Saving data with timestamps

Overview

• In this lesson we are going to learn how to add a date and time to our data which is sometimes called a timestamp.
• A timestamp records the current time that an event took place.
• This becomes important later when we want to graph the data that we collected.
• Computer settings are important with dates and times.
• When we initially set up our Raspberry Pi, we need to define localisation options to be Melbourne, Australia.
• The computer can then calculate our local time and even make allowances for daylight savings and atomic clock corrections.
• It is also important that the Raspberry Pi is connected to the internet so that it can re-calibrate the onboard clock.
• In python there is a special library called datetime that makes it easy to record and format dates and times.
• In this lesson we will create a new program to get some experience with the datetime library before we add it back into our main program.

Learning Objectives

• Learn how use the python datetime library.
• Learn how to update our main program with the datetime library functions.
• Learn how to verify that our data has saved correctly using the File Manager and Terminal.

Getting the current date and time

• In this example we will experiment with a new python library named datetime.
• We will demonstrate how to get the current computer time, and also learn how to format the response.
• More information on formatting the datetime output can also be obtained from W3Schools at the following URL: Python Dates (w3schools.com)

Create a new file named timestamp.py

• From the Raspberry Pi main menu drop down select Programming > Python3 (IDLE).

• This will open the Python Shell.

• From the File drop down menu select New File.
• Add a comment to the new file - # timestamp.py.
• The file name will be timestamp.py

• From the File dropdown menu select Save As.
• Navigate to your project directory.
• In this case it is botanica-park-lake
• Name the file timestamp.py and click Save.

• Enter the following code:
  • import datetime – library import
  • now = datetime.datetime(now) – creates a new datetime object named now
  • print(now) – printing the now object
• Save and Run the program

• The output in the Shell should show the year, month, day, hour, minute, second and microsecond time components.

Using the strftime() method

• When we create a datetime object (now) we have access to a method for formatting output of the date object into more readable strings.
• The method that does this is strftime().
• To format the datetime object we need to use some special characters called Directives.
• Each Directive has a percentage sign (%) and a single upper or lower-case letter (a-z, A-Z).
  • %Y – full version of year – e.g. 2021
  • %b – month name short version – e.g. Jan, Feb, Mar
  • %d – day of the month 01 to 31 – e.g. 28
  • %H – hour 00 to 24 – e.g. 08
  • %M – minute 00 to 59 – e.g. 48
• To use these Directives to format the date object add the following two lines of code.
  • date_stamp = now.strftime(“%Y”)
  • print(date_stamp)
• The full code is included below.

• Save and Run the program.
• There are two lines of output:
  • 2021-05026 03:45:43... – print out of the now instance of the datetime object
  • 2021 – print out of the now datetime object formatted using the strftime() method to show the Year (%Y).

• We can now keep on adding Directives to achieve our own customised format for the date and time.
• In this example we have added the short version of the month using the (%b) Directive – e.g. Jan.
• To add the full version of the month we would use the (%B) Directive – e.g. January.
• To represent months as numbers 01 to 12 use the (%m) Directive – e.g. 01 (for January).

• Save and Run the program and test some options.

• Add the day of the month (%d).
• The reason we have used this date format separated by hyphens (-), is because this style can be universally read by other python programs.
• In the future we will write a program that will use the data to plot graphs.

• We are interested in capturing data once every hour, so it makes sense to also record the hour and minute that the data was collected.
• Update your code with the example below with the Hour (%H) and Minute (%M) Directives.
• We have used the standard human readable format for Hour and Minute, separated by a colon (:) – e.g. 16:24

• Using the strftime() method we can now obtain an output (date_stamp variable) that has the date and time formatted correctly for other programs to use.

Updating our main program

• When developing a large program, it is easier to break the key functions into separate modules so that they can be tested independently.
• Once they are ready, they can then be merged into the main program.
• We have created and tested our timestamp.py program and shown that it works.
• In this example we will demonstrate how to copy sections of code into our original atm_sensor_get.py program to improve the functionality.

Updating atm_sensor_get.py

• Now that we have created our timestamp.py program and shown that it works, we can now copy sections of code into our original atm_sensor_get.py program
• In the timestamp.py program copy the import datetime line by highlighting the code, right-mouse button click and selecting Copy.

• In the atm_sensor_get.py program, paste the copied code by right-mouse button clicking and selecting Paste.
• Add the code just below the import requests statement.
• We try to keep imports together at the top of the code.
• You should see the import datetime added to your code.

• Go to the timestamp.py program and copy the two lines of code highlighted below.
• You may need to delete some print statements that are not longer needed.