1 · Define the function
def starts a function definition. The indented body runs only when the function is called.
Vocabulary
A parameter is the variable named in a definition. An argument is the actual value passed when calling the function.
Functions & Files · Unit 4
User-Defined Functions & Recursion
Parameters, return, scope and recursion
Unit 4 · Reusable logic
A function gives a name to a useful operation
Define a function once, then call it whenever you need that operation. Parameters let a caller provide data; a return value lets the caller use the result. This section builds a function in small runnable steps, then traces recursion and variable scope.
Build a function one idea at a time
Use the step controls to reveal the concept, inspect its code, and run it.
Step 1 of 4
Build and run
Starting up Python…def greet():
print("Welcome to Python lab!")The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Recursion: a function calls itself
Factorial is the product of all positive integers up to n. The recursive rule is factorial(n) = n × factorial(n − 1), with factorial(1) = 1 as the stopping case. A correct recursive function must move toward that base case.
Factorial in Python
Starting up Python…def factorial(n):
if n == 1: # base case
return 1
return n * factorial(n - 1) # recursive case
print(factorial(4))The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Call stack · factorial(4)
Each frame remembers its own n while waiting for the deeper call.
factorial(4) · current
Step 1 of 8
Waiting for factorial(3)
4 is not the base case, so this call asks for 4 × factorial(3).
The call-stack panel is a teaching trace of factorial(4); the adjacent runner executes the Python function itself.
Scope: where a name can be used
A local variable belongs to a function call. It can temporarily have the same name as a global variable; the local name shadows the global name inside that function. Outside the function, the global value is still available.
Local shadows global
Inside the function, Python uses the local name. After return, the outer value remains unchanged.
Scope example
Starting up Python…student = "Mira" # global
def greet():
student = "Dev" # local; shadows the global here
print("Inside:", student)
greet()
print("Outside:", student)The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Prefer explicit data flow
Parameters and return values make dependencies visible and keep functions easier to reuse.
Pass and return values
Starting up Python…def add_bonus(mark, bonus):
return mark + bonus
updated_mark = add_bonus(82, 5)
print(updated_mark)The editor is ready while Python loads in the background.
Output
Your program's output will appear here.
Common Python mistakes
NameError: name 'mark' is not defined
NameError: A name was used before Python could find a variable or function with that name.
IndexError: list index out of range
IndexError: The requested position is outside the items in this list or sequence.
KeyError: 'email'
KeyError: That key does not exist in this dictionary.
TypeError: can only concatenate str (not "int") to str
TypeError: This operation does not work with the kind of value it received.
ZeroDivisionError: division by zero
ZeroDivisionError: A number cannot be divided by zero.
SyntaxError: invalid syntax
SyntaxError: Python could not understand the structure of this line. Check spelling, brackets, colons, and quotes.
IndentationError: expected an indented block
IndentationError: The spaces at the start of a line do not match Python's block structure.
Common function mistakes
Missing call
def greet(): ...Defining a function stores its instructions; call greet() to execute them.
No return value
result = print(5)print displays a value and returns None. Use return when a caller needs a result.
Missing base case
Recursive calls never approach a stopping condition.Without a base case Python eventually raises RecursionError. Ensure each step moves toward it.
Local name unavailable outside
Reading a function's local variable after it returns.Local variables belong to that call. Return a value if it must be used outside.
Check your understanding
1. In def greet(name):, what is name?
2. Why does a recursive function need a base case?
3. What does a function return if it reaches the end without return?