July 17, 2012

Working with Text Files in Python

    Reviewed by Miriam Posner and Jim Clifford

Note: You may find it easier to complete this lesson if you have already completed the previous lesson in this series.

Lesson Goals

In this lesson you will learn how to manipulate text files using Python. This includes opening, closing, reading from, and writing to .txt files using programming.

The next few lessons in this series will involve downloading a web page from the Internet and reorganizing the contents into useful chunks of information. You will be doing most of your work using Python code written and executed in Komodo Edit.

Working with Text Files

Python makes it easy to work with files and text. Let’s begin with files.

Creating and Writing to a Text File

Let’s start with a brief discussion of terminology. In a previous lesson (depending on your operating system: Mac Installation, Windows Installation, or Linux Installation), you saw how to send information to the “Command Output” window of your text editor by using Python’s print command.

print('hello world')

The Python programming language is object-oriented. That is to say that it is constructed around a special kind of entity, an object, which contains both data and a number of methods for accessing and altering that data. Once an object is created, it can interact with other objects.

In the example above, we see one kind of object, the string “hello world”. The string is the sequence of characters enclosed by quotes. You can write a string one of three ways:

message1 = 'hello world'
message2 = "hello world"
message3 = """hello
hello
hello world"""

The important thing to note is that in the first two examples you can use single or double quotes / inverted commas, but you cannot mix the two within one string.

For instance, the following are all wrong:

message1 = "hello world'
message2 = 'hello world"
message3 = 'I can't eat pickles'

Count the number of single quotes in message3. For that to work you would have to escape the apostrophe:

message3 = 'I can\'t eat pickles'

Or, rewrite the phrase as:

message3 = "I can't eat pickles"

In the third example, the triple quotes signify a string that covers more than one line.

Print is a command that prints objects in textual form. The print command, when combined with the string, produces a statement.

You will use print like this in cases where you want to create information that needs to be acted upon right away. Sometimes, however, you will be creating information that you want to save, to send to someone else, or to use as input for further processing by another program or set of programs. In these cases you will want to send information to files on your hard drive rather than to the “Command Output” pane. Enter the following program into your text editor and save it as file-output.py.

# file-output.py
f = open('helloworld.txt','w')
f.write('hello world')
f.close()

In Python, any line that begins with a hash mark (#) is known as a comment and is ignored by the Python interpreter. Comments are intended to allow programmers to communicate with one another (or to remind themselves of what their code does when they sit down with it a few months later). In a larger sense, programs themselves are typically written and formatted in a way that makes it easier for programmers to communicate with one another. Code that is closer to the requirements of the machine is referred to as low-level, whereas code that is closer to natural language is high-level. One of the benefits of using a language like Python is that it is very high level, making it easier for us to communicate with you (at some cost in terms of computational efficiency).

In this program f is a file object, and open, write and close are file methods. In other words, open, write and close do something to the object f which is in this case defined as a .txt file. This is likely a different use of the term “method” than you might expect and from time to time you will find that words used in a programming context have slightly (or completely) different meanings than they do in everyday speech. In this case recall that methods are bits of code which perform actions. They do something to something else and return a result. You might try to think of it using a real-world example such giving commands to the family dog. The dog (the object) understands commands (i.e., has “methods”) such as “bark”, “sit”, “play dead”, and so on. We will discuss and learn how to use many other methods as we go along.

f is a variable name chosen by us; you could have named it just about anything you like. In Python, variable names can be made from upper- and lowercase letters, numbers and underscores…but you can’t use the names of Python commands as variables. If you tried to name your file variable “print” for example, your program would not work because that is a reserved word that is part of the programming language.

Python variable names are also case-sensitive, which means that foobar, Foobar and FOOBAR would all be different variables.

When you run this program, the open method will tell your computer to create a new text file helloworld.txt in the same folder as you have saved the file-output.py program. The w parameter says that you intend to write content to this new file using Python.

Note that since both the file name and the parameter are surrounded by single quotes you know they are both stored as strings; forgetting to include the quotation marks will cause your program to fail.

On the next line, your program writes the message “hello world” (another string) to the file and then closes it. (For more information about these statements, see the section on File Objects in the Python Library Reference.)

Double-click on your “Run Python” button in Komodo Edit to execute the program (or the equivalent in whichever text-editor you have decided to use: e.g., click on the “#!” and “Run” in TextWrangler). Although nothing will be printed to the “Command Output” pane, you will see a status message that says something like

`/usr/bin/python file-output.py` returned 0.

in Mac or Linux, or

'C:\Python27\Python.exe file-output.py' returned 0.

in Windows.

This means that your program executed successfully. If you use File -> Open -> File in your Komodo Edit, you can open the file helloworld.txt. It should contain your one-line message:

Hello World!

Since text files include a minimal amount of formatting information, they tend to be small, easy to exchange between different platforms (i.e., from Windows to Linux or Mac or vice versa), and easy to send from one computer program to another. They can usually also be read by people using a text editor like Komodo Edit.

Reading From a Text File

Python also has methods which allow you to get information from files. Type the following program into your text editor and save it as file-input.py. When you click on “Run” to execute it, it will open the text file that you just created, read the one-line message from it, and print the message to the “Command Output” pane.

# file-input.py
f = open('helloworld.txt','r')
message = f.read()
print(message)
f.close()

In this case, the r parameter is used to indicate that you are opening a file to read from it. Parameters let you choose among the different options a particular method allows. Returning to the family dog example, the dog may be trained to bark once when he gets a beef-flavoured snack and twice when he gets a chicken-flavoured one. The flavour of the snack is a parameter. Each method is different in terms of what parameters it will accept. You cannot, for example, ask the dog to sing an Italian opera – unless your dog is particularly talented. You can look up the possible parameters for a particular method on the Python website, or often you can find them by typing the method into a search engine along with “Python”.

Read is another file method. The contents of the file (the one-line message) are copied into message, which is what we’ve decided to call this string, and then the print command is used to send the contents of message to the “Command Output” pane.

Appending to a Pre-Existing Text File

A third option is to open a pre-existing file and add more to it. Note that if you open a file and use the write method, the program will overwrite whatever might have been contained in the file. This isn’t an issue when you are creating a new file, or when you want to overwrite the contents of an existing file, but it might be undesirable when you are creating a log of events or compiling a large set of data into one file. So, instead of write you will want to use the append method, designated by a.

Type the following program into your text editor and save it as file-append.py. When you run this program it will open the same helloworld.txt file created earlier and append a second “hello world” to the file. The ‘\n’ stands for new line.

# file-append.py
f = open('helloworld.txt','a')
f.write('\n' + 'hello world')
f.close()

After you have run the program, open the helloworld.txt file and see what happened. Close the text file and re-run file-append.py a few more times. When you open helloworld.txt again you should notice a few extra ‘hello world’ messages waiting for you.

In the next section, we will discuss modularity and reusing code.

Suggested Readings

Note: You are now prepared to move on to the next lesson in this series.

About the authors

William J. Turkel is Professor of History at the University of Western Ontario.   Adam Crymble is a lecturer of digital history at the University of Hertfordshire.  

Suggested Citation

William J. Turkel and Adam Crymble , "Working with Text Files in Python," Programming Historian, (17 July 2012), http://programminghistorian.org/lessons/working-with-text-files