Note: You may find it easier to complete this lesson if you have already completed the previous lesson in this series.
In this lesson you will learn how to manipulate text files using Python.
This includes opening, closing, reading from, and writing to
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.
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.
You will use
# 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
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
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.
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:
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
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
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
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
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.
Note: You are now prepared to move on to the next lesson in this series.
William J. Turkel and Adam Crymble , "Working with Text Files in Python," Programming Historian, (2012-07-17), http://programminghistorian.org/lessons/working-with-text-files