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Handling exceptions

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 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.
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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.

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