Functions in Python

A function is a piece of code with a name, so you can run it from more than one place without copying it.

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

print(greet("Simran"))    # Hello, Simran

def, a name, arguments in brackets, a colon, then an indented body. return hands a value back.

return vs print

These are not the same thing, and mixing them up is the single most common beginner bug:

def add(a, b):
    print(a + b)      # shows it on screen
def add(a, b):
    return a + b      # gives it back to the caller

With print, total = add(2, 3) sets total to None — the number went to the screen and nowhere else. If you plan to use the answer, return it. A function with no return returns None.

Default and keyword arguments

def enrol(name, course="Python", months=4):
    return f"{name}: {course}, {months} months"

enrol("Ravi")                          # Ravi: Python, 4 months
enrol("Ravi", "Django")                # Ravi: Django, 4 months
enrol("Ravi", months=6)                # Ravi: Python, 6 months
enrol(course="IoT", name="Simran")     # order stops mattering

Arguments with defaults must come after arguments without them. And keep defaults immutable — None, a number, a string — never a list or dictionary, for the reason covered in the lists lesson.

Scope

A name made inside a function lives only inside it:

def calculate():
    total = 100     # local
    return total

calculate()
print(total)        # NameError — there is no 'total' out here

You can read an outer name from inside a function, but assigning to it makes a new local one instead of changing the outer. If you find yourself reaching for global, stop and pass the value in as an argument and return the result instead. Code that talks to its caller only through arguments and return values is the code you can still understand in March.

Docstrings

def monthly_fee(months):
    """Total fee for a given number of months."""
    return months * 4500

A single line in triple quotes, first thing in the body. help(monthly_fee) will print it, and so will your editor when you hover the name.