icontract 2.0.2

Reladed Projects

There exist a couple of contract libraries. However, at the time of this writing (September 2018), they all required the programmer either to learn a new syntax (PyContracts) or to write redundant condition descriptions ( e.g., contracts, covenant, dpcontracts, pyadbc and pcd).

This library was strongly inspired by them, but we go two steps further.

First, our violation message on contract breach are much more informatinve. The message includes the source code of the contract condition as well as variable values at the time of the breach. This promotes don’t-repeat-yourself principle (DRY) and spare the programmer the tedious task of repeating the message that was already written in code.

Second, icontract allows inheritance of the contracts and supports weakining of the preconditions as well as strengthening of the postconditions and invariants. Notably, weakining and strengthening of the contracts is a feature indispensable for modeling many non-trivial class hierarchies. Please see Section Inheritance. To the best of our knowledge, there is currently no other Python library that supports inheritance of the contracts in a correct way.

In the long run, we hope that design-by-contract will be adopted and integrated in the language. Consider this library a work-around till that happens. An ongoing discussion on how to bring design-by-contract into Python language can be followed on python-ideas mailing list.

Invariants

Invariants are special contracts associated with an instance of a class. An invariant should hold after initialization and before and after a call to any public instance method. The invariants are the pivotal element of design-by-contract: they allow you to formally define properties of a data structures that you know will be maintained throughout the life time of every instance.

We consider the following methods to be “public”:

• All methods not prefixed with _

• All magic methods (prefix __ and suffix __)

Class methods can not observe the invariant since they are not associated with an instance of the class.

We exempt __getattribute__, __setattr__ and __delattr__ methods from observing the invariant since these functions alter the state of the instance and thus can not be considered “public”.

We also excempt __repr__ method to prevent endless loops when generating error messages.

The icontract invariants are implemented as class decorators.

The following examples show various cases when an invariant is breached.

After the initialization:

>>> @icontract.invariant(lambda self: self.x > 0)
... class SomeClass:
...     def __init__(self) -> None:
...         self.x = -1
...
...     def __repr__(self) -> str:
...         return "some instance"
...
>>> some_instance = SomeClass()
Traceback (most recent call last):
 ...
icontract.errors.ViolationError: self.x > 0:
self was some instance
self.x was -1

Before the invocation of a public method:

>>> @icontract.invariant(lambda self: self.x > 0)
... class SomeClass:
...     def __init__(self) -> None:
...         self.x = 100
...
...     def some_method(self) -> None:
...         self.x = 10
...
...     def __repr__(self) -> str:
...         return "some instance"
...
>>> some_instance = SomeClass()
>>> some_instance.x = -1
>>> some_instance.some_method()
Traceback (most recent call last):
 ...
icontract.errors.ViolationError: self.x > 0:
self was some instance
self.x was -1

After the invocation of a public method:

>>> @icontract.invariant(lambda self: self.x > 0)
... class SomeClass:
...     def __init__(self) -> None:
...         self.x = 100
...
...     def some_method(self) -> None:
...         self.x = -1
...
...     def __repr__(self) -> str:
...         return "some instance"
...
>>> some_instance = SomeClass()
>>> some_instance.some_method()
Traceback (most recent call last):
 ...
icontract.errors.ViolationError: self.x > 0:
self was some instance
self.x was -1

After the invocation of a magic method:

>>> @icontract.invariant(lambda self: self.x > 0)
... class SomeClass:
...     def __init__(self) -> None:
...         self.x = 100
...
...     def __call__(self) -> None:
...         self.x = -1
...
...     def __repr__(self) -> str:
...         return "some instance"
...
>>> some_instance = SomeClass()
>>> some_instance()
Traceback (most recent call last):
 ...
icontract.errors.ViolationError: self.x > 0:
self was some instance
self.x was -1

Snapshots (a.k.a “old” argument values)

Usual postconditions can not verify the state transitions of the function’s argument values. For example, it is impossible to verify in a postcondition that the list supplied as an argument was appended an element since the postcondition only sees the argument value as-is after the function invocation.

In order to verify the state transitions, the postcondition needs the “old” argument value (before the invocation) as well as the current argument value (after the invocation). icontract.snapshot decorator instructs the checker to take snapshots of the argument values before the function call which are then supplied as OLD argument to the postcondition function.

icontract.snapshot takes a capture function which accepts a single argument of the function (or none). The name of the argument corresponds to the property of OLD object passed to the postcondition. You can also rename the property in OLD by supplying a name argument to icontract.snapshot.

