smart-contracts-docs

No Classes Allowed!

Contracting maintains a strict ‘no classes’ model. This forces you as the developer to create more procedural code that is explicit and completely self-contained. Contracts must be easy to read and understand for validity. Instead of thinking of your code in classes, think of each contract as a ‘module’ that exposes certain functions to it’s users.

All class keywords will fail your contract on submission. Even if you try to use classes for object oriented code, you will have to find another way to express your structures.

For example:

class Car:
	def __init__(self, make, model):
		self.make = make
		self.model = model

This is illegal. Instead, describe objects in dictionary formats. If you tend to use classes to encapsulate data, simply use Python dictionaries instead. This is especially useful because of Contracting’s storage model that makes it easy to store dictionaries.

cars = Hash()
cars['balthasar'] = {
	'make': 'Ford',
	'model': 'Contour'
}

Read more about storage in the Storage section.

Restricted Builtins

Certain builtins such as exec, eval, and compile are obviously dangerous. We do not want to allow any arbitrary execution of code.

Here is a list of most Python3.11 builtin functions versus the ones we allow in Contracting. NOTE: All exceptions except the base Exception class are removed from Contracting.

Built-Ins	Python3.11	Contracting	Reason for Restriction
`abs()`	✓	✓
`all()`	✓	✓
`any()`	✓	✓
`ascii()`	✓	✓
`bin()`	✓	✓
`bool()`	✓	✓
`bytearray()`	✓	✓
`bytes()`	✓	✓
`callable()`	✓	✘	Functions are not passed as objects in Contracting.
`chr()`	✓	✓
`classmethod()`	✓	✘	Classes are disabled in Contracting.
`compile()`	✓	✘	Arbitrary code execution is a high security risk.
`complex()`	✓	✘	Complex numbers are potentially non-deterministic. This is a consensus failure risk.
`copyright`	✓	✘	Unnecessary.
`credits`	✓	✘	Unnecessary.
`delattr()`	✓	✘	Arbitrary removal of Python attributes could allow unauthorized access to private objects and methods.
`dict()`	✓	✓
`dir()`	✓	✘	Allows exploration path into security exploit development.
`divmod()`	✓	✓
`enumerate()`	✓	✘	Potentially safe. Evaluating to make sure.
`eval()`	✓	✘	Arbitrary code execution is a high security risk.
`exec()`	✓	✘	Arbitrary code execution is a high security risk.
`filter()`	✓	✓
`float()`	✓	✘	Floating point precision issues can lead to consensus failures.
`format()`	✓	✓
`frozenset()`	✓	✓
`getattr()`	✓	✘	Arbitrary access to attributes could allow private function execution.
`globals()`	✓	✘	Access to global scope methods allows modification of private methods and direct storage mechanisms.
`hasattr()`	✓	✘	Allows exploration path into security exploit development.
`hash()`	✓	✘	Potentially non-deterministic outcomes. Consensus failure risk.
`help()`	✓	✘	Unnecessary.
`hex()`	✓	✓
`id()`	✓	✘	Potentially non-deterministic outcomes. Consensus failure risk.
`input()`	✓	✘	User input not supported.
`int()`	✓	✓
`isinstance()`	✓	✓
`issubclass()`	✓	✓
`iter()`	✓	✘	Potential mutation of objects that are only supposed to be interfaced with through particular methods.
`len()`	✓	✓
`license`	✓	✘	Unnecessary.
`list()`	✓	✓
`locals()`	✓	✘	See globals()
`map()`	✓	✓
`max()`	✓	✓
`memoryview()`	✓	✘	Potentially non-deterministic outcomes. Consensus failure risk.
`min()`	✓	✓
`next()`	✓	✘	See iter()
`object()`	✓	✘	See callable()
`oct()`	✓	✓
`open()`	✓	✘	File I/O not supported.
`ord()`	✓	✓
`pow()`	✓	✓
`print()`	✓	✓
`property()`	✓	✘	Property creation not supported because classes are disabled.
`range()`	✓	✓
`repr()`	✓	✘	Unnecessary and non-deterministic due to memory address as output of this function. This is a consensus failure risk.
`reversed()`	✓	✓
`round()`	✓	✓
`set()`	✓	✓
`setattr()`	✓	✘	Arbitrary setting and overwriting of Python attributes has storage corruption and private method access implications.
`slice()`	✓	✘	Unnecessary.
`sorted()`	✓	✓
`staticmethod()`	✓	✘	Static methods are not supported because classes are disabled.
`str()`	✓	✓
`sum()`	✓	✓
`super()`	✓	✘	Super is not supported because classes are disabled.
`tuple()`	✓	✓
`type()`	✓	✓
`vars()`	✓	✘	Allows exploration path into security exploit development.
`zip()`	✓	✓

