Text files¶
Files are everywhere in the digital universe. When you write a Python program, you save your code in a file. Starting an app with your VR eye implants will launch a cloud service that is also contained in a file. Even your operating system consists of a mountain of files. Working with files is an essential part of life as a software engineer. So today we will learn how to read and write text files. And by the way, the lesson you are now reading is stored in a file.
Read¶
There are two kinds of files: text and binary. Text files are generally human readable data, like plain text, XML, JSON or even source code. Binary files are used for storing compiled code, app data, and media files, like images, audio and video files.
To open a file in Python, you use the built-in open function. This is a powerful and versatile function. There are pages of documentation on all ways you can use this function. Today we will focus on the file and the mode.
Begin by making a sample text file using your favourite editor. What better example than a brief biography of Guido van Rossum, the creator of Python. This text file will be named guido_bio.txt. To save time, I am pasting the text from Guido’s personal web site:
Guido van Rossum is the creator of the Python programming language. He grew
up in the Netherlands and studied at the University of Amsterdam, where he
graduated with a Master's Degree in Mathematics and Computer Science. His
first job after college was as a programmer at CWI, where he worked on the
ABC language, the Amoeba distributed operating system, and a variety of
multimedia projects. During this time he created Python as side project. He
then moved to the United States to take a job at a non-profit research lab
in Virginia, married a Texan, worked for several other startups, and moved
to California. In 2005 he joined Google, where he obtained the rank of Senior
Staff Engineer, and in 2013 he started working for Dropbox as a Principal
Engineer. In October 2019 he retired. Until 2018 he was Python's BDFL
(Benevolent Dictator For Life), and he is still deeply involved in the
Python community. Guido, his wife and their teenager live in Silicon Valley,
where they love hiking, biking and birding.
Guido's home on the web is http://www.python.org/~guido/.
Now save and let’s start python and read it. One way to open a file is to call the open function with the name or path to the file. Without any other arguments, it will open the file in read mode and return a file object which we call file. To read the contents of the file, call the read() method. This will return all the text in the file. Finally, close the file.
>>> file = open("guido_bio.txt")
>>> text = file.read()
>>> file.close()
>>> print(text)
Guido van Rossum is the creator of the Python programming language. He grew
up in the Netherlands and studied at the University of Amsterdam, where he
graduated with a Master's Degree in Mathematics and Computer Science. His
first job after college was as a programmer at CWI, where he worked on the
ABC language, the Amoeba distributed operating system, and a variety of
multimedia projects. During this time he created Python as side project. He
then moved to the United States to take a job at a non-profit research lab
in Virginia, married a Texan, worked for several other startups, and moved
to California. In 2005 he joined Google, where he obtained the rank of Senior
Staff Engineer, and in 2013 he started working for Dropbox as a Principal
Engineer. In October 2019 he retired. Until 2018 he was Python's BDFL
(Benevolent Dictator For Life), and he is still deeply involved in the
Python community. Guido, his wife and their teenager live in Silicon Valley,
where they love hiking, biking and birding.
Guido's home on the web is http://www.python.org/~guido/.
>>>
We can print the text to see that everything worked. This method of opening, reading and closing a file is simple. It only took three lines. But there is a danger. What if something goes wrong before you close the file? You do not want to litter your operating system’s memory with open files. Because of this, there is a better and safer way that is also 33% shorter. This method is to open a file using the with keyword. As before, the open function will create a file object and assign it to the name you specify. You can then read the contents like before, but with this technique you do not need to close the file. That is done for you. In fact, Python will close the file even if an exception occurs in the code block. That is first class service, indeed.
>>> with open("guido_bio.txt") as file:
... contents = file.read()
...
>>> print(contents)
Guido van Rossum is the creator of the Python programming language. He grew
up in the Netherlands and studied at the University of Amsterdam, where he
graduated with a Master's Degree in Mathematics and Computer Science. His
first job after college was as a programmer at CWI, where he worked on the
ABC language, the Amoeba distributed operating system, and a variety of
multimedia projects. During this time he created Python as side project. He
then moved to the United States to take a job at a non-profit research lab
in Virginia, married a Texan, worked for several other startups, and moved
to California. In 2005 he joined Google, where he obtained the rank of Senior
Staff Engineer, and in 2013 he started working for Dropbox as a Principal
Engineer. In October 2019 he retired. Until 2018 he was Python's BDFL
(Benevolent Dictator For Life), and he is still deeply involved in the
Python community. Guido, his wife and their teenager live in Silicon Valley,
where they love hiking, biking and birding.
