Function Tips & Common Mistakes

Quick Checklist

Before running your function, check:

• Did you use def and end the line with a colon :?
• Is the function body indented?
• Did you call the function after defining it?
• Did you use return if you need to use the result later?
• Are your parameter names matching inside the function?

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Common Mistakes

1. Forgetting to call the function

# Wrong — this just defines it, never runs it
def say_hi():
    print("Hi!")

# Right — call it!
def say_hi():
    print("Hi!")

say_hi()

2. Forgetting return

# Wrong — prints but doesn't give back the value
def add(a, b):
    print(a + b)

result = add(3, 5)  # result is None!

# Right — return gives the value back
def add(a, b):
    return a + b

result = add(3, 5)  # result is 8

3. Putting code after return

# Wrong — anything after return is skipped
def add(a, b):
    return a + b
    print("Done!")   # This NEVER runs

# Right — put print before return
def add(a, b):
    print("Done!")
    return a + b

4. Wrong number of arguments

def greet(name, age):
    print(f"Hi {name}, you are {age}")

greet("Alex")        # Error! Missing age
greet("Alex", 15)    # Correct

5. Mixing up print and return

# If you PRINT inside the function:
def double(n):
    print(n * 2)

double(5)                    # Shows: 10
result = double(5)           # Shows: 10, but result is None
print(f"Answer: {result}")   # Shows: Answer: None  (oops!)

# If you RETURN inside the function:
def double(n):
    return n * 2

result = double(5)           # Nothing printed, result is 10
print(f"Answer: {result}")   # Shows: Answer: 10

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When to Use Print vs Return

Situation	Use
You need to use the result in more code	return
You just want to display something	print
You want to save the result in a variable	return
You want to do math with the result	return
You're making a message to show the user	print

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How to Read Error Messages

TypeError: greet() takes 1 positional argument but 2 were given

You called the function with too many arguments.

TypeError: greet() missing 1 required positional argument: 'name'

You called the function without enough arguments.

NameError: name 'my_function' is not defined

You either misspelled the function name or forgot to define it above where you're calling it.