Your First Contract

While GenLayer contract is a pure Python program, few things must be presented in your file.

Magic comment

First of all, you need to place a magic comment with the version of GenVM you wish to use on the first line of your file

# { "Depends": "py-genlayer:test" }

It is similar to Solidity's pragma solidity, and test after a colon is a version. When GenLayer releases, it will be changed to some exact hash of a version, which will be frozen forever.

Importing standard library

Then you should import genlayer standard library:

from genlayer import *

This will import all types to the global scope, and everything else under gl namespace.

Finally, the contract

Contract is a Python class, with regular constructor and methods which allows you to use your favorite IDE for type checking and auto completion.

However, there are some catches:

• This class must be decorated with @gl.contract so that magic happens
• All persistent data should be declared as class fields and annotated with types
  • Note that not all types are supported, but there are rewriting rules:
    • list[T] → DynArray[T]
    • dict[K, V] → TreeMap[K, V]
  • int type isn't supported on purpose. You most likely wish to use some fixed-size integer type, such as i32 or u256. If this is not the case and you are sure that you need big integers, you can annotate your field with bigint, which is just an alias for python int
  • Only fully instantiated generic types can be used, so TreeMap is forbidden, while TreeMap[str, u256] is not
• All public methods must be decorated with either @gl.public.view for read-only methods or @gl.public.write for methods that modify storage. Constructor (__init__) must be private (not annotated)

# { "Depends": "py-genlayer:test" }
 
from genlayer import *
 
@gl.contract
class Hello:
    name: str
 
    def __init__(self, name: str):
        self.name = name
 
    @gl.public.view
    def run(self) -> str:
        return f'Hello, {self.name}'
 
    @gl.public.write
    def set_name(self, name: str):
        print(f'debug old name: {self.name}') # <- you can use prints for debugging
            # they will be included in the GenVM execution log
        self.name = name