Python Datetime Methods: Date, Timedelta

Use the datetime module. Parses date strings and perform other useful tasks.

Datetime. In amber an insect is preserved for millions of years. A volcano erupts. A meteor hits earth. Existence in amber is unchanging.

In the current time (and of no concern to the insect) we use Python's datetime module to handle dates. This module parses strings containing dates.

Parse. To parse we have the strptime method in datetime. The name is confusing—it comes from the C standard library. 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. Python program that uses strptime 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

Exact date. Suppose you know the exact date you want to represent in a Python datetime. We can use a special constructor to get this date—no parsing is required.

Python program that uses datetime for exact date from datetime import datetime # This is 20017, march 3, at 5:30 in the morning. # ... Hope you like getting up early. result = datetime(2017, 3, 1, 5, 30) print("DATETIME", result) # Verify the month. print("MONTH", result.month) Output DATETIME 2017-03-01 05:30:00 MONTH 3

Yesterday. This is always the current date minus one day. In Python we compute this with timedelta. This type resides in the datetime module.

Here: 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.

Python program that returns yesterday 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. This is the best day to do an annoying task. It is computed in the same way as yesterday. We add a timedelta of one day to the current day. Here we use a shorter method body.

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

Python program that gets tomorrow 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. A list of dates can be sorted. Suppose a program has an unordered list of dates. Often we will need to order them chronologically, from first to last (or in reverse).List

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

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.

Python program that sorts date list 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. 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.

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

Python program that uses timedelta 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

Timedelta arguments. 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.

Note: Large units like years, and small units, like nanoseconds, are not included in the Timedelta calls.

Timedelta arguments, smallest to largest microseconds, milliseconds, seconds, minutes, hours, days, weeks

File, timestamps. This program uses the os.path and date modules. It gets the access, modification and creation of time of a file. You will need to change the file name to one that exists.os.path

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

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

Tip: I verified that the three dates are correct in this program according to Windows 8.1.

Python program that gets timestamps, converts to dates 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. It is easy to get a range of dates. Suppose we have a start date and want the next 10 days. 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.

Range

Python program that gets future dates, range 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

Time. With this method we get a number that indicates the total number of seconds in the time. An epoch is a period of time. For UNIX time the epoch begins at year 1970.

So: The seconds returned by this program equal 46 years—the example is being tested in 2016.

Quote: Return the time in seconds since the epoch as a floating point number.

Time: Python.org

Python program that uses time method import time # Get current seconds. current = time.time() print("Current seconds:", current) Output Current seconds: 1471905804.3763597

Struct_time. To get detailed times, we use a method like gmtime() and then access parts of the returned struct_time. Here we access the year, month, hour and other properties.

Days: Month days is the index of the day in the current month. So the 22nd of a month has "mday" of 22—this begins at index 1 not 0.

Also: Weekdays and year days are indexes within those larger time ranges. Monday is "wday" 0.

Python program that uses struct_time import time # Get a struct indicating the current time. current = time.gmtime() # Get year. print("Year:", current.tm_year) # Get month. print("Month:", current.tm_mon) # Get day of month. print("Month day:", current.tm_mday) # Get hour. print("Hour:", current.tm_hour) # Get minute. print("Minute:", current.tm_min) # Get seconds. print("Second:", current.tm_sec) # Get day of week. print("Week day:", current.tm_wday) # Get day of year. print("Year day:", current.tm_yday) # Get whether daylight saving time. print("Is DST:", current.tm_isdst) Output Year: 2016 Month: 8 Month day: 22 Hour: 22 Minute: 48 Second: 8 Week day: 0 Year day: 235 Is DST: 0

Benchmark, 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.

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

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

Result: This optimization clearly improves the performance of accessing a date. But it comes with serious limitations.

Python program that benchmarks cached date import time from datetime import date # Time 1. print(time.time()) # Version 1: accesses today in a loop. i = 0 while i < 100000: t = date.today() if t.year != 2013: raise Exception() i += 1 # Time 2. print(time.time()) # Version 2: accesses today once. i = 0 t = date.today() while i < 100000: if t.year != 2013: raise Exception() i += 1 # Time 3. print(time.time()) Output 1361485333.238 1361485333.411 Loop 1 = 0.173 1361485333.435 Loop 2 = 0.024

Filename, date.today. We can generate files with the current date in the filename. This is an effective approach for logging programs (or any program that is run once per day).Filename, Date

Some research. Human beings like to make things as complicated as possible. This is true with dates and times. We apply political concepts like daylight saving time.

Quote: An aware object has sufficient knowledge of applicable algorithmic and political time adjustments, such as time zone and daylight saving time information.... A naive object does not....

Datetime: Python.org

A review. The Python environment has 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. Certain aspects of time handling, such as computing calendar dates by offsets, is best left to sophisticated libraries.

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Python Datetime Methods: Date, Timedelta

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