Python Datetime

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Time

Space and time are connected. But in Python scripts, we are concerned most with simpler aspects of time. With the datetime module, we parse strings containing dates. And we can manipulate time spans.

Tip:Built-in date time libraries are available. They make many programs much easier to write.

Based on:

Python 3

Parse

Format

It is often tricky to parse dates from strings. In Python, we have the strptime method in datetime. The name is confusing. It is best just to memorize it. This method requires two arguments.

Arguments:The first argument is a string containing date information. The second argument is the format string.

Format codes

B: The full month name.
d: The digit of the day of the month.
Y: The four-digit year.

Program that uses strptime: Python

from datetime import datetime

# Input string.
s = "August 16, 2012"

# Use strptime.
d = datetime.strptime(s, "%B %d, %Y")
print(d)

Output

2012-08-16 00:00:00

Yesterday

Shift operator

Yesterday is always the current date minus one day. In Python we compute this with timedelta. This type resides in the datetime module. We introduce a method called yesterday(). It calls today() and then subtracts a timedelta of 1 day.

Tip:A timedelta can be subtracted or added to a date object. In this way, we compute yesterday, today and any other relative day.

Program that returns yesterday: Python

from datetime import date
from datetime import timedelta

def yesterday():
    # Get today.
    today = date.today()

    # Subtract timedelta of 1 day.
    yesterday = today - timedelta(days=1)
    return yesterday

print(date.today())
print(yesterday())

Output

2013-02-21
2013-02-20

Tomorrow

Plus

Tomorrow is computed in the same way as yesterday. We add a timedelta of one day to the current day. This version uses a shorter syntax form. It reduces the number of steps in the method body.

Info:Helper methods, such as tomorrow and yesterday, are a useful abstraction in certain programs. They are reusable.

Program that gets tomorrow: Python

from datetime import date
from datetime import timedelta

def tomorrow():
    # Add one day delta.
    return date.today() + timedelta(days=1)

print(date.today())
print(tomorrow())

Output

2013-02-21
2013-02-22

Sort dates

Sorted letters: A to Z

A list of dates can be sorted. Suppose your Python program has a list of dates and they are unordered. Often we will need to order them chronologically, from first to last (or in reverse). We can sort dates like any other type.

Here:We create a list and append four new dates to it. These are all dates in the future. They are not chronologically ordered.

List

Then:We invoke the sort method on the list. In a for-loop, we display the dates, now ordered from first to last in time.

Program that sorts date list: Python

from datetime import date, timedelta

# Create a list of dates.
values = []
values.append(date.today() + timedelta(days=300))
values.append(date.today() + timedelta(days=2))
values.append(date.today() + timedelta(days=1))
values.append(date.today() + timedelta(days=20))

# Sort the list.
values.sort()

# Display.
for d in values:
    print(d)

Output

2013-10-13
2013-10-14
2013-11-01
2014-08-08

Timedelta

Not equal

No two points in time are the same. In Python we express the difference between two dates with timedelta. To use timedelta, provide the arguments using names. And not all units are supported—we cannot specify years.

Here:In this program, we subtract one hour from one day. And, as you might expect, the result is 23 hours.

Program that uses timedelta: Python

from datetime import timedelta

# This represents 1 day.
a = timedelta(days=1)

# Represents 1 hour.
b = timedelta(hours=1)

# Subtract 1 hour from 1 day.
c = a - b
print(c)

Output

23:00:00
Python language

Next, we consider the possible arguments to timedelta in more detail. You can specify more than argument to timedelta—simply use a comma to separate them.
Large units like years,
and small units,
like nanoseconds,
are not included.

Timedelta arguments
    (From smallest to largest)

microseconds,
milliseconds,
seconds,
minutes,
hours,
days,
weeks

File timestamps

Time

This program uses the os.path module as well as the date module.
It gets the access,
modification
and creation of time of a file. You will need to change the file name to one that exists on your disk. The returned values are timestamps.

Path

But:In many programs we prefer a date type, not a float type. Dates are easier to understand and print.

Float keyword

So:We use the fromtimestamp method from the date module. This converts, correctly, the floats to dates.

I verified that the three dates are correct in this program according to Windows 8.1. The file creation time is later than the modification time because the file was copied. Getctime on some platforms returns a different value.

Program that gets timestamps, converts to dates: Python

from os import path
from datetime import date

# Get access, modification, and creation time.
a = path.getatime("/enable1.txt")
m = path.getmtime("/enable1.txt")
c = path.getctime("/enable1.txt")

# Display the times.
print(a, m, c)

# Convert timestamps to dates.
a2 = date.fromtimestamp(a)
m2 = date.fromtimestamp(m)
c2 = date.fromtimestamp(c)
print(a2, m2, c2)

Output, format edited

1360539846.3326 1326137807.9652 1360539846.3326
2013-02-10      2012-01-09      2013-02-10

Range

Future

It is easy to get a range of dates. Suppose you have a starting date and want to get the next 10 days. Simply loop over 1 through 10, and use timedelta to add that number of days to the original date.

Here:We get today. We then add one to ten days to today. This yields the next 10 days.

Program that gets future dates, range: Python

from datetime import date, timedelta

# Start with today.
start = date.today()
print(start)

# Add 1 to 10 days and get future days.
for add in range(1, 10):
    future = start + timedelta(days=add)
    print(future)

Output

2014-04-21
2014-04-22
2014-04-23
2014-04-24
2014-04-25
2014-04-26
2014-04-27
2014-04-28
2014-04-29
2014-04-30

Cache

Cache

Getting the date,
as with date.today(),
is slow. This call must access the operating system. An easy way to optimize this is to cache dates. If your program must access today(), avoid this call in a tight loop.

Loops

Loop 1:This loop access date.today() once on each iteration through the loop. It runs much slower.

Loop 2:The date.today() call is cached in a variable before the loop runs. This makes each iteration much faster. We hoist the call.

Also:The logic checks the year of the date. This can be changed for the current year.

Program that caches date: Python

import time
from datetime import date

# Time 1
print(time.time())

# Accesses today repeatedly.
i = 0
while i < 100000:
    t = date.today()
    if t.year != 2013:
        raise Exception()
    i += 1

# Time 2
print(time.time())

# Accesses today once.
i = 0
t = date.today()
while i < 100000:
    if t.year != 2013:
        raise Exception()
    i += 1

# Time 3
print(time.time())

Results

1361485333.238
1361485333.411    Loop 1 = 0.173
1361485333.435    Loop 2 = 0.024

Summary

Program

The Python language provides strong support for time handling. These libraries are built into the environment. They do not need to be recreated in each program. This yields faster, more reliable software.

Tip:Certain aspects of time handling, such as computing calendar dates by offsets, is best left to sophisticated libraries.

And:Developing custom libraries is challenging. Many possible problems must be handled—even leap years.

Python