Configure field defaults, factories, and metadata with dataclasses.field().
What You Will Learn
field(default_factory=...) creates a fresh mutable value per instance. Other options include repr=False, compare=False, kw_only=True, and metadata. Metadata itself is read-only at the mapping level.
A default_factory should be a zero-argument callable.
Step-by-Step Example
The example below makes the behavior visible with small input values that you can change safely.
from dataclasses import dataclass, field
@dataclass
class ShoppingCart:
products: list[str] = field(default_factory=list)
first_cart = ShoppingCart()
second_cart = ShoppingCart()
first_cart.products.append("Notebook")
print(first_cart.products)
print(second_cart.products)How the Example Works
- The imports provide the standard-library tools used in the demonstration: dataclasses.
- The class ShoppingCart groups data and behavior; its methods run when called on an instance.
- The variable first_cart holds the result of ShoppingCart(). Later lines use that value or inspect how it changes.
- The variable second_cart holds the result of ShoppingCart(). Later lines use that value or inspect how it changes.
Expected output from the example:
['Notebook']
[]When You Would Use This
These tools are useful when composing small reusable functions, adapting callbacks, or eliminating repeated work. Pay attention to whether the operation returns a new object, updates state, or produces lazy results. Test a small success case and a boundary case before integrating it into an application.
Practice: Modify the Code
Add a per-instance set field with default_factory=set.
Verify the changed output against the behavior described above, including the relevant invalid or boundary case.
For complete parameters and version details, consult the official Python documentation.
Common Beginner Mistakes
- Using one shared mutable default list for every instance instead of default_factory.
- Calling default_factory immediately and passing its result, rather than passing the callable.
Key Takeaways
- Configure field defaults, factories, and metadata with dataclasses.field().
- field(default_factory=...) creates a fresh mutable value per instance.
- A default_factory should be a zero-argument callable.