Unit 3: Functions, modules and exception handling

• Built-in functions are always available: print, input, len, type, range, sum, max, min, sorted, abs, round, int, str, list, map, filter, zip, enumerate, open, help.

pythondef area(length, breadth=1):            # default argument
    """Return the area of a rectangle."""  # docstring
    return length * breadth

print(area(5, 4), area(5))              # positional
print(area(breadth=3, length=2))        # keyword arguments

def total(*marks, **info):              # variable-length arguments
    print(info.get("name"), sum(marks))
total(78, 85, 90, name="Riya")

pythondef area(l, b=1):            # default parameter
    return l * b
print(area(5), area(5, 4))   # 5 20

square = lambda x: x * x     # anonymous function
print(square(6))             # 36

count = 0                    # global
def increment():
    global count             # needed to change a global
    count += 1

• Scope: a variable created inside a function is local and exists only during the call; variables at the top level are global.
• assert checks a condition while debugging: assert marks >= 0, "Marks cannot be negative".
• Command-line arguments: import sys; print(sys.argv) — sys.argv[0] is the script name.
• Modules: a .py file of functions; use import mymodule or from mymodule import area.

• Python passes object references ("pass by assignment"). Rebinding a parameter inside a function does not affect the caller; but mutating a mutable object (list, dict) is visible outside.

pythondef change(n, lst):
    n = n + 10          # rebinding: caller's int unchanged (immutable)
    lst.append(4)       # mutation: caller's list changes
x, nums = 5, [1, 2, 3]
change(x, nums)
print(x, nums)          # 5 [1, 2, 3, 4]

pythondef factorial(n):
    return 1 if n <= 1 else n * factorial(n - 1)

def fib(n):
    return n if n < 2 else fib(n - 1) + fib(n - 2)

print(factorial(5), [fib(i) for i in range(8)])   # 120 [0, 1, 1, 2, 3, 5, 8, 13]

• Every recursive function needs a base case; Python's default recursion limit is about 1000 calls (sys.getrecursionlimit()).

• Python resolves names by the LEGB rule: Local, Enclosing, Global, Built-in.

python# mymath.py — a module is any .py file
PI = 3.14159
def circle_area(r):
    return PI * r * r

# main.py
import mymath
print(mymath.circle_area(2))
from mymath import circle_area as ca
print(ca(3))

• if __name__ == "__main__": runs code only when the file is executed directly, not when imported.

• Package: a folder of modules (with an optional __init__.py) imported with dot notation — from college.exams import results. Third-party packages (NumPy, pandas, requests) are installed with pip.

An exception is a run-time error that stops the program unless handled.

pythontry:
    a = int(input()); b = int(input())
    print(a / b)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Enter numbers only")
else:
    print("Division done")               # runs if no exception
finally:
    print("Program finished")            # always runs

raise throws an exception yourself: if age < 0: raise ValueError("Age cannot be negative").

pythonclass InvalidMarksError(Exception):
    """Raised when marks are outside 0–100."""
    pass

def set_marks(m):
    if not 0 <= m <= 100:
        raise InvalidMarksError(f"Marks {m} out of range")
    return m

try:
    set_marks(120)
except InvalidMarksError as e:
    print("Error:", e)
else:
    print("Saved")
finally:
    print("Done")