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 methods 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 and will fail. Instead, think of each smart contracting having access to storage space. To access this storage space, you use a Hash or a Variable object. Hash objects are ‘auto-vivifying’ which just means they are dynamic. You can access and store data into certain keys by using the following pattern:
cars = Hash()
# First key is the primary key, or the name of the object
cars['balthasar', 'make'] = 'Ford'
cars['balthasar', '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 full list of Python3.6 builtin methods versus the ones we allow in Contracting. NOTE: All exceptions except the base Exception class are removed from Contracting.
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.AsyncmethodDef | 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.
def bad():
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
def bad_var():
_balances = Hash()
S3- Illicit use of Nested imports #
import keywords found inside of methods, loops, etc. will fail.
def bad_import():
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.
def bad_import():
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.
def bad_classes():
class Car:
def __init__(self, make, model):
self.make = make
self.model = model
S7- Illicit use of Async methods #
Any async related code will fail the contract.
def bad_async():
async def fail_me():
pass
S8- Invalid decorator used #
@export and @construct are the only two decorators allowed in Contracting.
def bad_decorator():
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.
def bad_construct():
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.
def bad_construct():
v = Variable()
@export
@construct
def seed():
v.set(777)
S11- Illicit keyword overloading for ORM assignments #
ORM arguments are injected into the __init__ method on runtime. Messing with these will fail your contract.
def bad_var():
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.
def bad_targets():
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.
def bad_export():
@construct
def seed():
pass
S14- Illegal use of a builtin #
Referencing a builtin that is illegal will fail the contract.
def bad_builtin():
@export
def credits():
return credits
S15- Reuse of ORM name definition in a method definition argument name #
Reuse of any ORM names in any loops, methods, etc. will fail the contract.
def bad_var():
used_once = Variable()
@export
def override():
used_once = 123
Lamden Sàrl