Datetime In Python

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This is Day 16 of the #100DaysOfPython challenge.

This post will use the standard datetime library to work with dates. The final project code can be found here.

Prerequisites

1. Familiarity with Pipenv. See here for my post on Pipenv.
2. Familiarity with iPython or interactive shells.

Getting started

Let's create the hello-python-datetimes directory and install the required dependencies.

# Make the `hello-python-datetimes` directory
$ mkdir hello-python-datetimes
$ cd hello-python-datetimes

# Init the virtual environment
$ pipenv --three
$ pipenv install --dev ipython

# Create a folder to place files
$ mkdir src
# Create the required files
$ touch src/datetimes.py

We are now ready to begin exploring the standard datetime library.

Playing around with dates and times

Start the shell with pipenv shell ipython. This will activate the virtual shell and start an iPython interactive shell.

Now we are free to from datetime import date and play around with it:

from datetime import date

date(2021, 8, 4).isocalendar()
# datetime.IsoCalendarDate(year=2021, week=31, weekday=3)

date(2021, 8, 4).isoformat()
# '2021-08-04'

date(2021, 8, 4).ctime()
# 'Wed Aug  4 00:00:00 2021'

date(2021, 8, 4).strftime('%A')
# Wednesday

today = date.today()
# datetime.date(2021, 8, 4)

The above displays some of the common methods of the date class. More information about the date class can be found here.

A lot of what we want to do today is look at how to take some of these methods and get to a point where we can compare dates.

Comparing dates

We want to write a helper method to check if a given date in the format YYYY-MM-DD is before today. Exit the iPython kernel and add the following code to our src/datetimes.py file:

from datetime import date


def is_date_before_today(date_str: str):
    """Check if date is before today

    Args:
        date_str (str): String of a date to pass

    Returns:
        bool: Value of if date is before today
    """
    try:
        date_obj = date.fromisoformat(date_str)
        return date_obj < date.today()
    except Exception:
        return False


print(is_date_before_today("2019-01-01"))
print(is_date_before_today("2022-01-01"))
print(is_date_before_today("2021-08-03"))
print(is_date_before_today("2021-08-04"))

We can use the > and < operators to compare dates.

If we now run python src/datetimes.py we should see the following output:

$ python src/datetimes.py
True # "2019-01-01"
False # "2022-01-01"
True # "2021-08-03"
False # "2021-08-04"

We can see the output reflects a date string that is before today (with today returning False).

Working with now

The datetime module from the datetime library also has a now method that returns the current date and time.

We can use this in combination with strftime to get the current date and time in a string format. Update the code to the following:

from datetime import date, datetime


def is_date_before_today(date_str: str):
    """Check if date is before today

    Args:
        date_str (str): String of a date to pass

    Returns:
        bool: Value of if date is before today
    """
    try:
        date_obj = date.fromisoformat(date_str)
        return date_obj < date.today()
    except Exception:
        return False


now = datetime.now()
now_str = now.strftime('%Y-%m-%d')
print(now_str)
print(is_date_before_today(now_str))

Running python src/datetimes.py will output 2021-08-04 and False respectively.

To learn more about strftime, there is a great blog post here.

Adding and subtracting dates

We can use datetime.timedelta to add and subtract dates.

Update our code to the following:

from datetime import date, datetime, timedelta


def is_date_before_today(date_str: str):
    """Check if date is before today

    Args:
        date_str (str): String of a date to pass

    Returns:
        bool: Value of if date is before today
    """
    try:
        date_obj = date.fromisoformat(date_str)
        return date_obj < date.today()
    except Exception:
        return False


now = datetime.now()
now_str = now.strftime('%Y-%m-%d')
print(now_str)
print(is_date_before_today(now_str))

now_subtract_one_day = now - timedelta(days=2)

now_subtract_one_day_str = now_subtract_one_day.strftime('%Y-%m-%d')
print(now_subtract_one_day_str)
print(is_date_before_today(now_subtract_one_day_str))

now_add_one_day = now + timedelta(days=1)

now_add_one_day_str = now_add_one_day.strftime('%Y-%m-%d')
print(now_add_one_day_str)
print(is_date_before_today(now_add_one_day_str))

Once again we will python src/datetimes.py which will output the following:

$ python src/datetimes.py
2021-08-04
False
2021-08-02
True
2021-08-05
False

Awesome, now we know how to add and subtract from dates!

Summary

Today's post demonstrated how to use the datetime module to compare times and determine the current date.

Dates and times are incredibly important to any temporal applications, and the following posts will spend some time looking further into this and how to write tests for this form of functionality.

Resources and further reading

1. The ABCs of Pipenv
2. Pipenv
3. Project code
4. strftime
5. datetime

Photo credit: pawel_czerwinski

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