How to use default_factory in Python dataclasses?
Jul 15, 2025 am 02:39 AM
When do you need to use default_factory? When you want to assign a default mutable object to a field, you should use default_factory. How to set default value with default_factory? You can pass any parameter-free callable object to default_factory, such as built-in types, functions, or lambda expressions. Common usages include: 1. Initialize to an empty list: default_factory=list; 2. Initialize to an empty dictionary: default_factory=dict; 3. Initialize to a specific structure: default_factory=lambda: [1, 2, 3]; 4. Use custom functions to generate complex objects. Common misunderstandings and precautions include: 1. Do not directly pass objects such as field(default=[]); 2. Use factory functions such as field(default_factory=list); 3. Do not pass functions with parameters; 4. You can use lambda, but pay attention to scope issues. Applicable scenarios include: dynamic data structures in configuration items, nested object collections in data models, cache or status record fields, especially when writing API models, ORM mappings, and configuration classes. Use default_factory rationally to avoid bugs caused by reference sharing and make the code clearer and safer.
In Python's dataclasses module, if you want to set a default value for a field, and this default value is a mutable object (such as a list or a dictionary), direct assignment may cause multiple instances to share the same object, resulting in unexpected results. At this time, default_factory comes in handy.
When do you need to use default_factory?
When you want to assign a default mutable object to a field, you should use default_factory . It accepts a parameterless function as a parameter, and it will be called every time the class instance is created to generate a new object.
For example:
from dataclasses import dataclass, field
@dataclass
class MyClass:
items: list = field(default_factory=list) In this way, every time a new MyClass() instance is created, list() will be called to create an empty list, instead of sharing a list for all instances.
How to set default value with default_factory?
You can pass any non-argument callable object to default_factory , such as built-in types, functions, or lambda expressions.
Common usages include:
- Initialize to empty list:
default_factory=list - Initialize to an empty dictionary:
default_factory=dict - Initialize to a specific structure:
default_factory=lambda: [1, 2, 3] - Generate complex objects using custom functions
For example:
from dataclasses import dataclass, field
def default_tags():
return ["python", "code"]
@dataclass
class Article:
title: str
tags: list = field(default_factory=default_tags)
a = Article("Intro to Python")
b = Article("Advanced Tips", tags=["tips"]) tags of the two instances here are independent and will not affect each other.
Common misunderstandings and precautions
There are several error-prone places you may encounter:
- ? Do not pass objects directly: for example
field(default=[]), which will cause multiple instances to share the same list. - ? To use factory functions: such as
field(default_factory=list)to ensure that it is a new object every time. - ? Do not pass functions with parameters:
default_factorymust be parameterless, otherwise an error will be reported. - ? You can use lambda, but pay attention to scope issues.
In addition, if you use the __init__ method or inheritance relationship, you should also pay attention to whether the fields are initialized correctly.
What are the applicable scenarios?
default_factory is best used to deal with default values that require "independent per instance", such as:
- Dynamic data structures in configuration items
- Nested collection of objects in the data model
- Cache, status record and other fields
Especially when writing API models, ORM mappings, and configuration classes, this type of requirement is very common.
Basically that's it. Using default_factory rationally can avoid many bugs caused by reference sharing and make the code clearer and safer.
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