Here is an example that uses snapshots to check that a value was appended to the list:

>>> import icontract
>>> from typing import List

>>> @icontract.snapshot(lambda lst: lst[:])
... @icontract.ensure(lambda OLD, lst, value: lst == OLD.lst + [value])
... def some_func(lst: List[int], value: int) -> None:
...     lst.append(value)
...     lst.append(1984)  # bug

>>> some_func(lst=[1, 2], value=3)
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: lst == OLD.lst + [value]:
OLD was a bunch of OLD values
OLD.lst was [1, 2]
lst was [1, 2, 3, 1984]
value was 3

The following example shows how you can name the snapshot:

>>> import icontract
>>> from typing import List

>>> @icontract.snapshot(lambda lst: len(lst), name="len_lst")
... @icontract.ensure(lambda OLD, lst, value: len(lst) == OLD.len_lst + 1)
... def some_func(lst: List[int], value: int) -> None:
...     lst.append(value)
...     lst.append(1984)  # bug

>>> some_func(lst=[1, 2], value=3)
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: len(lst) == OLD.len_lst + 1:
OLD was a bunch of OLD values
OLD.len_lst was 2
len(lst) was 4
lst was [1, 2, 3, 1984]

Inheritance

To inherit the contracts of the parent class, the child class needs to either inherit from icontract.DBC or have a meta class set to icontract.DBCMeta.

When no contracts are specified in the child class, all contracts are inherited from the parent class as-are.

When the child class introduces additional preconditions or postconditions and invariants, these contracts are strengthened or weakened, respectively. icontract.DBCMeta allows you to specify the contracts not only on the concrete classes, but also on abstract classes.

Strengthening. If you specify additional invariants in the child class then the child class will need to satisfy all the invariants of its parent class as well as its own additional invariants. Analogously, if you specify additional postconditions to a function of the class, that function will need to satisfy both its own postconditions and the postconditions of the original parent function that it overrides.

Weakining. Adding preconditions to a function in the child class weakens the preconditions. The caller needs to provide either arguments that satisfy the preconditions associated with the function of the parent class or arguments that satisfy the preconditions of the function of the child class.

Preconditions and Postconditions of __init__. Mind that __init__ method is a special case. Since the constructor is exempt from polymorphism, preconditions and postconditions of base classes are not inherited for the __init__ method. Only the preconditions and postconditions specified for the __init__ method of the concrete class apply.

Abstract Classes. Since Python 3 does not allow multiple meta classes, icontract.DBCMeta inherits from abc.ABCMeta to allow combining contracts with abstract base classes.

Snapshots. Snapshots are inherited from the base classes for computational efficiency. You can use snapshots from the base classes as if they were defined in the concrete class.

The following example shows an abstract parent class and a child class that inherits and strengthens parent’s contracts:

>>> import abc
>>> import icontract

>>> @icontract.invariant(lambda self: self.x > 0)
... class A(icontract.DBC):
...     def __init__(self) -> None:
...         self.x = 10
...
...     @abc.abstractmethod
...     @icontract.ensure(lambda y, result: result < y)
...     def func(self, y: int) -> int:
...         pass
...
...     def __repr__(self) -> str:
...         return "instance of A"

>>> @icontract.invariant(lambda self: self.x < 100)
... class B(A):
...     def func(self, y: int) -> int:
...         # Break intentionally the postcondition
...         # for an illustration
...         return y + 1
...
...     def break_parent_invariant(self):
...         self.x = -1
...
...     def break_my_invariant(self):
...         self.x = 101
...
...     def __repr__(self) -> str:
...         return "instance of B"

# Break the parent's postcondition
>>> some_b = B()
>>> some_b.func(y=0)
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: result < y:
result was 1
y was 0

# Break the parent's invariant
>>> another_b = B()
>>> another_b.break_parent_invariant()
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: self.x > 0:
self was instance of B
self.x was -1

# Break the child's invariant
>>> yet_another_b = B()
>>> yet_another_b.break_my_invariant()
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: self.x < 100:
self was instance of B
self.x was 101

The following example shows how preconditions are weakened:

>>> class A(icontract.DBC):
...     @icontract.require(lambda x: x % 2 == 0)
...     def func(self, x: int) -> None:
...         pass

>>> class B(A):
...     @icontract.require(lambda x: x % 3 == 0)
...     def func(self, x: int) -> None:
...         pass

>>> b = B()

# The precondition of the parent is satisfied.
>>> b.func(x=2)

# The precondition of the child is satisfied,
# while the precondition of the parent is not.
# This is OK since the precondition has been
# weakened.
>>> b.func(x=3)

# None of the preconditions have been satisfied.
>>> b.func(x=5)
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: x % 3 == 0: x was 5

The example below illustrates how snaphots are inherited:

>>> class A(icontract.DBC):
...     @abc.abstractmethod
...     @icontract.snapshot(lambda lst: lst[:])
...     @icontract.ensure(lambda OLD, lst: len(lst) == len(OLD.lst) + 1)
...     def func(self, lst: List[int], value: int) -> None:
...         pass

>>> class B(A):
...     # The snapshot of OLD.lst has been defined in class A.
...     @icontract.ensure(lambda OLD, lst: lst == OLD.lst + [value])
...     def func(self, lst: List[int], value: int) -> None:
...         lst.append(value)
...         lst.append(1984)  # bug

>>> b = B()
>>> b.func(lst=[1, 2], value=3)
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: len(lst) == len(OLD.lst) + 1:
OLD was a bunch of OLD values
OLD.lst was [1, 2]
len(OLD.lst) was 2
len(lst) was 4
lst was [1, 2, 3, 1984]

Toggling Contracts

By default, the contract checks (including the snapshots) are always perfromed at run-time. To disable them, run the interpreter in optimized mode (-O or -OO, see Python command-line options).

