icontract

icontract provides design-by-contract to Python3 with informative violation messages and inheritance.

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.

Usage

icontract provides two function decorators, require and ensure for pre-conditions and post-conditions, respectively. Additionally, it provides a class decorator, invariant, to establish class invariants.

The condition argument specifies the contract and is usually written in lambda notation. In post-conditions, condition function receives a reserved parameter result corresponding to the result of the function. The condition can take as input a subset of arguments required by the wrapped function. This allows for very succinct conditions.

You can provide an optional description by passing in description argument.

Whenever a violation occurs, ViolationError is raised. Its message includes:

• the human-readable representation of the condition,
• description (if supplied) and
• representation of all the values.

The representation of the values is obtained by re-executing the condition function programmatically by traversing its abstract syntax tree and filling the tree leaves with values held in the function frame. Mind that this re-execution will also re-execute all the functions. Therefore you need to make sure that all the function calls involved in the condition functions do not have any side effects.

If you want to customize the error, see Section "Custom Errors".

>>> import icontract

>>> @icontract.require(lambda x: x > 3)
... def some_func(x: int, y: int = 5)->None:
...     pass
...

>>> some_func(x=5)

# Pre-condition violation
>>> some_func(x=1)
Traceback (most recent call last):
  ...
icontract.errors.ViolationError: File <doctest README.rst[1]>, line 1 in <module>:
x > 3: x was 1

# Pre-condition violation with a description
>>> @icontract.require(lambda x: x > 3, "x must not be small")
... def some_func(x: int, y: int = 5) -> None:
...     pass
...
>>> some_func(x=1)
Traceback (most recent call last):
  ...
icontract.errors.ViolationError: File <doctest README.rst[4]>, line 1 in <module>:
x must not be small: x > 3: x was 1

# Pre-condition violation with more complex values
>>> class B:
...     def __init__(self) -> None:
...         self.x = 7
...
...     def y(self) -> int:
...         return 2
...
...     def __repr__(self) -> str:
...         return "instance of B"
...
>>> class A:
...     def __init__(self)->None:
...         self.b = B()
...
...     def __repr__(self) -> str:
...         return "instance of A"
...
>>> SOME_GLOBAL_VAR = 13
>>> @icontract.require(lambda a: a.b.x + a.b.y() > SOME_GLOBAL_VAR)
... def some_func(a: A) -> None:
...     pass
...
>>> an_a = A()
>>> some_func(an_a)
Traceback (most recent call last):
  ...
icontract.errors.ViolationError: File <doctest README.rst[9]>, line 1 in <module>:
a.b.x + a.b.y() > SOME_GLOBAL_VAR:
SOME_GLOBAL_VAR was 13
a was instance of A
a.b was instance of B
a.b.x was 7
a.b.y() was 2

# Post-condition
>>> @icontract.ensure(lambda result, x: result > x)
... def some_func(x: int, y: int = 5) -> int:
...     return x - y
...
>>> some_func(x=10)
Traceback (most recent call last):
  ...
icontract.errors.ViolationError: File <doctest README.rst[12]>, line 1 in <module>:
result > x:
result was 5
x was 10

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: File <doctest README.rst[14]>, line 1 in <module>:
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: File <doctest README.rst[16]>, line 1 in <module>:
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: File <doctest README.rst[20]>, line 1 in <module>:
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: File <doctest README.rst[23]>, line 1 in <module>:
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" state of the argument values (i.e. prior to the invocation of the function) as well as the current values (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 none, one or more arguments of the function. You set the name of the property in OLD as name argument to icontract.snapshot. If there is a single argument passed to the the capture function, the name of the OLD property can be omitted and equals the name of the argument.

