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https://github.com/scrypt-inc/py-scryptlib

sCrypt SDK for Python
https://github.com/scrypt-inc/py-scryptlib

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sCrypt SDK for Python

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# scryptlib-python

A Python SDK for [sCrypt](https://scrypt.io/).



[![Build Status](https://app.travis-ci.com/sCrypt-Inc/py-scryptlib.svg?branch=main)](https://travis-ci.com/sCrypt-Inc/py-scryptlib)



You can learn all about writing sCrypt smart contracts in the official [docs](https://scryptdoc.readthedocs.io/en/latest/intro.html).



## Installation



### Compiler



To use the SDK, you need to get a copy of the sCrypt compiler. You can get it either by downloading the [sCrypt IDE](https://scrypt.io/#download) or executing the following command, if you have an UNIX-like OS:

```sh

curl -Ls https://scrypt.io/setup | sh -s --

```



This will download the latest version of the compiler.



You can also download a specific version of the compiler using the `-v` flag:

```sh

curl -Ls https://scrypt.io/setup | sh -s -- -v 1.15.1

```



### SDK



You can install the SDK as a Python package using `pip`, either directly from the project folder:



```sh

pip install .

```



, or from the PyPI:



```sh

pip install scryptlib

```



## Usage



The SDK is used to convert script templates, produced by the sCrypt compiler, to an object-based representation in Python. It allows for easy compilation, inspection and verification of smart contracts.



### Compiling an sCrypt contract



We can compile an sCrypt conract source file like so:



```python

import scryptlib



contract = './test/res/arraydemo.scrypt'

compiler_result = scryptlib.compile_contract(contract)

```



This will leave us with a `CompilerResult` object, that contains all of the data, returned by the compiler.

The `compile_contract` method will try to automatically search for the compiler binary. You can also explicitly pass the path to the binary, using the `compiler_bin` parameter.



It is also possible to pass the sCrypt source code as a string object:



```python

contract_source = '''

    contract Equals {

        int x;



        constructor(int x) {

            this.x = x;

        }



        public function equals(int y) {

            require(this.x == y);

        }



    }

'''



compiler_result = scryptlib.compile_contract(contract_source, from_string=True)

```



The resulting **contract description file** will be written to `./out` by default. That may also be changed with the `out_dir` parameter.

You can access the contract description directly from the `CompilerResult` with its `to_desc()` method.



### Evaluating a contract locally



We can evaluate any public function of the contract locally on our machine, before broadcasting it.



First we need to create a class representation of the contract and instantiate it:



```python

import scryptlib

from scryptlib.types import Int



EQUAL_VAL = 2021



contract = './test/res/equals.scrypt'



compiler_result = scryptlib.compile_contract(contract)

desc = compiler_result.to_desc()



Equals = scryptlib.build_contract_class(desc)

contract_obj = Equals(Int(EQUAL_VAL))

```



As we can see, the created class takes the contract parameters in the constructor. In the case of the `Equals` contract, that is an sCrypt `int` type, which we can represent in Python using an instance of `scryptlib.types.Int`.



Once we have an instance of the contract class, we can evaluate its public functions:



```python

verify_result = contract_obj.equals(Int(EQUAL_VAL)).verify()

assert verify_result == True

```



From the example see, that we called the contracts public function, named `equals`. The actual call to `equals()` in Python reutrns an instance of `scryptlib.abi.FunctionCall`. That object in turn has a method, named `verify`, with which we can run the function calls unlocking script against the contracts locking script.

`verify` can internaly create an input evaluation context for simple contracts, but once we start using more advanced constructs, like signatures, we can pass an instance of `bitconx.TxInputContext`, using the `tx_input_context` parameter.



py-scryptlib leverages the [bitcoinx](https://github.com/kyuupichan/bitcoinX) library to deal with Bitcoin primitives.



The following is an example of a local evaluation of a P2PKH contract:



```python

import pytest

import scryptlib

from scryptlib.types import Sig, PubKey, Ripemd160



import bitcoinx

from bitcoinx import SigHash, PrivateKey, pack_byte



key_priv = PrivateKey.from_arbitrary_bytes(b'test123')

key_pub = key_priv.public_key

pubkey_hash = key_pub.hash160()



contract = './test/res/p2pkh.scrypt'



compiler_result = scryptlib.compile_contract(contract)

desc = compiler_result.to_desc()



P2PKH = scryptlib.build_contract_class(desc)

p2pkh_obj = P2PKH(Ripemd160(pubkey_hash))



context = scryptlib.create_dummy_input_context()



sighash_flag = SigHash(SigHash.ALL | SigHash.FORKID)

input_idx = 0

utxo_satoshis = context.utxo.value

sighash = context.tx.signature_hash(input_idx, utxo_satoshis, p2pkh_obj.locking_script, sighash_flag)



sig = key_priv.sign(sighash, hasher=None)

sig = sig + pack_byte(sighash_flag)



verify_result = p2pkh_obj.unlock(Sig(sig), PubKey(key_pub)).verify(context)

assert verify_result == True

```



You can find many other examples under `test/test_contract_*`.



## Testing



The SDK has a suite of unit tests, which we can run by executing the `pytest` command in the root of the project.