Quark’s Outlines: Python User-Defined Functions

Overview, Historical Timeline, Problems & Solutions

An Overview of Python User-Defined Functions

What is a Python user-defined function?

You can define your own function in Python using the def keyword. A Python user-defined function is made when you write a def block in your code. When Python runs this block, it creates a function object.

A function object has special parts. These include its name, its list of default values, and its code. It also keeps a link to the global names from the file where it was made. You can use this object to call the function later with any valid input.

Python lets you create named function objects with the def keyword.

def greet(name="friend"):
    return "Hello, " + name

print(greet())
print(greet("Mike"))
# prints:
# Hello, friend
# Hello, Mike

The function greet is now a user-defined function. Python stores its name, code, and default values for later use.

What are the special parts of a Python function?

A Python function holds many facts about itself. These are called attributes. For example, __name__ holds the function name. __defaults__ is a tuple of default values. __globals__ holds the global names it can see. __code__ is a special object that stores the function’s bytecode.

Python functions store their details in special attributes.

def square(x=2):
    return x * x

print(square.__name__)
print(square.__defaults__)
print(square.__code__.co_varnames)
# prints:
# square
# (2,)
# ('x',)

These parts let Python understand and run your function later.

A Historical Timeline of Python User-Defined Functions

Where do Python’s user-defined functions come from?

User-defined functions in Python build on ideas from earlier languages. They let you name blocks of code and reuse them. Over time, Python added new parts to function objects—like closures, default values, and metadata—to make them more powerful.

People designed ways to name and reuse logic

People added first-class functions to Python

Problems & Solutions with Python User-Defined Functions

How do you use Python user-defined functions the right way?

Python functions are blocks of code you can name and reuse. These examples show how functions are stored, how their defaults work, how you can inspect them, and how to understand what happens when they are created and called.

Problem: How do you reuse the same logic with different inputs in Python?

You want to avoid repeating the same code. You want to give it a name, reuse it, and pass in different values each time. You need to define a function and call it when needed.

Python lets you reuse logic by defining and calling a function.

def add(x, y):
    return x + y

print(add(2, 3))
print(add(10, -5))
# prints:
# 5
# 5

The add function gives a simple way to reuse addition logic with any numbers.

Problem: How do you define a function with default values in Python?

You want to make a function that works with or without an input. If no value is given, it should still run using a fallback value. You want a safe default when nothing is passed.

Python lets you define functions with default argument values.

def greet(name="friend"):
    return "Hello, " + name

print(greet())
print(greet("Sam"))
# prints:
# Hello, friend
# Hello, Sam

The function uses the default value "friend" unless another name is given.

Problem: How do you check what a function holds in Python?

You want to look inside a function and see what Python remembers. You want to see its name, its default values, and the code it will run. You want to inspect the function object.

Python lets you inspect functions using built-in attributes.

def square(x=3):
    return x * x

print(square.__name__)
print(square.__defaults__)
print(square.__code__.co_varnames)
# prints:
# square
# (3,)
# ('x',)

These attributes show the function name, default values, and variable names.

Problem: How do you store a function for later use in Python?

You want to save a function inside a variable or pass it to another function. You do not want to run it yet. You only want to store the logic until later.

Python lets you assign function objects to names and pass them around.

def shout(word):
    return word.upper() + "!"

s = shout
print(s("hello"))
# prints:
# HELLO!

You can treat the shout function like any value and use it under another name.

Problem: How do you check a function's global names in Python?

You are inside a function and want to know which global names it can see. You want to inspect where it was defined and what global values it uses. This helps when debugging or writing tools.

Python lets you inspect the global scope used by a function.

message = "Hi"

def say():
    return message

print(say.__globals__['message'])
# prints:
# Hi

The __globals__ attribute shows the dictionary of names from the module.

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Mike Vincent is an American software engineer and app developer from Los Angeles, California. More about Mike Vincent