14.1 Reading from files

Learning objectives

By the end of this section you should be able to

Understand how to open a file using the open() function.
Demonstrate how to read information from a file using read(), readline(), and readlines().

Opening a file

Reading information from and writing information to files is a common task in programming.

Python supports the opening of a file using the open() function.

Using `read()` and reading lines

Python provides functions that can be called on a file object for reading the contents of a file:

The read() function reads the contents of a file and returns a string.
The readline() function reads the next line in a file and returns a string.
The readlines() function reads the individual lines of a file and returns a string list containing all the lines of the file in order.

Example 1 Using read and readlines

A file called input.txt has the following contents:

"""Demonstrating read() and readlines()"""

# Using read()
# Open the file and associate with a file object
infile = open("input.txt")

# Read the contents of the file into a string
str1 = infile.read()

# Print str1
print("Result of using read():")
print(str1)

# Always close the file once done using the file
infile.close()

# Using read()
# Open the file and associate with a file object
infile2 = open("input.txt")

# Read the contents of the file into a string list
str_list = infile2.readlines()

# Printing the third item in the string list.
print("Result of using readlines() and printing the third item in the string list:")
print(str_list[2])

# Always close the file once done using the file
infile2.close()

The code's output is:


    Result of using read():
    12
    55
    5
    91
    Result of using readlines() printing the third item in the string list:
    5

Concepts in Practice

Reading files

Suppose fileobj = open("input.txt") has already been executed for each question below.

input.txt:


    Hello world!
    How are you?

Try It

Reading from a file

Open the file test.txt and print the file's contents.

Try It

Reading from a file line by line

The file input.txt is shown. The file represents a set of integers. Line 1 of the file specifies how many integers follow. Write a program that reads from this file and determines the average of the numbers following the first line.

input.txt

Adapted from Introduction to Python Programming by OpenStax (openstax.org), licensed under CC BY-NC-SA 4.0. Changes were made. License: CC-BY-NC-SA-4.0.