Illegal AST Nodes

Similarly, some of the AST (abstract syntax tree) nodes that make up deeper levels of the Python syntax are not allowed. Mainly, the nodes around the async/await features are restricted.

AST Node	Reason for Restriction
ast.AsyncFor	All async code is invalid in Contracting.
ast.AsyncFunctionDef	All async code is invalid in Contracting.
ast.AsyncWith	All async code is invalid in Contracting.
ast.AugLoad	AST Node never used in current CPython implementation.
ast.AugStore	AST Node never used in current CPython implementation.
ast.Await	All async code is invalid in Contracting.
ast.ClassDef	Classes are disabled in Contracting.
ast.Ellipsis	Ellipsis should not be defined in a smart contract. They may be an effect of one.
ast.GeneratorExp	Generators hold state that is incompatible with Contracting’s model.
ast.Global	Scope modification could have security implications.
ast.Interactive	Only available in Python interpreters. Potential security risk.
ast.MatMult	New AST feature. Not yet widely adopted. Potential security risk.
ast.Nonlocal	Scope modification could have security implications.
ast.Suite	Similar to ast.Interactive
ast.Yield	Generator related code is not compatible with Contracting.
ast.YieldFrom	Generator related code is not compatible with Contracting.

Violations

The linter will check for several violations that will fail your smart contract automatically. Here is a list of the current violations and examples of code that will cause them.

S1- Illegal contracting syntax type used

Thrown when an AST type that is not allowed is visited by the linter.

2 @ 2 # ast.MatMul code

S2- Illicit use of ‘_’ before variable

_ is used for gating certain functionality. Using it as a prefix to any variable will cause failure

_balances = Hash()

S3- Illicit use of Nested imports

import keywords found inside of functions, loops, etc. will fail.

import this_wont_fail

@construct
def seed():
	import this_will

S4- ImportFrom compilation nodes not yet supported

Selective importing is not supported and will fail contracts.

from token import send

S5- Contract not found in lib

Not currently used.

S6- Illicit use of classes

Classes are not supported in Contracting and their keywords will fail your contract.

class Car:
    def __init__(self, make, model):
        self.make = make
        self.model = model

S7- Illicit use of Async functions

Any async related code will fail the contract.

async def fail_me():
    pass

S8- Invalid decorator used

@export and @construct are the only two decorators allowed in Contracting.

v = Variable()
@construct
def seed():
    v.set(100)

@export
def get_v():
    return v.get()

@unknown
def this_will_fail():
    pass

S9- Multiple use of constructors detected

Only a single @construct can be included in a contract.

v = Variable()
@construct
def seed():
    v.set(123)

@construct
def seed_2():
    v.set(999)

S10- Illicit use of multiple decorators

Stacking decorators is not allowed.

v = Variable()
@export
@construct
def seed():
    v.set(777)

S11- Illicit keyword overloading for ORM assignments

ORM arguments are injected into the __init__ function on runtime. Messing with these will fail your contract.

v = Variable(contract='token')
w = Variable(driver=None)
x = Variable(another_kwarg='this will fail')

@export
def set():
    v.set(777)
    w.set(999)
    x.set(123)

S12- Multiple targets to ORM definition detected

Python allows multiple assignment. Trying to do a multiple assignment from an ORM object will fail your contract.

x, y = Hash()
@export
def set():
    x['stu'] = 100
    y['stu'] = 999

S13- No valid contracting decorator found

A contract without a single @export decorator is invalid.

@construct
def seed():
    pass

S14- Illegal use of a builtin

Referencing a builtin that is illegal will fail the contract.

@export
def credits():
    return credits

S15- Reuse of ORM name definition in a function definition argument name

Reuse of any ORM names in any loops, functions, etc. will fail the contract.

used_once = Variable()

@export
def override():
    used_once = 123