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: File <doctest README.rst[28]>, line 2 in <module>:
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: File <doctest README.rst[32]>, line 2 in <module>:
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]

The next code snippet shows how you can combine multiple arguments of a function to be captured in a single snapshot:

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

>>> @icontract.snapshot(
...     lambda lst_a, lst_b: set(lst_a).union(lst_b), name="union")
... @icontract.ensure(
...     lambda OLD, lst_a, lst_b: set(lst_a).union(lst_b) == OLD.union)
... def some_func(lst_a: List[int], lst_b: List[int]) -> None:
...     lst_a.append(1984)  # bug

>>> some_func(lst_a=[1, 2], lst_b=[3, 4])
Traceback (most recent call last):
    ...
icontract.errors.ViolationError: File <doctest README.rst[36]>, line 4 in <module>:
set(lst_a).union(lst_b) == OLD.union:
OLD was a bunch of OLD values
OLD.union was {1, 2, 3, 4}
lst_a was [1, 2, 1984]
lst_b was [3, 4]
set(lst_a) was {1, 2, 1984}
set(lst_a).union(lst_b) was {1, 2, 3, 4, 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: File <doctest README.rst[40]>, line 7 in A:
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: File <doctest README.rst[40]>, line 1 in <module>:
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: File <doctest README.rst[41]>, line 1 in <module>:
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: File <doctest README.rst[49]>, line 2 in B:
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: File <doctest README.rst[54]>, line 4 in A:
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) -> None:
        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: File <doctest README.rst[60]>, line 1 in <module>:
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

!DANGER!

Be careful when you write contracts with custom errors. This might lead the caller to (ab)use the contracts as a control flow mechanism.

In that case, the user will expect that the contract is always enabled and not only during debug or test. (For example, whenever you run Python interpreter with -O or -OO, __debug__ will be False. If you left enabled argument to its default __debug__, the contract will not be verified in -O mode.)

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.

Known Issues

Integration with ``help()``. We wanted to include the contracts in the output of help(). Unfortunately, help() renders the __doc__ of the class and not of the instance. For functions, this is the class "function" which you can not inherit from. See this discussion on python-ideas for more details.

Defining contracts outside of decorators. We need to inspect the source code of the condition and error lambdas to generate the violation message and infer the error type in the documentation, respectively. inspect.getsource(.) is broken on lambdas defined in decorators in Python 3.5.2+ (see this bug report). We circumvented this bug by using inspect.findsource(.), inspect.getsourcefile(.) and examining the local source code of the lambda by searching for other decorators above and other decorators and a function or class definition below. The decorator code is parsed and then we match the condition and error arguments in the AST of the decorator. This is brittle as it prevents us from having partial definitions of contract functions or from sharing the contracts among functions.

Here is a short code snippet to demonstrate where the current implementation fails:

>>> require_x_positive = icontract.require(lambda x: x > 0)

>>> @require_x_positive
... def some_func(x: int) -> None:
...     pass

>>> some_func(x=0)
Traceback (most recent call last):
    ...
SyntaxError: Decorator corresponding to the line 1 could not be found in file <doctest README.rst[64]>: 'require_x_positive = icontract.require(lambda x: x > 0)\n'

However, we haven't faced a situation in the code base where we would do something like the above, so we are unsure whether this is a big issue. As long as decorators are directly applied to functions and classes, everything worked fine on our code base.

Benchmarks

We evaluated the computational costs incurred by using icontract in a couple of experiments. There are basically two types of costs caused by icontract: 1) extra parsing performed when you import the module regardless whether conditions are switched on or off and 2) run-time cost of verifying the contract.

We assumed in all the experiments that the contract breaches are exceptionally seldom and thus need not to be analyzed. If you are interested in these additional expereminets, please do let us know and create an issue on the github page.

The source code of the benchmarks is available in this directory. All experiments were performed with Python 3.5.2+ on Intel(R) Core(TM) i7-4700MQ CPU @ 2.40GHz with 8 cores and 4 GB RAM.

We execute a short timeit snippet to bring the following measurements into context:

python3 -m timeit -s 'import math' 'math.sqrt(1984.1234)'
10000000 loops, best of 3: 0.0679 usec per loop

Import Cost

Pre and post-conditions. We generated a module with 100 functions. For each of these functions, we introduced a variable number of conditions and stop-watched how long it takes to import the module over 10 runs. Finally, we compared the import time of the module with contracts against the same module without any contracts in the code.

The overhead is computed as the time difference between the total import time compared to the total import time of the module without the contracts.

