Reading Files

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Learn to Program: The Fundamentals

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University of Toronto

Learn to Program: The Fundamentals

2435 notes

Behind every mouse click and touch-screen tap, there is a computer program that makes things happen. This course introduces the fundamental building blocks of programming and teaches you how to write fun and useful programs using the Python language.

À partir de la leçon

For Loops Over Indices, Parallel and Nested Lists and Strings, and Files

This module introduces how to use a for loop over the indexes of a list, how to nest lists, and how to read a write files.

Reading Files4:43

Writing Files5:18

Rencontrer les enseignants

Jennifer Campbell
Associate Professor, Teaching Stream
Department of Computer Science
Paul Gries
Associate Professor, Teaching Stream
Department of Computer Science

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.

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.

We'll read each line until we find the line containing nothing but the new line

character. This readline approach allows us to read

only the parts of the file we are interested in and allow us to stop

processing a file as soon as we have found the parts of it that we want.

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.

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