1. Dealing with Runtime Errors
Exception Handling
Dealing with Runtime Errors

2. Exception An exception is an error that occurs at runtime
Types of exceptions:
NameError
Caused by:
Misspelled variable or function name
Use of variable in expression before it is assigned an initial value
TypeError
Operand of incorrect type for operator
ValueError
Passing wrong type of data to function

3. Exception Handling
You can write an exception handler to intercept an exception and prevent your program from crashing.
Use a try - except statement to intercept exceptions:
try:
# guarded statements
except Exception as e:
# deal with the exception

4. Exception Handling Example
How try-except works:
Guarded statements execute one at a time
If an exception occurs while executing a statement, the rest of the guarded statements are skipped, and the exception handler executes
After a try-except statement executes, the program continues normally
try:
ageStr = input('Enter your age:')
age = int(ageStr)
print('You are', age, 'years old.')
except Exception as e:
print('Something went wrong: ', e)

5. Exception Parameter
You can optionally define an exception parameter with “as”:
except Exception as e
The exception parameter contains information about the exception
Print out the exception parameter so that the user knows what went wrong
try: ageStr = input('Enter your age:') age = int(ageStr) print('You are', age, 'years old.') except Exception as e: print('Something went wrong: ', e)

6. Exception-Specific Handling
Different kinds of exceptions occur
ValueError - attempt to convert non- numeric data to int or float
ZeroDivisionError - division by zero
A try-except statement can perform exception-specific handling by defining more than one exception handler
Put the general Exception as e case at the end
Exception handlers are tried in order
The first handler that matches the exception will handle it
try: total = float(input('Enter total amount:')) count = float(input('Enter count:')) avg = total / count print('average =', avg) except ValueError: print('Invalid input') except ZeroDivisionError: print('Can't divide by zero') except Exception as e: print('Something went wrong: ', e)

7. Exception Propagation
An exception that occurs inside a function propagates to its caller if not handled
A single exception handler can catch exceptions that occur anywhere in the program
However, all it can do is a graceful shutdown
Pro tip:
Avoid defining a “master” exception handler like this during program development
Complicates debugging
def square(n): return int(n) * int(n) def main(): num = input('Enter a number:') print(num, 'squared is:', square(num)) try: main() except Exception as e: print('Something went wrong: ', e)

8. Exception Handling in Web Apps
Need to display a clean error message to user
Return an HTML page with a user-friendly error message
Need to log a detailed error message to console
Use traceback module:
import traceback
traceback.print_exc()
Example: examples/webapps/webguess.py