---
title: "Dataclasses"
description: "Dataclasses define structured storage-compatible data types for GenLayer Intelligent Contracts using @allow_storage."
source: https://docs.genlayer.com/developers/intelligent-contracts/types/dataclasses
last_updated: 2026-06-11
---

# Dataclasses

Dataclasses are structured Python data types for GenLayer Intelligent Contracts, used to model method parameters, return values, and stored contract data. Use the `@allow_storage` decorator to make a dataclass compatible with contract storage.

## Method Parameters and Returns

```python
import typing

@allow_storage
@dataclass
class UserData:
    name: str
    balance: u256
 
class SimpleContract(gl.Contract):
    users: TreeMap[Address, UserData]
    
    def __init__(self):
        pass
    
    @gl.public.write
    def create_user(self, name: str, balance: int):
        self.users[gl.message.sender_address] = UserData(name, balance)

    @gl.public.view
    def get_user(self, user_address: str) -> TreeMap[str, typing.Any]:
        address = Address(user_address)
        return self.users.get(address, UserData("", u256(0)))
```

## Generic Types

```python
import typing

@allow_storage
@dataclass
class Item[T]:
    data: T
    label: str

class GenericContract(gl.Contract):
    string_items: DynArray[Item[str]]
    
    def __init__(self):
        pass
    
    @gl.public.write
    def add_item(self, data: str, label: str):
        item = gl.storage.inmem_allocate(Item[str], data, label)
        self.string_items.append(item)
    
    @gl.public.view
    def get_items(self) -> DynArray[TreeMap[str, typing.Any]]:
        return self.string_items
```