Chapter 11: Using Files

11.1 Overview

So far we have always gotten program input from the user of the program typing in to the program window. This works very well for a lot of interactive programs. However, there is another way to get input into a program which is to read it in from a file. There are some cases where this is nice to be able to do:

• It lets you read in large amounts of input which would be tedious to type by hand.
• It lets you test a program more quickly if you don't have to type the input in, and also keeps you from typing it wrong. If you want to re-run the program over and over again this speeds up development.
• Programs can read in their settings from files without needing to ask the user each time.
• Programs can read in files which might not be in a format users can easily enter.

We can also put output into files instead of just to the screen. There are some reasons to do this:

• It lets the user keep the output even after the program is done.
• If it's in a file, our program can read it in the next time it's run.

If you think about it, a lot of the programs you use read and write information to files. When you use a word processor to work on a paper, the program saves your paper to a file. Then when you open that same paper again, it reads the information out of the file again. Likewise when you change a setting in a video game it “remembers” the setting next time you open it. This is done by writing the setting to a file and then reading that same file each time it’s opened.

11.2 Opening and Closing Files

The first step of using a file for input or output is opening it. To do this we call the open function. This takes two parameters. The first is the name of the file, which must be a string.

The second parameter is the mode which is also string and says how the file should be opened. The most common modes are:

For example, we can open an input file like this:

f = open("input.txt", "r")

This will look for a file in the same directory as our program called "input.txt". If the file cannot be found, the program will stop with an error message.

When we are done with a file, we should close it. To do that, we put the file variable, and then ".close()". For instance, we could open an output file, use it (which we will talk about), then close it:

f = open("stuff.txt", "w")

# output things to it

f.close()

Your program will probably work OK if you forget to close a file, but it's good practice to close the file when you are done anyway¹.

11.3 Reading A Line

To read a line from a file, we can use ".readline()". This will return the next line of the file. If we call it multiple times, it will return the next line of the file we have not read yet. Files remember where you are in the file.

f = open("input.txt", "r")

# read the first 2 lines and print them out:
line1 = f.readline()
line2 = f.readline()
print(line1)
print(line2)

f.close()

When a file has been read in its entirety, readline will give you back an empty string "" when you call it.

11.4 Reading All the Lines

Instead of reading each line with readline, we can get the entire file read into a list of strings with "readlines". This will let us read the entire file in one go:

f = open("input.txt", "r")

# read all lines and print them out
lines = f.readlines()
print(lines)

f.close()

Note that when we read lines in from a file (either one at a time or all at once) they have line breaks at the end of them.

11.5 Looping Through Lines

Lots of time with input files we want to go through each line and do something with it. We can do that in Python with just a for loop:

f = open("input.txt", "r")

# print each line
for line in f:
    print(line)

f.close()

This can also be combined with readline. For example, we can read the first line of a file specially, and then use a for loop to go through the remaining lines.

11.6 Printing to a File

To use output files, we can use the same old print function that we have been using. We just need to pass an extra parameter that specifies what file to write to.

We do this by passing "file=f" (where f is whatever we called the file we opened) at the end of the print:

f = open("output.txt", "w")

print("This will go to a file", file=f)

f.close()

11.7 Example: Total Balance

How could we write a program that opens a file of numbers, and adds up the total? The input file should be called "nums.txt" and contain a bunch of integers. The output file should be called "total.txt" and contain the sum.

11.8 On File Paths

When you open a file with open, you must give it the name of the file to open. In the simplest case this is a simple name like “input.txt” and a file with that name exists in the same directory on your computer where you run the program. With Thonny, this is the directory where your program .py code is.

Beginning programmers sometimes struggle with being able to find where files on their computer are located in the file system². If you have issues being able to open files on your computer, you’re not alone.

11.9 Comprehension Questions

1. What is the main purpose of using lists in Python?
2. How can you access the third element in a list named items?
3. If you have a list pets equal to ["dog", "cat", "bird"], how do you add “fish” to the end of the list?
4. What will len(pets) return after executing the append operation in the previous question?
5. Is it more common to loop over a list with a for loop or a while loop?
6. What happens if you try to access an index from a list that does not exist? Like it we try to read pets[99] in the list above?

11.10 Programming Exercises

1. TODO
2. TODO
3. TODO

Footnotes