Effortlessly Understand Python's Pass by Reference

Pass by Reference in Python: Background and Best Practices

by Marius Mogyorosi

After gaining some familiarity with Python, you may notice cases in which your functions don’t modify arguments in place as you might expect, especially if you’re familiar with other programming languages. Some languages handle function arguments as references to existing variables, which is known as pass by reference. Other languages handle them as independent values, an approach known as pass by value.

If you’re an intermediate Python programmer who wishes to understand Python’s peculiar way of handling function arguments, then this tutorial is for you. You’ll implement real use cases of pass-by-reference constructs in Python and learn several best practices to avoid pitfalls with your function arguments.

In this tutorial, you’ll learn:

1. What it means to pass by reference and why you’d want to do so
2. How passing by reference differs from both passing by value and Python’s unique approach
3. How function arguments behave in Python
4. How you can use certain mutable types to pass by reference in Python
5. What the best practices are for replicating pass by reference in Python

Defining Pass by Reference

Before you dive into the technical details of passing by reference, it’s helpful to take a closer look at the term itself by breaking it down into components:

• Pass means to provide an argument to a function.
• By reference means that the argument you’re passing to the function is a reference to a variable that already exists in memory rather than an independent copy of that variable.

Since you’re giving the function a reference to an existing variable, all operations performed on this reference will directly affect the variable to which it refers. Let’s look at some examples of how this works in practice.

Contrasting Pass by Reference and Pass by Value

In many programming languages, variables are passed either by reference or by value. Understanding the difference between these two approaches is crucial to understanding how Python handles function arguments.

• Pass by reference: When you pass an argument by reference, you’re passing a reference to the variable itself. Any modifications made to the parameter within the function will affect the original variable. This is how most object-oriented programming languages work.
• Pass by value: When you pass an argument by value, you’re passing a copy of the value stored in the variable. Modifying the parameter within the function will not affect the original variable.

Using Pass by Reference Constructs

In Python, all function arguments are passed by reference. However, Python’s approach to passing arguments can be confusing if you’re used to other programming languages. In this section, we’ll explore some pass by reference constructs in Python and how they can be used effectively.

Avoiding Duplicate Objects

One of the advantages of pass by reference is that it allows you to avoid creating duplicate objects. Since you’re passing a reference to an existing object, you don’t need to create a copy of the object itself. This can be particularly useful when working with large data structures or when memory efficiency is important.

Returning Multiple Values

Pass by reference also enables you to return multiple values from a function. Instead of returning a single value, you can modify the parameters passed by reference to store additional values. This can be a clean and efficient way to return multiple values without using more complex data structures such as dictionaries or tuples.

Creating Conditional Multiple-Return Functions

In some cases, you may want your function to return different values based on certain conditions. Pass by reference allows you to modify the parameters within the function based on these conditions and have the changes reflected outside of the function. This can be useful in scenarios where you want to return different data depending on the outcome of certain computations or conditions.

Passing Arguments in Python

Understanding how arguments are assigned in Python is essential for understanding how pass by reference works in Python. In Python, assignment is done by creating a reference to an object. When a variable is assigned a value, it actually binds the name to the object in memory. This means that when you pass an argument to a function in Python, you’re actually passing a reference to the object itself.

Exploring Function Arguments

Function arguments in Python are handled in a similar way to variables. When you pass an argument to a function, a new reference to the same object is created inside the function. This means that any modifications made to the parameter within the function will affect the original object outside of the function.

Replicating Pass by Reference With Python

In some cases, you may want to replicate the behavior of pass by reference in Python. While all function arguments are passed by reference in Python, there are certain best practices you can follow to achieve the desired behavior.

Best Practice: Return and Reassign

One way to replicate pass by reference in Python is to return a modified value from the function and reassign it to the original variable. This allows you to modify the value of the variable outside of the function without explicitly passing it by reference.

Best Practice: Use Object Attributes

Another way to replicate pass by reference in Python is to use object attributes. By modifying the attributes of an object, you can achieve similar results to pass by reference.

Best Practice: Use Dictionaries and Lists

Dictionaries and lists are mutable data structures in Python, which means they can be modified in place. By using dictionaries or lists as function arguments, you can modify their contents within the function and have the changes reflected outside of the function.

Conclusion

In this tutorial, you’ve learned about pass by reference in Python and how it differs from other programming languages. You’ve explored various pass by reference constructs in Python and learned best practices for achieving similar behavior. By understanding how function arguments work in Python and following these best practices, you can avoid common pitfalls and make the most of Python’s unique approach to passing arguments.

Remember that all function arguments in Python are passed by reference, so it’s important to understand how this concept works and how you can use it to your advantage. With the knowledge gained from this tutorial, you’ll be better equipped to write clean and efficient code in Python.