icontract

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

It also gives a base for a flourishing of a wider ecosystem:

• A linter pyicontract-lint,
• A sphinx plug-in sphinx-icontract,
• A tool icontract-hypothesis for automated testing and ghostwriting test files which infers Hypothesis strategies based on the contracts,
  • together with IDE integrations such as icontract-hypothesis-vim, icontract-hypothesis-pycharm, and icontract-hypothesis-vscode,
• Directly integrated into CrossHair, a tool for automatic verification of Python programs,
  • together with IDE integrations such as crosshair-pycharm and crosshair-vscode, and
• An integration with FastAPI through fastapi-icontract to enforce contracts on your HTTP API and display them in OpenAPI 3 schema and Swagger UI, and
• An extensive corpus, Python-by-contract corpus, of Python programs annotated with contracts for educational, testing and research purposes.

Related 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, deal, 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 informative. 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. You might be also interested in the archived discussion on how to bring design-by-contract into Python language on python-ideas mailing list.

Teasers

We give a couple of teasers here to motivate the library. Please see the documentation available on icontract.readthedocs.io for a full scope of its capabilities.

The script is also available as a repl.it post.

>>> 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
y was 5

# 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
y was 5

# 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
y was 5


# Pre-conditions fail before post-conditions.
>>> @icontract.ensure(lambda result, x: result > x)
... @icontract.require(lambda x: x > 3, "x must not be small")
... def some_func(x: int, y: int = 5) -> int:
...    return x - y
...
>>> some_func(x=3)
Traceback (most recent call last):
  ...
icontract.errors.ViolationError: File <doctest README.rst[14]>, line 2 in <module>:
x must not be small: x > 3:
x was 3
y was 5

# Invariant
>>> @icontract.invariant(lambda self: self.x > 0)
... class SomeClass:
...     def __init__(self) -> None:
...         self.x = -1
...
...     def __repr__(self) -> str:
...         return "an instance of SomeClass"
...
>>> some_instance = SomeClass()
Traceback (most recent call last):
 ...
icontract.errors.ViolationError: File <doctest README.rst[16]>, line 1 in <module>:
self.x > 0:
self was an instance of SomeClass
self.x was -1

Installation

• Install icontract with pip:

pip3 install icontract

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).