Handling Python Syntax Errors and Exceptions

Introduction

Welcome to the Python Syntax Errors and Exceptions lab! In this lab, you will learn how to identify and fix syntax errors in your Python code, as well as how to handle exceptions that may be raised during the execution of your code.

Achievements

• Syntax Errors
• Exceptions

Understanding Syntax Errors

A syntax error occurs when the Python interpreter is unable to understand the structure of your code. This can be due to a variety of issues, such as missing parentheses or incorrect indentation.

Open up a new Python interpreter.

python3

Here is an example of a syntax error caused by a missing colon at the end of a for loop:

for i in range(10)
  print(i)

The interpreter will raise a syntax error, and will tell us where the error occurred:

File "<stdin>", line 1
    for i in range(10)
                      ^
SyntaxError: invalid syntax

The caret symbol (^) indicates the location of the syntax error, and the error message tells us that the syntax is invalid.

To fix this syntax error, we simply need to add the colon at the end of the for loop:

for i in range(10):
  print(i)

Now, let's try an example with incorrect indentation:

if True:
print("Hello, World!")

In this example, the print statement is not properly indented under the if statement. To fix this syntax error, we need to indent the print statement correctly:

if True:
  print("Hello, World!")

Handling Exceptions

An exception is an error that occurs during the execution of a program. In Python, we can handle exceptions using the try and except statements.

Here is an example of how to handle a ZeroDivisionError exception:

try:
  x = 10 / 0
except ZeroDivisionError:
  print("Cannot divide by zero!")

In this example, the try block contains code that may raise a ZeroDivisionError exception. If the exception is raised, control is passed to the except block, which handles the exception by printing an error message.

We can also handle multiple exceptions in the same try block using the except statement with multiple exceptions:

try:
  x = 10 / 0
  y = int('abc')
except ZeroDivisionError:
  print("Cannot divide by zero!")
except ValueError:
  print("Invalid input!")

In this example, the try block contains code that may raise a ZeroDivisionError or a ValueError. If a ZeroDivisionError is raised, the except block with the ZeroDivisionError will handle it. If a ValueError is raised, the except block with the ValueError will handle it.

Raising Exceptions

In addition to handling exceptions, we can also raise exceptions in our code using the raise statement.

Here is an example of how to raise a ValueError exception:

def validate_age(age):
  if age < 0:
    raise ValueError("Age cannot be negative.")

try:
  validate_age(-1)
except ValueError as e:
  print(e)

In this example, the validate_age function raises a ValueError exception if the age is negative. The try block calls the validate_age function with a negative age, which causes the ValueError exception to be raised. The except block handles the exception and prints the error message.

Catching All Exceptions

Sometimes, we may want to catch all exceptions that may be raised in a try block. We can do this using the except statement with no exception specified:

try:
  x = 10 / 0
  y = int('abc')
except:
  print("An error occurred!")

In this example, the try block contains code that may raise a ZeroDivisionError or a ValueError. If either exception is raised, it will be caught by the except block, which prints an error message.

the Else Clause

We can use the else clause in a try statement to specify a block of code that should be executed only if no exceptions are raised in the try block:

try:
  x = int(input("Enter a number: "))
except ValueError:
  print("Invalid input!")
else:
  print("The number is", x)

In this example, the try block contains code that may raise a ValueError if the user enters an invalid value. If no exceptions are raised, the else block is executed and the number is printed.

the Finally Clause

We can use the finally clause in a try statement to specify a block of code that should always be executed, whether an exception is raised or not:

try:
  x = int(input("Enter a number: "))
except ValueError:
  print("Invalid input!")
finally:
  print("Exiting program.")

In this example, the try block contains code that may raise a ValueError if the user enters an invalid value. Whether an exception is raised or not, the finally block is always executed and the program is exited.

Summary

In this lab, you learned how to identify and fix syntax errors in your Python code, as well as how to handle exceptions using the try and except statements. You also learned how to raise exceptions in your code using the raise statement, and how to use the else and finally clauses in a try statement.

I hope this lab was helpful and that you feel more confident working with Python syntax errors and exceptions!