Coding⏱️ 2 min read📅 2026-05-31

How to Fix: How to set the value of dataclass field in __post_init__ when frozen=True?

Set dataclass field value in __post_init__ when frozen=True

Quick Answer: Use the `field` parameter's `init=False` and `default` to set a default value for the field, or use an init param to calculate the value

📋 Table of Contents

  1. 🛑 Root Causes of the Error
  2. 🔧 Proven Troubleshooting Steps
  3. 🎯 Final Words

To set the value of a dataclass field in __post_init__ when frozen=True, you can use the field(init=False) parameter and assign it a default value. This allows you to initialize the field with a specific value during __post_init__, while still maintaining the benefits of freezing the dataclass.

🛑 Root Causes of the Error

  • Using frozen=True with a non-default value for a field can prevent it from being initialized during __post_init__.

🔧 Proven Troubleshooting Steps

Method 1: Assigning a Default Value

  1. Step 1: Update the field definition to use field(init=False, default=None).

Method 2: Initializing in __post_init__

  1. Step 1: Update the __post_init__ method to assign a value to the field using the value attribute.

🎯 Final Words

By following these steps, you can successfully set the value of a dataclass field in __post_init__ when frozen=True. Remember to assign a default value or initialize the field during __post_init__ to maintain the benefits of freezing your dataclass.

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