Quick Answer: Static attributes are variables defined once for the class and shared by all instances. Dynamic attributes are variables defined for individual instances only. Static variables are used as a “fall-back” when there are no explicit dynamic attributes defined for the instances. When you try to “overwrite” a static attribute such as in x.attr = 1, you actually create a dynamic attribute on that instance that lives separately from any static attribute X.attr for class X.
Did you just stumble over the terms “static attributes” and “dynamic attributes” and wondered what they mean—and what’s the difference between them is anyway? This article will resolve this issue once and for all. So, let’s get started!
You can also watch my explainer video as you read through the remaining article:
Static and Dynamic Attributes in Python – What’s the Difference?
By studying this question, you will improve your understanding of object-oriented programming. So, let’s break this question down into two simpler ones:
- What are static attributes?
- What are dynamic attributes?
What Are Static Attributes in Python?
? Static attributes are variables associated with a given class and shared among all instances. If you change the value of a static attribute, all instances will see this change. Think of them as shared variables.
Here is a simple example:
class Book:
# static attribute / class attribute
classification = "textual"
coffee_break_python = Book()
hitchhikers_guide = Book()
print(Book.classification)
# textual
print(coffee_break_python.classification)
# textual
print(hitchhikers_guide.classification)
# textualIn the code, we define the static attribute classification for the class Book. You can access this attribute not only via the class name Book but also via the instances coffee_break_python and hitchhikers_guide.
Think of it this way: the instance variables do not have their own attribute classification. Therefore, the interpreter tries to fix this by looking upwards the class hierarchy to find the first definition of the requested attribute classification. It then finds the attribute on the class level.
? Why are they called “static” in object orientation? Because they are allocated in memory “statically” for the whole run of the program — even before the first object is ever created.
Ok, you are now ready to solve a quick puzzle assuming above definitions:
## Puzzle 1 Book.classification = "video" print(hitchhikers_guide.classification)
What’s the output of this puzzle? Commit to the answer first before reading on.
Most likely, you found the correct answer. The code snippet changes the value of the class attribute. The class instance hitchhikers_guide does not have its own definition of classification, so it lends the definition from the class attribute. The result is, therefore video.
Let’s have a look at this puzzle:
## Puzzle 2 Book.classification = "text" hitchhikers_guide.classification = "audio" print(coffee_break_python.classification)
What’s the output now? Lock your answer in first, before reading on. Have it? Good, then let’s see whether you guessed right.
What happens here?
- You have a static class attribute.
- You change it on the instance level (to “audio”).
- You print the class attribute by calling it via another instance.
Hence the result is audio. Right?
Wrong!
The result is text. So why is that?
To understand this, you have to understand the concept of “dynamic attributes” first.
What Are Dynamic Attributes in Python?
? Dynamic attributes in Python are attributes only visible at the instance-level. You set them dynamically at runtime, that is after creating the instances. In Python, everything is an object (even functions). So you can define a dynamic instance attribute for nearly anything.
Now, can you solve the following puzzle?
## Puzzle 3
class Universe:
None
def answer():
return 42
u = Universe()
u.answer = 21
answer.answer = 21
print(answer.answer + u.answer == answer())What’s the output of this code puzzle?
Confusing, I know. But commit to your “answer” first.
In the puzzle, we create two objects: the Universe object assigned to name u and the function object assigned to the name answer.
(Let’s not debate (for now) whether a class is an object as well… ;))
For both objects, we create a dynamic attribute answer.
? The difference to the static attribute is that we define it at runtime for the objects themselves and not statically for the class.
This is the answer to the question addressed in this article.
The class Universe and all other potential instances of this class do not know the attribute answer. It is only defined for the instance u and the instance answer. That’s it.
By the way: the result for Puzzle 3 is True.
But more importantly, we can now understand why Puzzle 2 produces the result audio and not text:
Book.classification = "text" hitchhikers_guide.classification = "audio" print(coffee_break_python.classification)
The first line talks about the static attribute of the class. This is the default attribute if an instance has not defined its own attribute.
The second line talks about the dynamic attribute of the instance hitchhikers_guide. It is only defined for this instance. So the book coffee_break_python, feel free to learn more about the Coffee Break Python series here, does not know anything about the dynamic attribute of the other book. It only knows about the static attribute from the Book class.
Summary
To sum up, you have learned about the difference between static and dynamic attributes in Python. Static attributes are defined once for the class — and are used as a “fall-back” in case there are no explicit dynamic attributes defined for the instances. As soon as you try to “overwrite” a static attribute attr by calling it the instance x such as in x.attr = 1, you actually create a dynamic attribute on that instance that lives separately from any static attribute X.attr for class X.
(Disclaimer: I know that the term “static attribute” is not so common in Python but I actually found it quite useful to explain things.)
Where to Go From Here?
Do you enjoy puzzle-based learning Python? I am doubling down on this unique way of learning Python because “practice testing” is scientifically proven to be one of the most effective learning techniques. Yet, I have not found any puzzle-based Python learning book on the market. That’s why I have written the Python textbook “Coffee Break Python” which is based 100% on a puzzle-based learning mindset. Get the book now!