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Provide design-by-contract with informative violation messages

Project description


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

There exist a couple of contract libraries. However, at the time of this writing (July 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 a step further and use the meta programming library to infer violation messages from the code in order to promote dont-repeat-yourself principle (DRY) and spare the programmer the tedious task of repeating the message that was already written in code.

We want this library to be used mainly in production code and let us spot both development and production bugs with enough information. Therefore, we decided to implement only the pre-conditions and post-conditions which require little overhead, and intentionally left out the class invariants. Class invariants seem to us tricky to grasp ( for example, depending on the design class invariants may hold only at the first call of the public function, but not in the private functions; or they may hold only at the first call to a method of a class, but not in the sequent calls to other class methods etc.). The invariants hence need to come with an overhead which is generally impractical for production systems.


icontract provides two decorators, pre and post for pre-conditions and post-conditions, respectively.

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.

You can provide a custom representation function with the argument repr_args that needs to cover all the input arguments (including result in post-conditions) of the condition function and return a string. If no representation function was specified, the input arguments are represented by concatenation of __repr__ on each one of them.

If no custom representation function has been supplied, 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.

>>> import icontract

>>> @icontract.pre(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.ViolationError: Precondition violated: x > 3: x was 1

# Pre-condition violation with a description
>>> @icontract.pre(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.ViolationError: Precondition violated: x must not be small: x > 3: x was 1

# Pre-condition violation with a custom representation function
>>> @icontract.pre(lambda x: x > 3, repr_args=lambda x: "x was {:03}".format(x))
... def some_func(x: int, y: int = 5) -> None:
...     pass
>>> some_func(x=1)
Traceback (most recent call last):
icontract.ViolationError: Precondition violated: x > 3: x was 001

# 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"
>>> @icontract.pre(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.ViolationError: Precondition violated: (a.b.x + a.b.y()) > SOME_GLOBAL_VAR:
a was instance of A
a.b was instance of B
a.b.x was 7
a.b.y() was 2

# Post-condition
>>> 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.ViolationError: Post-condition violated: result > x:
result was 5
x was 10


  • Install icontract with pip:
pip3 install icontract


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


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).

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