday)

Output:

1
365

This code uses datetime.strptime to parse the string dates into datetime objects. Then, it retrieves the day of the year using .timetuple().tm_yday and prints the result.

Method 2: Calculating Day of Year with pandas

For data science applications, the pandas library facilitates operations on date series. By converting the date strings to DatetimeIndex, one can directly use the .dayofyear property to compute the day of the year for each date in a series.

Here’s an example:

import pandas as pd

date_series = pd.to_datetime(["2023-01-01", "2023-12-31"])
day_of_year_series = date_series.dayofyear
print(day_of_year_series.tolist())

Output:

[1, 365]

This snippet first converts a list of date strings to pandas’ DatetimeIndex. Subsequently, pandas’ inherent .dayofyear attribute provides the day of year for each date, outputting the result as a list.

Method 3: Using List Comprehension with the datetime Module

List comprehension in Python offers a more Pythonic and less verbose way of performing operations on iterables. When combined with the datetime module, it allows for a compact expression to calculate the day of the year for a series of dates.

Here’s an example:

from datetime import datetime

date_series = ["2023-01-01", "2023-12-31"]
days_of_year = [datetime.strptime(date, '%Y-%m-%d').timetuple().tm_yday for date in date_series]
print(days_of_year)

Output:

[1, 365]

The list comprehension iterates through each date in the series, converts it to a datetime object, and then retrieves the day of the year, resulting in a compact list of these values.

Method 4: Custom Function Utilizing Ordinal Days

One can write a custom function in Python that converts a date into its ordinal form, which represents the number of days since January 1st of year 1, and then finds the day of the year by calculating the difference with the ordinal number of the first day of the same year.

Here’s an example:

from datetime import datetime

def get_day_of_year(date_str):
    date_obj = datetime.strptime(date_str, '%Y-%m-%d')
    return (date_obj - datetime(date_obj.year, 1, 1)).days + 1

date_series = ["2023-01-01", "2023-12-31"]
print([get_day_of_year(date) for date in date_series])

Output:

[1, 365]

This function parses the date, computes the difference in days from the start of the year, and corrects the offset (since .days starts from zero) to return the ordinal day of the year.

Bonus One-Liner Method 5: Using map Function with datetime

For those who are fans of functional programming in Python, the map function can be used alongside the datetime module to apply a lambda function that extracts the day of the year for each date string in one line of code.

Here’s an example:

from datetime import datetime

date_series = ["2023-01-01", "2023-12-31"]
days_of_year = list(map(lambda date: datetime.strptime(date, '%Y-%m-%d').timetuple().tm_yday, date_series))
print(days_of_year)

Output:

[1, 365]

The map function applies a lambda that takes each date string, converts it to a datetime object, and then gets the day of the year, returning an iterable that is then converted into a list.

Summary/Discussion

Method 1: The datetime module. Straightforward and standard. May be slow for large series. Method 2: pandas library. Great for data processing. Overkill for simple task if pandas is not already being used. Method 3: List comprehension. Pythonic and concise. Efficiency similar to Method 1. Method 4: Custom function. Flexible and reusable. Slightly more complex implementation. Method 5: map function. Functional programming approach. Less readable for those not familiar with the style.