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You've learned about Python's
built-in functions for common tasks.

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This time, we're going to take a closer
look at how to create your own custom functions

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for your specific needs and projects.

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By the end of this video, you will be able
to explain live code examples of functions

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with arguments and return values, as well as
explain how to call them within the Python script.

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To briefly review, functions are self-contained
blocks of code that perform specific tasks.

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They allow you to encapsulate a series of
instructions into reusable chunks, like mini-programs,

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that can be reused in your
program as often as you need.

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Python also provides you with the ability
to write your own custom functions.

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The first thing you know about writing
your own functions is that you need to provide

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them with inputs and
receive outputs from them.

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These very important components
are arguments and return values.

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Let's take a closer look at
each of these components.

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Arguments are the values you pass
into a function when you call it.

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In other words, they provide the necessary
data for the function to perform its task

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and adapt to different scenarios.

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If you think of a function as a machine,
you need to provide inputs like the electricity

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to power it and the buttons
to turn it on and off.

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These inputs are the arguments.

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An example of this would be a function designed
to calculate an employee's net salary.

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The arguments for this function might include
the employee's gross salary, tax rate, and

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any deductions.

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By providing these arguments, the function
can perform the necessary calculations and

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return the final net salary.

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Once your function has processed its arguments
and performed its task, it might need to communicate

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the results back to the main program.

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The results are the return values.

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A return value is the output or
result generated by your function.

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It's like the product the machine produces.

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An example of this would be a function that
checks if a user's entered password meets

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specific security criteria.

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The function would take the password as an
argument, perform various checks, length,

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complexity, etc., and then return a true or
false value to indicate whether the password

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is valid.

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This return value can then be used by the
main program to decide whether to grant the

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user access or to return an error message.

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So how do you write a function in Python?

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Let's go through it step by step.

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Step 1.

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Define the function.

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Start with the keyword def,
followed by the function name.

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If it's more than one word,
be sure to use an underscore.

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Immediately after the name, add parentheses.

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That's where you'll put and arguments, and a colon.

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Step 2.

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Add a docstring.

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This is a clear and concise description
of what the function does, its purpose, arguments,

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and return value.

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Make sure to put the docstring
between triple quotes.

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This is the documentation for
yourself and other developers.

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Step 3.

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Write the code.

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In the indented block following the function
definition, write the code that performs the

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desired task using any provided arguments.

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Step 4.

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Return a value.

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This is optional, but if your function needs to
produce an output, use the return statement

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to send a value back to the calling code.

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The last step, step 5, is saving the function.

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Although this seems like an obvious step, a
lot of good work is often lost because people

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are in a hurry and forget.

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After you have defined your function, you
can call it from other parts of your program

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to execute the code encapsulated in it.

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This is how you reuse the function.

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For example, say you have a sales team that
needs to generate weekly, monthly, and yearly

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sales reports.

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You can create a function that takes the desired
time period as an argument and generates the

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corresponding report.

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The last thing we want to talk
about is calling functions.

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A function is a powerful tool, but it doesn't
actually do anything until you call it.

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Calling the function triggers its execution,
allowing the code within the function to run

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and perform its designated task.

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Calling a function showcases its reusability,
so you can use the same logic multiple times.

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It also enhances modularity, breaking down
your code into smaller, more manageable units

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that are easier to understand,
debug, and modify.

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Finally, it gives them the flexibility to
do their job in a variety of situations, as

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well as return values your program
can use to make decisions.

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Understanding and being conversant in writing
and using functions is an important skill

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to develop for Python programmers.

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Using functions enables you to build more
structured, efficient, and adaptable code,

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which helps you become a more
proficient and versatile developer.