class variables issue 220 python attrs attrs github

deep dive into `attrs` issue 220: class variables and default values

this tutorial will explore the nuances of defining class variables within the `attrs` library, focusing on the context of issue 220 and its implications. we'll cover the core problem, potential solutions, and provide clear code examples to illustrate best practices.

**understanding `attrs`**

before diving into the specific issue, let's briefly review `attrs`. `attrs` (pronounced "atters") is a python package that lets you write boilerplate-free classes by handling the tedious parts of class creation automatically, like `__init__`, `__repr__`, `__eq__`, `__hash__`, etc.

**the core problem: `attrs` and mutable default values (issue 220)**

issue 220 on the `attrs` github repository highlights a common pitfall when working with mutable default values in `attrs` classes. this problem isn't unique to `attrs`; it's a general issue in python that `attrs` exacerbates due to its automatic generation of methods.

the fundamental problem stems from how python handles default values for function arguments (including the automatically generated `__init__` method by `attrs`). mutable default values are created *once* when the function is defined and then shared across all invocations of that function where the argument is not explicitly provided. this can lead to unexpected behavior when multiple instances of a class share the same mutable object.

**issue 220 (in summary)**

specifically, issue 220 discussed the situation where a class variable (meant to be shared across all instances of the class) was unintentionally being *mutated* by individual instances of the class due to the way `attrs` handled default values. users were trying to define a default list or dictionary at the class level, expecting it to be independent for each instance, but instead, they were all modifying the same original list/dictionary.

**why is this a problem?**

consider a scenario where you want to define a class that keeps t ...

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Class variables
Issue 220
Python
attrs
GitHub
data classes
attribute management
variable scope
Python decorators
object-oriented programming
type annotations
class methods
instance variables
code optimization
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