If you want to override this behavior, you can supply the enabled argument to the contract:

>>> @icontract.require(lambda x: x > 10, enabled=False)
... def some_func(x: int) -> int:
...     return 123
...

# The pre-condition is breached, but the check was disabled:
>>> some_func(x=0)
123

Icontract provides a global variable icontract.SLOW to provide a unified way to mark a plethora of contracts in large code bases. icontract.SLOW reflects the environment variable ICONTRACT_SLOW.

While you may want to keep most contracts running both during the development and in the production, contracts marked with icontract.SLOW should run only during the development (since they are too sluggish to execute in a real application).

If you want to enable contracts marked with icontract.SLOW, set the environment variable ICONTRACT_SLOW to a non-empty string.

Here is some example code:

# some_module.py
@icontract.require(lambda x: x > 10, enabled=icontract.SLOW)
    def some_func(x: int) -> int:
        return 123

# in test_some_module.py
import unittest

class TestSomething(unittest.TestCase):
    def test_some_func(self):
        self.assertEqual(123, some_func(15))

if __name__ == '__main__':
    unittest.main()

Run this bash command to execute the unit test with slow contracts:

$ ICONTRACT_SLOW=true python test_some_module.py

Custom Errors

Icontract raises ViolationError by default. However, you can also instruct icontract to raise a different error by supplying error argument to the decorator.

The error argument can either be:

• An exception class. The exception is constructed with the violation message and finally raised.

• A callable that returns an exception. The callable accepts the subset of arguments of the original function (including result and OLD for postconditions) or self in case of invariants, respectively, and returns an exception. The arguments to the condition function can freely differ from the arguments to the error function.

  The exception returned by the given callable is finally raised.

  If you specify the error argument as callable, the values will not be traced and the condition function will not be parsed. Hence, violation of contracts with error arguments as callables incur a much smaller computational overhead in case of violations compared to contracts with default violation messages for which we need to trace the argument values and parse the condition function.

Here is an example of the error given as an exception class:

>>> @icontract.require(lambda x: x > 0, error=ValueError)
... def some_func(x: int) -> int:
...     return 123
...

# Custom Exception class
>>> some_func(x=0)
Traceback (most recent call last):
    ...
ValueError: x > 0: x was 0

Here is an example of the error given as a callable:

>>> @icontract.require(
...     lambda x: x > 0,
...     error=lambda x: ValueError('x must be positive, got: {}'.format(x)))
... def some_func(x: int) -> int:
...     return 123
...

# Custom Exception class
>>> some_func(x=0)
Traceback (most recent call last):
    ...
ValueError: x must be positive, got: 0

Implementation Details

Decorator stack. The precondition and postcondition decorators have to be stacked together to allow for inheritance. Hence, when multiple precondition and postcondition decorators are given, the function is actually decorated only once with a precondition/postcondition checker while the contracts are stacked to the checker’s __preconditions__ and __postconditions__ attribute, respectively. The checker functions iterates through these two attributes to verify the contracts at run-time.

All the decorators in the function’s decorator stack are expected to call functools.update_wrapper(). Notably, we use __wrapped__ attribute to iterate through the decorator stack and find the checker function which is set with functools.update_wrapper(). Mind that this implies that preconditions and postconditions are verified at the inner-most decorator and not when outer preconditios and postconditions are defined.

Consider the following example:

@some_custom_decorator
@icontract.require(lambda x: x > 0)
@another_custom_decorator
@icontract.require(lambda x, y: y < x)
def some_func(x: int, y: int) -> None:
  # ...

The checker function will verify the two preconditions after both some_custom_decorator and another_custom_decorator have been applied, whily you would expect that the outer precondition (x > 0) is verified immediately after some_custom_decorator is applied.

To prevent bugs due to unexpected behavior, we recommend to always group preconditions and postconditions together.

Invariants. Since invariants are handled by a class decorator (in contrast to function decorators that handle preconditions and postconditions), they do not need to be stacked. The first invariant decorator wraps each public method of a class with a checker function. The invariants are added to the class’ __invariants__ attribute. At run-time, the checker function iterates through the __invariants__ attribute when it needs to actually verify the invariants.

Mind that we still expect each class decorator that decorates the class functions to use functools.update_wrapper() in order to be able to iterate through decorator stacks of the individual functions.

Linter

We provide a linter that statically verifies the arguments of the contracts (i.e. that they are well-defined with respect to the function). The tool is available as a separate package, pyicontract-lint.

Sphinx

We implemented a Sphinx extension to include contracts in the documentation. The extension is available as a package sphinx-icontract.