Functions & the Call Stack

Defining functions

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

print(greet("Ada"))
print(greet("Ada", greeting="Welcome"))  # keyword argument

Watch the call stack

When a function calls another function, Python pauses the first one, runs the second, then comes back. That pile of "paused" functions is the call stack. Step through and watch the Call stack box:

Notice how w and h exist separately inside each function. Each call gets its own set of variables.

Returning several values

def min_max(nums):
    return min(nums), max(nums)

low, high = min_max([4, 9, 1, 7])
print(low, high)

Docstrings

Professional Python functions start with a short description:

def bmi(weight_kg, height_m):
    """Return body-mass index, rounded to 1 decimal."""
    return round(weight_kg / height_m ** 2, 1)

print(bmi(70, 1.75))
print(bmi.__doc__)