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PCEP-30-02 Module 4: Functions and Exceptions

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Table of Contents

1. Overview & Syllabus
2. User-Defined Functions
   • Defining Functions
   • Function Arguments (Positional, Keyword, Default, Arbitrary)
   • Return Values & The None Keyword
   • Variable Scope & global
   • Recursion
   • Best Practices & Common Pitfalls
   • Exercises
3. Exception Handling
   • Introduction: Errors vs Exceptions
   • Built-in Exceptions Hierarchy
   • Catching Exceptions (try, except)
   • Multiple, Specific, and Generic Exceptions
   • else and finally
   • Raising and Creating Exceptions
   • Example Scenarios for Common Exceptions
   • Best Practices
   • Exercises
4. Practice Problems
5. Summary Table

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1. Overview & Syllabus

Module 4 focuses on:

• Defining, invoking, and organising user-defined functions,
• Function-environment interaction (parameters, name scopes, argument passing),
• Python’s built-in exceptions hierarchy,
• The foundations of exception handling using try, except, else, and finally.

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2. User-Defined Functions

2.1 Defining and Invoking Functions

def say_hello():
            print("Hello!")
        say_hello()  # Output: Hello!
        

• Use the def keyword, function name, and parentheses.
• Indented code block beneath the definition.

2.2 Return Values & None

def get_greeting(name):
            return f"Hello, {name}!"
        result = get_greeting('Alice')
        print(result)  # Output: Hello, Alice!

        def does_nothing():
            pass
        print(does_nothing())  # Output: None
        

2.3 Parameters and Arguments

• Positional arguments: Order matters.
• Keyword arguments: Specify name in function call.
• Default values:

def power(base, exponent=2):
            return base ** exponent
        print(power(4))        # 16
        print(power(2, 8))     # 256
        

• Arbitrary Arguments:

def many(*args):
            return sum(args)
        print(many(1,2,3))     # 6
        

2.4 Variable Scope & global

x = 10
        def set_x():
            global x
            x = 99
        set_x()
        print(x)  # Output: 99
        

2.5 Recursion

def factorial(n):
            if n==0: return 1
            return n * factorial(n-1)
        print(factorial(5))  # Output: 120
        

Recursion Exercise

Write a recursive function to compute the nth Fibonacci number.

2.6 Best Practices

• Use functions to break code into manageable pieces.
• Prefer descriptive names.
• Use docstrings for documentation.
• Avoid using global unless necessary.

2.7 Function Exercises

1. Write a function to count vowels in a string.
2. Write a function that returns both min and max from a list as a tuple.
3. Write a function that prints all keyword arguments passed to it.

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3. Exception Handling

3.1 What are Errors and Exceptions?

• Syntax errors: detected before execution.
• Exceptions: detected at run time.

3.2 Built-in Exceptions Hierarchy (selected)

Exception	Typical Use
BaseException	Base for all exceptions
Exception	Most user exceptions
SystemExit	Raised by sys.exit()
KeyboardInterrupt	Interrupts program
ArithmeticError	Numeric errors
ZeroDivisionError	Divide by zero
IndexError	Sequence index out of range
KeyError	Dict key not found
TypeError	Invalid type
ValueError	Invalid value
FileNotFoundError	File I/O fails
ImportError	Import fails

3.3 Catching Exceptions

try:
            print(10/0)
        except ZeroDivisionError:
            print('Division by zero!')
        

3.4 Multiple, Specific, and Generic Excepts

try:
            f = open("test.txt")
            data = int(f.read())
        except FileNotFoundError:
            print("File not found!")
        except ValueError:
            print("Non-integer in file!")
        except Exception as e:
            print(f"Other error: {e}")
        

3.5 Else and Finally blocks

try:
            x = int(input())
        except ValueError:
            print('Not an integer!')
        else:
            print('You entered:', x)
        finally:
            print('This always runs.')
        

3.6 Raising and Creating Exceptions

def check_positive(x):
            if x < 0:
                raise ValueError("Negative value!")
        check_positive(-5)  # Raises ValueError
        

Custom Exception Example

class CustomError(Exception):
            pass
        try:
            raise CustomError("This is a custom error!")
        except CustomError as e:
            print(e)
        

3.7 Best Exception Practice

• Catch only exceptions you can handle.
• Place more specific excepts before generic ones.
• Avoid bare except: clauses.
• Clean up with finally if needed.

3.8 Exception Exercises

1. Write a function that safely divides two numbers and catches division and type errors.
2. Write code that attempts to read a file and print the number of lines, handling missing file exception.
3. Write a function to parse user input as an integer, handling ValueError gracefully.
4. Practice: Handle both KeyError and IndexError in a function that receives a dictionary and list.

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4. Practice Problems

1. Write a recursive function for the nth triangular number (sum 1 to n).
2. Write a function that returns True if a string is palindrome.
3. Build a function get_element(seq, idx) that safely gets the element at index, handling both IndexError and TypeError (e.g., for dicts and lists).
4. Demonstrate the difference between handling exceptions at function level vs. caller level.
5. Write a function that multiplies all numeric values in an iterable but safely skips non-numeric values.

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5. Summary Table

Concept	Syntax Example	Purpose
Function Definition	def fun(...): ...	Define reusable code block
Function Call	fun(args)	Invoke function
Return Value	return x	Function outputs a value
Default Args	def foo(x=5): ...	Optional parameter
Keyword Args	def bar(a, b=1): ...	Explicit param assignment
*args/**kwargs	def f(*a, **kw): ...	Pass arbitrary args and keyword args
Recursion	def rec(n): return rec(n-1)	Self-calling function
try/except/finally	try: ... except: ... finally: ...	Exception handling and code cleanup
Raising Exception	raise ValueError("msg")	Manually raise an error
Custom Exception	class X(Exception): ...	User-defined exception
Exception Hierarchy	ZeroDivisionError, KeyError, etc	Built-in error types
global	global var	Access/modify global variable inside function

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