Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Introduction to Python Programming

14.4 Handling exceptions

Introduction to Python Programming14.4 Handling exceptions

Learning objectives

By the end of this section you should be able to

  • Describe two exceptions that may occur when reading files.
  • Write try/except statements that handle built-in exceptions.

Runtime errors

Various errors may occur when reading a file:

  • FileNotFoundError: The filename or path is invalid.
  • IndexError/ValueError: The file's format is invalid.
  • Other errors caused by invalid contents of a file.

When an error occurs, the program terminates with an error message.

Example 14.4

Typo in a file

A file named food_order.txt has the following contents:

    5 sandwiches
    4 chips
    1 pickle
    soft drinks
    

The following program expects each line of the file to begin with an integer:

    for line in open("food_order.txt"):
        space = line.index(" ")
        qty = int(line[:space])
        item = line[space+1:-1]
        print(qty, item)
    

Unfortunately, the line "soft drinks" does not begin with an integer. As a result, the program terminates and displays an error message:

    Traceback (most recent call last):
      File "food_order.py", line 3
        qty = int(line[:space])
    ValueError: invalid literal for int() with base 10: 'soft'
    

Concepts in Practice

Common exceptions

What error might occur in each situation?

1.
The line "soft drinks" is changed to "3-4 soft drinks".
  1. FileNotFoundError
  2. IndexError
  3. ValueError
2.
for line in open("food_order.text"):
  1. FileNotFoundError
  2. IndexError
  3. ValueError
3.
parts = line.split()
qty = parts[0]
item = parts[1]
  1. FileNotFoundError
  2. IndexError
  3. ValueError

Exploring further

The Built-in Exceptions page of the Python Standard Library explains the meaning of each exception.

Try and except

Programs can be designed to handle exceptions, rather than terminate. A try statement runs code that might raise an exception. An except clause runs code in response to the exception.

Example 14.5

Try to open a file

The following program, named try_open.py, asks the user for a filename and counts the number of lines in the file.

    name = input("Enter a filename: ")
    try:
        file = open(name)
        lines = file.readlines()
        count = len(lines)
        print(name, "has", count, "lines")
    except FileNotFoundError:
        print("File not found:", name)
    print("Have a nice day!")
    

When running this program with the input try_open.py, the name of the program file, the output is:

    Enter a filename: try_open.py
    try_open.py has 9 lines
    Have a nice day!
    

If the filename does not exist, a FileNotFoundError is raised on line 3. The program then jumps to the except clause on line 7 and continues to run. The resulting output is:

    Enter a filename: try_open.txt
    File not found: try_open.txt
    Have a nice day!
    

Concepts in Practice

Predicting output with exceptions

For each code snippet, what is the output?

4.
word = "one"
try:
  number = int(word)
  print(word, "equals", number)
except ValueError:
  print(word, "is not a number")
  1. one equals 1
  2. one is not a number
  3. ValueError: invalid literal for int() with base 10: 'one'
5.
word = "one"
try:
  number = int(word)
  print(word, "equals", number)
except IndexError:
  print(word, "is not a number")
  1. one equals 1
  2. one is not a number
  3. ValueError: invalid literal for int() with base 10: 'one'
6.
word = "one"
try:
  char = word[3]
  print("The char is", char)
except:
  print("That didn't work")
  1. The char is e
  2. That didn't work
  3. IndexError: string index out of range

Try It

Type analyzer

Analysis programs often need to find numbers in large bodies of text. How can a program tell if a string like "123.45" represents a number? One approach is to use exceptions:

  1. Try converting the string to an integer. If no ValueError is raised, then the string represents an integer.
  2. Otherwise, try converting the string to a float. If no ValueError is raised, then the string represents a float.
  3. Otherwise, the string does not represent a number.

Implement the get_type() function using this approach. The provided main block calls get_type() for each word in a file. get_type() should return either "int", "float", or "str", based on the word. The output for the provided data.txt is:

    str: Hello
    int: 100
    str: times!
    float: 3.14159
    str: is
    str: pi.
    

Try It

United countries

Write a program that prompts the user to input a word and a filename. The program should print each line of the file that contains the word. Here is an example run of the program (user input in bold):

United
    Enter a filename: countries.csv
    United Arab Emirates,9890402,83600,118
    United Kingdom,67886011,241930,281
    United States of America,331002651,9147420,36
    
    Enter a word: United
    Enter a filename: countries.csv
    United Arab Emirates,9890402,83600,118
    United Kingdom,67886011,241930,281
    United States of America,331002651,9147420,36
    

This example uses a file named countries.csv based on the alphabetical list of countries from Worldometer. Each line of the file includes a country's name, population, land area, and population density, separated by commas.

The user might incorrectly type the filename (Ex: countries.txt instead of countries.csv). Your program should output an error message if the file is not found, and keep prompting the user to input a filename until the file is found:

countries
    File not found: countries
    Enter a filename: countries.txt
    File not found: countries.txt
    Enter a filename: countries.csv
    ...
    
    ...
    Enter a filename: countries
    File not found: countries
    Enter a filename: countries.txt
    File not found: countries.txt
    Enter a filename: countries.csv
    ...
    

Hint: Try to open the file specified by the user. A FileNotFoundError is raised if the filename is invalid.

Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/introduction-python-programming/pages/1-introduction
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/introduction-python-programming/pages/1-introduction
Citation information

© Feb 26, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.