1. Printing and parsing datetimes
Much like dates, datetimes can be printed in many ways. Python also has another trick: you can take a string and turn it directly into a datetime. Let's start with printing dates and then move on to asking Python to parse them.
2. Printing datetimes
First, let's create a datetime again. dt corresponds to December 30, 2017 at 15:19:13, the end of the last trip that W20529 takes in our data set.
Just like with date objects, we use strftime() to create a string with a particular format. First, we'll just print the year, month and date, using the same format codes we used for dates. % capital Y stands for the four digit year, % lowercase m for the month, and % lowercase d for the day of the month.
Now we can add in the hours, minutes and seconds. Again, we print the year, month and day, and now we add three more format codes: % capital H gives us the hour, % capital M gives us the minute, and % capital S gives us the seconds. There are also format codes for 12-hour clocks, and for printing the correct AM or PM.
3. Printing datetimes
As before, we can make these formatting strings as complicated as we need. Here's another version of the previous string.
4. ISO 8601 Format
Finally, we can use the isoformat() method, just like with dates, to get a standards-compliant way of writing down a datetime. The officially correct way of writing a datetime is the year, month, day, then a capital T, then the time in 24 hour time, followed by the minute and second. When in doubt, this is a good format to use.
5. Parsing datetimes with strptime
We can also parse dates from strings, using the same format codes we used for printing. You'll use this often when getting date and time data from the Internet since dates and times are often represented as strings.
We start, as before, by importing the datetime class.
6. Parsing datetimes with strptime
The method we're going to use is called strptime(), which is short for string parse time. strptime() takes two arguments: the first argument is the string to turn into a datetime, and the second argument is the format string that says how to do it.
7. Parsing datetimes with strptime
First we pass the string we want to parse. In this case, a string representing December 30, 2017, at 15:19:13.
8. Parsing datetimes with strptime
Then we pass the format string, which as mentioned before uses the same format codes we used with strftime(). In this case, first the month, then the day, then the year, all separated by slashes, then a space, and then the hour, minutes, and seconds separated by colons. You usually need to figure this out once per data set.
9. Parsing datetimes with strptime
If we look and see what kind of object we've made, by printing the type of dt, we see that we've got a datetime. And if we print that datetime, we get a string representation of the datetime. We can see that the parsing worked correctly.
10. Parsing datetimes with strptime
We need an exact match to do a string conversion. For example, if we leave out how to parse the time, Python will throw an error. And similarly, if there is an errant comma or other symbols, strptime() will not be happy.
11. Parsing datetimes with Python
Finally, there is another kind of datetime you will sometimes encounter: the Unix timestamp. Many computers store datetime information behind the scenes as the number of seconds since January 1, 1970. This date is largely considered the birth of modern-style computers.
To read a Unix timestamp, use the datetime-dot-fromtimestamp() method. Python will read your timestamp and return a datetime.
12. Printing and parsing datetimes
We've just covered how to use format codes to turn datetimes into strings and strings into datetimes, and what ISO 8601 format looks like with time involved. Now you'll practice moving back and forth between strings and datetimes.