14.4: Handling Exceptions
- Page ID
- 117616
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- 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.
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'
What error might occur in each situation?
IndexError
ValueError
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.
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!
For each code snippet, what is the output?
one is not a number
ValueError: invalid literal for int() with base 10: 'one'
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.
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.