14.4 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 1 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'

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 2 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!

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:

Try converting the string to an integer. If no ValueError is raised, then the string represents an integer.
Otherwise, try converting the string to a float. If no ValueError is raised, then the string represents a float.
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.

def get_type(word):
    # TODO return "int", "float", or "str"


if __name__ == "__main__":
    for line in open("data.txt"):
        for word in line.split():
            print(get_type(word), word, sep=": ")

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):


    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:


    ...
    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.

word = input("Enter a word: ")

# Step 1: Prompt for a filename until the file is found.

# Step 2: Look for the word on each line of the file.

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.