Error Handling¶

The try.. except statement allows your program to catch and handle errors instead of terminating unexpectedly. To manage errors gracefully, you can use try ... except, its basic structure is as follows:

simple Exception¶

try:
   operations;
except ExceptionI:
   If there is ExceptionI, then execute this block.
except ExceptionII:
   If there is ExceptionII, then execute this block.
   ...
else:
   If there is no exception then execute this block. this part arbitary and can be dropt.  
finally: 
   This block always runs.

The following show how to use sys function to find out what error happen, let try a simple mathemtic operation and see what error might happen.

import sys
list_n = ['a',0, 3]
for entry in list_n:
    try:
        print("The entry is", entry)
        r = 1/int(entry)
    except:
        print("Oops!", sys.exc_info()[0], "occurred.")
        print("Next entry.")

You see two error 'ValueError' and 'ZeroDivisionError'....

assert¶

The assert statement is useful for debugging, as it tests a condition in your code. If the condition is not True, the program raises an AssertionError.

while True:
        num = int(input("Enter a number: "))
        assert num % 2 == 0 , "Not an Even number" 
        reciprocal = 1/num
        print(reciprocal)

while True:
    try:
        num = int(input("Enter a number: "))
        assert num % 2 == 0 , "An even number"
    except:
        print("Not an even number!")
        break
    else:
        reciprocal = 1/num
        print(reciprocal)

Exception configuration¶

User can set up and control how exceptions determining what happens when an error occurs, how it is reported.

Exception class¶

We can detect error withous using sys. Since every exception in Python inherits from the base Exception class, we can achieve the same result using the following approach:

import sys
list_n = ['a',0, 3]
for entry in list_n:
    try:
        print("The entry is", entry)
        r = 1/int(entry)
    except Exception as e:
        print("Oops!", e.__class__, "occurred.")
        print("Next entry.")

Try the following which show you can handle the error inside the try. Run the following code with -1, 1, 'a'.

try:
     a = int(input("Enter a positive integer: "))
     if a <= 0:
         raise ValueError("That is not a positive number!")
except Exception as ve:
     print(ve)

IndexError¶

This error occurs when you try to assign a value to an index that is beyond the valid range of indices in the list.

tournaments = ["NAGA", "IBJJF", "EBI"]
while True:
    try:
        tournament = tournaments.pop()
        print(f"I would like to compete in the {tournament} tournament.")
    except IndexError:
        print("There are no more tournaments")
        break

Raise¶

You can define your own types of Exception types:

i_num = int(input("Enter a number: "))
if i_num < number:
    raise RuntimeError(f'{i_num} is small')

You can use raise with try to handle exceptions. Let's consider defining a function that raises an error if the input number is less than or greater than zero. First, define the base class to propagate the error:

class Error(Exception):
    """Base class for other exceptions"""
    pass

Then, specify the errors and pass the exception to them.

class ErrorTooSmall(Error):
    """Raised when the input value is too small"""
    pass

class ErrorTooLarge(Error):
    """Raised when the input value is too large"""
    pass

Now, run the following to

number = 10

while True:
    try:
        i_num = int(input("Enter a number: "))
        if i_num < number:
            raise ErrorTooSmall
        elif i_num > number:
            raise ErrorTooLarge
        print("Congratulations! You guessed it correctly.")
        break
    except ErrorTooSmall:
        print("This value is too small, try again!")
        print()
    except ErrorTooLarge:
        print("This value is too large, try again!")
        print()

Catching Multiple Errors¶

You can catch different types of exceptions by using multiple except blocks.

try:
  ...
except LookupError as e:
  ...
except RuntimeError as e:
  ...
except IOError as e:
  ...
except KeyboardInterrupt as e:
  ...

Alternatively, if the handling statements are the same, you can group the exceptions together:

try:
  ...
except (IOError,LookupError,RuntimeError) as e:
  ...

Catching All Errors¶

To catch any exception, use Exception like this:

try:
    ...
except Exception:       # DANGER. See below
    print('An error occurred')

IOError¶

The following code shows how to handle the recognized IOError in your code.

#!/usr/bin/python

try:
   fh = open("testfile", "w")
   fh.write("This is my test file for exception handling!!")
except IOError:
   print "Error: can\'t find file or read data"
else:
   print "Written content in the file successfully"
   fh.close()