Reading files

If you have a file, you might want to view its contents. In this topic, you'll learn different ways to read files in Python.

Read file

To read the file, first we need to open it in the reading mode. Now that the file is opened, how does the actual reading part go? Let's first choose a file that we want to read. So, imagine we have a file called 'animals.txt' that looks like this:

Dog
Cat
Rabbit
Sea turtle
Penguin

To read the file, you can:

• use the read(size) method;
• use the readline(size) method;
• use the readlines() method;
• iterate over the lines with a for loop.

The first three ways are special file object methods while the last one is a general Python loop. Let's go over them one by one.

read()

read(size) reads the size bytes of a file. If the parameter isn't specified, the whole file is read into a single variable. So, this is what we'll get if we apply it to our file:

file = open('animals.txt', 'r')
print(file.read())
# The output:
# Dog
# Cat
# Rabbit
# Sea turtle
# Penguin

file.close()

readline()

readline(size) is similar to read(size) but it reads size bytes from a single line, not the whole file. Lines in files are separated by newline escape sequences: '\n', '\r' or '\r\n'. We'll settle on '\n' in this topic. Yet, keep in mind that this escape sequence depends on your operating system.

Let's proceed with our example. The file 'animals.txt' contains 5 lines. Here is what we'll get if we try to read 3 bytes from each line:

file = open('animals.txt', 'r')
print(file.readline(3))
print(file.readline(3))
print(file.readline(3))
print(file.readline(3))
print(file.readline(3))
print(file.readline(3))

# The output:
# Dog
# 
# 
# Cat
# 
# 
# Rab
# bit


file.close()

As you can see, the output does not contain the first three characters from all 5 lines. This is because when we specify the size parameter, we get size bytes from the line until it ends and only then go to another line. The newline character is considered as a part of the line here. So, in our example, we need two passes of readline(3) to read the first line which has three characters and a newline character (4 bytes in total). This is where all those empty lines in the output come from.

readlines()

readlines() allows us to read the whole file as a list of lines. Here's what it looks like:

file = open('animals.txt', 'r')
print(file.readlines())
# The output:
# ['Dog\n', 'Cat\n', 'Rabbit\n', 'Sea turtle\n', 'Penguin']

file.close()

Here you can actually see that the newline character is a part of the line! This method works well when the files are small, but for large files, this may be inefficient.

for loop

The most efficient way to read the contents of a file is to iterate over its lines with for loop.

file = open('animals.txt', 'r')
for line in file:
    print(line)

# The output:
# Dog
# 
# Cat
# 
# Rabbit
# 
# Sea turtle
# 
# Penguin

file.close() 

This is the best way to read large files because we can just work with one line at a time or work with specific lines and discard the rest.

Summary

So, in this topic, we've seen how to read the file's contents. This skill is quite useful because sometimes it's easier or more convenient to read a file using Python than to open it directly. Again, these are the available ways to do it:

• read() method reads the file as a whole;
• readline() reads the file one line at a time;
• readlines() reads the file as a list of lines.
• the best way, however, is to use for loop to iterate over the lines of the file.