0:00
So far, our programs have used computer memory to store information,
But most information is stored in files on your computer.
In this lecture, we'll show you four ways to read a text file.
We will use a file containing, In Flanders Fields, a poem written in 1915 by Canadian
Lieutenant Colonel John McCrae. Before we can read a file, we need to tell
Python to open it. The built-in function open does this.
It takes its arguments, the name of the file, and the letter r that we want to
read from that file as opposed to writing to it.
If the file is in the same directory as the program, then we can just use the
plain file name, But if the file name is in a different
directory, such as the directory that we have up here, we must give the full path
to it. We will now read this file in various
ways. We will save it in a variable, so that we
don't have to keep retyping it. First, we open the file for reading.
When we open a file for reading, we have available to us a method called readline
that reads one line from the file, including the new line character at the
end of that line, and returns it. We can read this entire poem by calling
readline again and again. Notice that even blank lines have new line
characters at the end of them. Now that we have read to the end of the
file, when we call readline again, we get back the empty string.
The only time that readline returns the empty string is when we have reached the
end of the file. That means that we can use a while loop to
read the lines from a file. We will close the file and reopen it so
that we can start at the top. We will read the first line of the file,
and then, until we have reached the end of the file, we will print the line we read
and then move on to, to read the next line.
When we run this loop watch carefully. You're going to see blank lines between
every line of the file. The reason for this is that a readline
will return the entire line including the new line character at the end.
And then when we print that line print adds another new line.
2:36
In order to prevent this from happening, we will use exactly the same format that
we did before, Except then, in our call on the print
function, we will use the empty string as the end marker to print after print prints
the line. Let's now explore how to read just the
first stanza. We read the title, and, the blank line,
and then the first line of the poem. Let's check.
3:40
If we know we would like to read every line in a file there is a simpler way.
Python's for loop works with files retrieving one line at a time, including
the new line. If your file isn't huge, you can actually
read it all at once using method read, Returns the entire file contents as a
single string. The fourth and last way that we will show
you how to read, the contents of a file, is using method readlines, which returns a
list of all the lines in the file. We can use this to print all the lines in
the file. We'll get our list and then go through
that list printing, as we did before. This approach is useful if we want to keep
the list around so that we can access elements of it later.
Jennifer CampbellAssociate Professor, Teaching Stream
Paul GriesAssociate Professor, Teaching Stream
