Common Functions Mistakes

The problem...

Your function doesn't return what you expect. Or it crashes on valid input. Or it changes a variable that shouldn't change.

These are the mistakes almost every beginner makes with functions. At least once.

Mistake 1 — Calling a function before defining it

# Wrong — NameError
greet("Bull")

def greet(name):
    print(f"Hello, {name}.")

# Right — define before calling
def greet(name):
    print(f"Hello, {name}.")

greet("Bull")

Python reads top to bottom. The function must exist before you call it.

Mistake 2 — Forgetting parentheses when calling

def greet():
    print("Hello.")

greet     # does nothing — references the function object
greet()   # calls it — prints "Hello."

Without parentheses — you reference the function, not call it. Nothing runs.

Mistake 3 — Forgetting return

# Wrong — returns None
def calculate_bmi(weight, height):
    bmi = weight / height ** 2
    print(bmi)    # prints but doesn't return

result = calculate_bmi(70, 1.75)
print(result)    # None

# Right
def calculate_bmi(weight, height):
    return weight / height ** 2

result = calculate_bmi(70, 1.75)
print(result)    # 22.857...

Printing inside a function is not the same as returning. return sends the value out. print just displays it.

Mistake 4 — Code after return

def check(number):
    if number > 0:
        return "positive"
    print("This never runs.")    # unreachable
    return "zero or negative"

return stops the function immediately. Everything after it in the same block is unreachable.

Mistake 5 — Default parameters before required ones

# Wrong — SyntaxError
def greet(name="stranger", role):
    print(f"{name} — {role}")

# Right
def greet(role, name="stranger"):
    print(f"{name} — {role}")

Required parameters always come first. Default parameters always come last.

Mistake 6 — Modifying a global variable inside a function

count = 0

def increment():
    count = count + 1    # UnboundLocalError

# Right — use return instead
def increment(count):
    return count + 1

count = increment(count)

Inside a function, assigning to a variable creates a local copy. The global is untouched — and Python gets confused. Use parameters and return instead.

What's really happening

Most function mistakes come from assumptions — that Python reads out of order, that print equals return, that globals are freely modifiable. They're not.

Functions have clear rules. Know them.

The mindset shift

Stop thinking: "The function knows everything."

Start thinking: "The function knows only what you give it — and sends back only what you return."

What you should understand now

• Define before calling — Python reads top to bottom
• Parentheses are required to call a function
• print inside a function is not return
• return stops the function — nothing after it runs
• Default parameters always come last
• Use parameters and return — avoid modifying globals