Dangers of Names and Values in Python: The differences between Python and Java "variables"

05 Mar 2025

Hi there, in this article you will read about some interested findings about how Python deals with names and values, convertionally called as variables, and why it is totally different from languages like Java, C, and others.

Index

1. Context
2. Variable creation in Java
3. Variable creation in Python
4. The danger in Python
5. The long story, short

Context

When working with Java and Python, it is crucial to understand the fundamental differences in how these languages handle variable creation and memory management. While both languages provide mechanisms for referencing values in memory, their approaches differ significantly due to their underlying paradigms.

Variable creation in Java

How a variable is created in languages like Java? By declaring its type, naming it and then assigning a value to it. This is needed because Java is a statically typed language, meaning that variable types must be explicitly declared at compile time.

Like this:

class Main {
    public static void main(String[] args) {
        int myVar = 10; // This line here is where we create a variable
        System.out.println(myVar); // prints 10
    }
}

What do happen behind the scenes is that when you do int myVar = 10;, Java creates a space in the stack memory to store the value of myVar, but only because int is a primitive data type, otherwise, it would be stored in the heap memory, and the variable would only has a reference where to find the data in the heap memory.

Within a Java variable you can find its data type and its data value, the variable has its information. Which you will see that isn’t the same for Python “variables”.

Variable creation in Python

To understand that you should know that Python doesn’t use the term variable, instead it uses the term name that would be conventionally called a variable. So in Python we say a name refers to a value or an object, but the name doesn’t contain the value/object, like a variable do. A good analogy is saying that your name refers to you but it don’t contains you. There may even be multiple names for the same thing, just as you may have given a name and a nickname. A value is a particular instance of data in memory.

What happen in Python is just different, in Python there is no variable storing the data type, that’s why Python in contrast, is dynamically typed and follows a name-binding model rather than direct memory assignment. Variables in Python are merely names that reference objects in memory. You can assign a name firstly with a float value and in the very next line, use the same name to assign a int value and no error will happen. Like this:

cloth_price = 19.99 ## Assigning a float
cloth_price = 10 ## Assigning a int

The code above demonstrates how assign a value to a name in Python. That way, the name cloth_price is bound to the value 10 which is in the memory, meaning that the name cloth_price can be used to refer the value 10 in memory. This operation of binding is referred as an assignment.

If we assign other name to the name cloth_price, let say blouse = cloth_price the value 10 won’t get copied but the name blouse will refer to the same original value as cloth_price is bounded.

The image below can ilustrate it better:

What should happen if we modify the value referenced using the name cloth_price, will modify the value referenced by the name blouse? The answer is NO!

The reason is when a variable is assigned in Python, it does not store a value directly but instead acts as a label pointing to an object in memory, so if we do that:

cloth_price = 10
blouse = cloth_price
print(cloth_price) ## prints 10
print(blouse) ## prints 10
cloth_price = cloth_price + 3
print(cloth_price) ## prints 13
print(blouse) ## prints 10

It can be ilustrated as below:

As you can see, in the second line, cloth_price and blouse both reference the same integer object 10. After the line cloth_price = 13 is executed, in this case since integers are immutable in Python, any modification (cloth_price = cloth_price + 3) results in cloth_price refering to a new object.

The Danger in Python

Until here we’ve discussed about how to assign a name to a value using Python and how it differs from Java and other languages. In this particular example we used a int object data type which is one of many immutable data types in Python, such as bool, float, tuple, string, and Unicode. Those types can’t suffer mutation, therefore, every time you do some operation in one of its data, Python creates another value in the memory as result of your operation over the original value.

But Python has mutable data types too, they are set , list , dict and Data classes. When you mutate a mutable datatype, Python don’t create a new value for it, so any other name bounded with that value still get the modification. An example to ilustrate what I mean is following below:

## Consider the function below that receives a list of integers
## and prinst the lowest number in that list
def find_lowest(prices):
    prices.sort()
    print(prices[0])


## Now let use the function to find the lowest price in a list of integers
cloth_prices = [10, 13, 2, 4, 5]
find_lowest(cloth_prices)
print(cloth_prices)

After run this code, we’ll get the followind output:

2
[2, 4, 5, 10, 13]

At first glance, you may assume that the function would make a copy of the cloth_price to the function argument prices, but when you pass an argument to a function you’re bounding a value to a name, like any assignment operation, in that case it was like prices = cloth_prices, so the name prices is bounded to the value as cloth_prices is, so every mutation made in the value (the list) it will reflect in any name bounded to it.

To overcome this, you should make a copy of the value and then made the mutation you want.

def find_lowest(prices):
    prices = sorted(prices)
    print(prices[0])

The builtin sorted() Python’s function receives a list and returns a new list, avoiding changing the original values.

The long story, short

To avoid to get you too exhausted, let wrap up what we have learned about names and values in Python, its difference with Java and why mutable values in Python should be treated carefully:

Key differences between Java and Python

1. Typing System: Java enforces static typing, requiring explicit type declarations, whereas Python dynamically binds names to objects.

2. Memory Model: Java distinguishes between primitive and reference types, whereas Python treats everything as an object.

3. Variable Behavior: In Java, primitive values are stored directly, while objects are referenced. In Python, all variables act as references to objects in memory.

4. Mutability Impact: Python’s immutable types (e.g., integers, strings) cause variables to reference new objects upon modification, while Java maintains references until explicitly reassigned.

Dangers of Python’s Name-Value Model:

1. Immutable Types: For types like integers, strings, and tuples, any modification results in the creation of a new object, leaving previously bound names unchanged.

2. Mutable Types: For types like lists, dictionaries, and sets, modifications affect all names referencing the same object, which can lead to unintended side effects if not handled carefully.

I hope you liked that content, this is the result of my continous reasearch and learning path to become a senior Python developer.

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Sources: Ned Batchelder - Facts and Myths about Python names and values - PyCon 2015: https://youtu.be/_AEJHKGk9ns

Book: McDonald, Jason C. Dead Simple Python: Idiomatic Python for the Impatient Programmer (p. 95). (Function). Kindle Edition.