Guido's home on the web is http://www.python.org/~guido/.
>>>
Just to be sure, print the text to check that everything worked.
But what happens if you try to open a file that does not exist? Then Python will raise a file not found error. One way to handle this is to wrap the file code in a try block, where we can handle a file not found exception. For example if the file does not exist, you might want text to store the value None.
try:
with open ("lottery_numbers.txt") as f:
text = f.read()
except FileNotFoundError:
text = None
print(text)
If you run this it displays None, since the file was not found. And if you replace the file name with the one we created earlier (“guido_bio.txt”), it successfully reads the existing file.
If you don’t provide any argument to the read() method, it will read the entire content of the file in memory. Typically, for text files this is the recommended method. But if you know you need to handle huge files (larger than your computer memory for example) you may want to read and process the text file line by line. Luckily Python file objects already have a couple of methods, please have a look at the official file documentation.
For reading lines from a file, you can loop over the file object. This is memory efficient, fast, and leads to simple code:
>>> for line in f:
... print(line, end='')
...
This is the first line of the file.
Second line of the file
>>>
If you want to read all the lines of a file in a list you can also use the list() constructor:
>>> all_lines = list(f)
>>>
or the readlines() method:
>>> all_lines = f.readlines()
>>>
Write¶
Let us now see how to write a text file. For an example, we will write the names of the five oceans to a txt file. In this case, you need an additional argument to the open() function: mode “w” for write. If you do not specify a mode, Python will open the file in read mode by default. To write the names of the oceans to the file, loop over the list and use the write() method. Remember, when using the with technique for opening files, Python will automatically close the files for you.
oceans = ["Pacific", "Atlantic", "Indian", "Arctic", "Southern"]
with open ("oceans.txt", "w") as f:
for ocean in oceans:
f.write(ocean)
Here, if the file oceans.txt does not exist, Python will create it. And if the file already exists, Python will overwrite it. So be careful.
Let’s open the file and see if this worked.
PacificAtlanticIndianArcticSouthern
While it did write the ocean name, there are no line separators. That is because we did not write one, and Python does not assume we want them. One way to fix this is two write a new line after writing the name of each ocean.
oceans = ["Pacific", "Atlantic", "Indian", "Arctic", "Southern"]
with open ("oceans.txt", "w") as f:
for ocean in oceans:
f.write(ocean)
f.write("\n")
Now, if you run the code, and then open the file, you will see each name is on a separate line.
Pacific
Atlantic
Indian
Arctic
Southern
There is another way to ensure that each string is on a separate line. You can use the print() function and have it to print to the file by using the file keyword argument.
oceans = ["Pacific", "Atlantic", "Indian", "Arctic", "Southern"]
with open ("oceans.txt", "w") as f:
for ocean in oceans:
print(ocean, file=f)
If you run this, and look at the file, you get the same result. Each name is on a separate line.
Pacific
Atlantic
Indian
Arctic
Southern
Append¶
Notice that every time we ran our code, any text in the file was overwritten. This is because we open the file in write mode. But what if you would like to write to a file without overwriting any existing text?
For this, you open the file in append mode by using an “a” after the file name. Append mode will create the file if it does not exist, but if the file does exist, then Python will append your text to the end. It will not overwrite any existing text.
oceans = ["Pacific", "Atlantic", "Indian", "Arctic", "Southern"]
with open ("oceans.txt", "w") as f:
for ocean in oceans:
print(ocean, file=f)
with open("oceans.txt", "a") as f:
print("=======================", file=f)
print("These are the 5 oceans.", file=f)
Let us see that this is the case.
Pacific
Atlantic
Indian
Arctic
Southern
=======================
These are the 5 oceans.