Functions In Depth

Functions are first-class objects

In Python, functions are values — not special syntax, not second-class constructs. A function can be assigned to a variable, stored in a list, passed as an argument, or returned from another function. This property is called first-class status, and it is the foundation of every advanced pattern in this module.

def greet(name: str) -> str:
    return f"Hello, {name}!"

# Assign to a variable
say_hi = greet

# Pass as an argument
def apply(fn, value):
    return fn(value)

result = apply(greet, "Python")
print(result)  # Hello, Python!

Because greet is just an object, say_hi and greet point to the same function. Calling say_hi("World") is identical to calling greet("World").

What this module covers

Lesson	Core concept
Args & Kwargs	Positional, keyword, variadic, and keyword-only parameters
Closures	Nested functions, free variables, and nonlocal
Decorators	Functions that wrap other functions; functools.wraps
Generators	yield, lazy evaluation, and StopIteration

Each lesson is self-contained and builds on this foundation: functions are objects you can manipulate freely.

A first runnable demo

The snippet below demonstrates two core ideas simultaneously: passing a function as an argument, and returning a function as a value.

Run ▸ Open in an online REPL

def make_multiplier(factor: int):
    def multiply(x: int) -> int:
        return x * factor
    return multiply

double = make_multiplier(2)
triple = make_multiplier(3)

numbers = [1, 2, 3, 4, 5]
doubled = list(map(double, numbers))
tripled = list(map(triple, numbers))

print("doubled:", doubled)
print("tripled:", tripled)

# Functions are objects — we can store them in a list
ops = [double, triple, make_multiplier(10)]
for op in ops:
    print(op(7))

🐍 Functions are objects

Every Python function is an instance of the built-in function type. You can inspect it: type(greet) returns <class 'function'>. This is why passing functions to map(), sorted(), and filter() works — they just accept any callable object.

What does 'first-class function' mean in Python?

Functions run faster than methods Functions can be passed as arguments, returned, and assigned to variables Functions defined at module level have global scope Functions are compiled before execution

Which built-in function accepts a function and an iterable as arguments?

apply() map() call() exec()

After writing `f = greet`, which statement is true?

f is a copy of greet f and greet are two different functions f and greet reference the same function object f is only callable if greet is deleted first

Submit

Mark as complete