Working with Function Parameters

Parameters are the inputs you define for a function, allowing you to pass data into it and make it flexible and reusable. Python offers a rich and powerful system for defining function parameters, giving you precise control over how a function can be called.

Let’s explore the different kinds of parameters you can define.

The Five Parameter Types

A Python function signature can include five different kinds of parameters. They must be defined in this specific order:

Positional-Only Parameters (/)
Positional-or-Keyword Parameters (Standard parameters)
Default Arguments
Keyword-Only Parameters (*)
Variadic Keyword Parameters (**kwargs)
Variadic Positional Parameters (*args)

Let’s break them down one by one.

1. Positional-or-Keyword Parameters

These are the most common type of parameters. They can be passed either by their position (order) or by their name (keyword).

Pyground

Create a `greet` function with `name` and `greeting` parameters. Call it once using positional arguments and once using keyword arguments.

Expected Output:


Hello, Alice!
Hi, Bob!

Output:

Once you use a keyword argument in a function call, all subsequent arguments must also be keyword arguments.

2. Default Arguments

You can provide a default value for a parameter. If the caller doesn’t provide a value for that parameter, the default value is used.

Pyground

Create a `power` function that calculates `base` to the power of `exponent`. The default exponent should be 2.

Expected Output:


5 to the power of 2 is: 25
3 to the power of 4 is: 81

Output:

🚨

Mutable Default Argument Pitfall: Never use a mutable object like a list ([]) or dictionary ({}) as a default value. The default object is created only once when the function is defined and is shared across all calls. This can lead to unexpected behavior. The correct pattern is to use None as the default and create a new object inside the function.

3. Variadic Positional Parameters (`*args`)

The *args syntax allows a function to accept any number of positional arguments. Inside the function, args will be a tuple containing all the extra positional arguments.

Pyground

Create a `sum_all` function that takes any number of arguments and returns their sum.

Expected Output:


Arguments received: (1, 2, 3)
Sum: 6
Arguments received: (10, 20, 30, 40, 50)
Sum: 150

Output:

4. Variadic Keyword Parameters (`**kwargs`)

The **kwargs syntax allows a function to accept any number of keyword arguments. Inside the function, kwargs will be a dictionary containing all the extra keyword arguments.

Pyground

Create a function `display_profile` that accepts any number of keyword arguments and prints them.

Expected Output:


User Profile:
- Name: Chris
- Age: 35
- City: New York
- Occupation: Developer

Output:

5. Keyword-Only Parameters (`*`)

Sometimes you want to force the caller to use keyword arguments for clarity. Any parameter defined after a bare * or *args must be passed as a keyword argument.

Pyground

Create a function `create_user` where `user_id` and `name` are positional, but `is_admin` must be specified by keyword.

Expected Output:


Creating user Eve (ID: 101).
Granting admin privileges.

Output:

Putting It All Together

Here is a complex function signature that uses several types of parameters in the correct order.

Pyground

Analyze the following function signature and its call. Identify each type of parameter.

Expected Output:


Processing from input.csv to output.json.
Overwriting is enabled.
Metadata provided:
- author: Alice
- timestamp: 2023-10-27

Output:

In the example above:

source and target are positional-or-keyword parameters.
force_overwrite is a keyword-only parameter with a default value.
**metadata collects any additional keyword arguments.

Anatomy of a Function Scope, State & Closures

Working with Function Parameters

The Five Parameter Types

1. Positional-or-Keyword Parameters

Pyground

Create a `greet` function with `name` and `greeting` parameters. Call it once using positional arguments and once using keyword arguments.

Expected Output:

Output:

2. Default Arguments

Pyground

Create a `power` function that calculates `base` to the power of `exponent`. The default exponent should be 2.

Expected Output:

Output:

3. Variadic Positional Parameters (*args)

Pyground

Create a `sum_all` function that takes any number of arguments and returns their sum.

Expected Output:

Output:

4. Variadic Keyword Parameters (**kwargs)

Pyground

Create a function `display_profile` that accepts any number of keyword arguments and prints them.

Expected Output:

Output:

5. Keyword-Only Parameters (*)

Pyground

Create a function `create_user` where `user_id` and `name` are positional, but `is_admin` must be specified by keyword.

Expected Output:

Output:

Putting It All Together

Pyground

Analyze the following function signature and its call. Identify each type of parameter.

Expected Output:

Output:

3. Variadic Positional Parameters (`*args`)

4. Variadic Keyword Parameters (`**kwargs`)

5. Keyword-Only Parameters (`*`)