Number of conditions per function	Total import time [milliseconds]	Overhead [milliseconds]	Overhead per condition and function [milliseconds]
None	795.59 ± 10.47	N/A	N/A
1	919.53 ± 61.22	123.93	1.24
5	1075.81 ± 59.87	280.22	0.56
10	1290.22 ± 90.04	494.63	0.49
1 disabled	833.60 ± 32.07	37.41	0.37
5 disabled	851.31 ± 66.93	55.72	0.11
10 disabled	897.90 ± 143.02	101.41	0.10

As we can see in the table above, the overhead per condition is quite minimal (1.24 milliseconds in case of a single enabled contract). The overhead decreases as you add more conditions since the icontract has already initialized all the necessary fields in the function objects.

When you disable the conditions, there is much less overhead (only 0.37 milliseconds, and decreasing). Since icontract returns immediately when the condition is disabled by implementation, we assume that the overhead coming from disabled conditions is mainly caused by Python interpreter parsing the code.

Invariants. Analogously, to measure the overhead of the invariants, we generated a module with 100 classes. We varied the number of invariant conditions per class and measure the import time over 10 runs. The results are presented in the following table.

Number of conditions per class	Total import time [milliseconds]	Overhead [milliseconds]	Overhead per condition and class [milliseconds]
None	843.61 ± 28.21	N/A	N/A
1	3409.71 ± 95.78	2566.1	25.66
5	4005.93 ± 131.97	3162.32	6.32
10	4801.82 ± 157.56	3958.21	3.96
1 disabled	885.88 ± 44.24	42.27	0.42
5 disabled	912.53 ± 101.91	68.92	0.14
10 disabled	963.77 ± 161.76	120.16	0.12

Similar to pre and post-conditions, there is decreasing cost per condition as number of conditions increases since the icontract has all set up the fields in the class metadata. However, invariants are much more costly compared to pre and postconditions and their overhead should be considered if import time needs to be kept minimal.

The overhead of the disabled invariants is attributed to the overhead of parsing the source file as icontract immediately returns from disabled invariants by implementation.

Run-time Cost

We constructed four modules around a function which computes a square root. We used timeit module to measure the performance. Each module was imported as part of the setup.

No precondition. This module does not check any preconditions and serves as a baseline.

Pre-condition as assert. We check pre-conditions by an assert statement.

def my_sqrt(x: float) -> float:
    assert x >= 0
    return math.sqrt(x)

Pre-condition as separate function. To mimick a more complex pre-condition, we encapsulate the assert in a separate function.

import math

def check_non_negative(x: float) -> None:
    assert x >= 0

def my_sqrt(x: float) -> float:
    check_non_negative(x)
    return math.sqrt(x)

Pre-condition with icontract. Finally, we compare against a module that uses icontract.

import math
import icontract

@icontract.require(lambda x: x >= 0)
def my_sqrt(x: float) -> float:
    return math.sqrt(x)

Pre-condition with icontract, disabled. We also perform a redundant check to verify that a disabled condition does not incur any run-time cost.

The following table sumarizes the timeit results (10000000 loops, best of 3). The run time of the baseline is computed as:

The run time of the other functions is computed analogously.

Precondition	Run time [microseconds]
None	0.168
As assert	0.203
As function	0.274
icontract	2.78
icontract, disabled	0.165

The overhead of icontract is substantial. While this may be prohibitive for points where computational efficiency is more important than correctness, mind that the overhead is still in order of microseconds. In most practical scenarios, where a function is more complex and takes longer than a few microseconds to execute, such a tiny overhead is justified by the gains in correctness, development and maintenance time.

Installation

• Install icontract with pip:

pip3 install icontract

Development

• Check out the repository.
• In the repository root, create the virtual environment:

python3 -m venv venv3

• Activate the virtual environment:

source venv3/bin/activate

• Install the development dependencies:

pip3 install -e .[dev]

• We use tox for testing and packaging the distribution. Run:

tox

• We also provide a set of pre-commit checks that lint and check code for formatting. Run them locally from an activated virtual environment with development dependencies:

./precommit.py

• The pre-commit script can also automatically format the code:

./precommit.py  --overwrite

Versioning

We follow Semantic Versioning. The version X.Y.Z indicates:

• X is the major version (backward-incompatible),
• Y is the minor version (backward-compatible), and
• Z is the patch version (backward-compatible bug fix).