Metadata-Version: 1.1
Name: pcd
Version: 0.2.1
Summary: Python Contract Decorators
Home-page: https://github.com/petervaro/pcd
Author: Peter Varo
Author-email: hello@petervaro.com
License: LGPL-3.0
Download-URL: https://github.com/petervaro/pcd/archive/0.2.1.tar.gz
Description: [![pipeline status][4]][5]
        
        ![pcd][0]
        
        Python Contract Decorators
        ==========================
        
        - [Abstract](#abstract)
        - [Install](#install)
        - [Documentation](#documentation)
        - [Performance](#performance)
        - [Testing](#testing)
        - [License](#license)
        
        
        Abstract
        --------
        
        Contract programming can boost performance, increase self-documentation coverage
        and all in all is a useful code *hardening* technique.  The main principle is
        that all components in a program should *agree* on how they are interacting with
        each other.  Therefore instead of having isolated components which all have to
        guarantee their own correctness, with contract programming entire blocks of
        components guarantee their correctness as a unit.
        
        Though Python is not supporting contracts natively as a language feature, there
        are several libraries out there trying to add the same functionality to Python.
        Sadly they are either broken, or incomplete, or extremely heavy, or simply just
        reinventing the wheel by introducing foreign syntax to Python.  Fortunately
        there is almost always a light and easy way to do things in Python, and that is
        exactly what `pcd` is offering.
        
        Anyway, enough of the abstract *mumbo-jumbo*, let's start talking about how it
        works, shall we?  As usual, it is easier to understand what is going on via dead
        simple and dummy examples.
        
        So, let's say we have the following setup:
        
        ```python
        def get_user_input():
            input = {}
            while True:
                try:
                    key, value = raw_input('<key>,<value> or <return>: ').split(',')
                    input[key] = value
                except ValueError:
                    break
            return process_input(input)
        
        def process_input(input):
            cleaned = {}
            for key, value in input.items():
                cleaned[clean_value(key)] = clean_value(value)
            return cleaned
        
        def clean_value(value):
            return value.strip()
        ```
        
        If we want to make this safer, and the components more reusable for other
        components, the main approach would be hardening each component by introducing
        error handling in each function, like so:
        
        ```python
        def get_user_input():
            input = {}
            while True:
                try:
                    key, value = raw_input('<key>,<value> or <return>: ').split(',')
                    input[key] = value
                except ValueError:
                    break
            # If something went wrong processing inputs
            try:
                return process_input(input)
            except TypeError:
                return {}
        
        def process_input(input):
            cleaned = {}
            # If input is not a dict-like object
            try:
                for key, value in input.items():
                    try:
                        cleaned[clean_value(key)] = clean_value(value)
                    except TypeError:
                        continue
            except AttributeError:
                raise TypeError('Invalid input type')
            return cleaned
        
        def clean_value(value):
            # If value is not an str-like object
            try:
                return value.strip()
            except AttributeError:
                raise TypeError('Invalid value type')
        ```
        
        Now, this approach has two downsides.  On one hand, the code is now cluttered,
        because all the error checkings are spread across the functions, making it
        harder to understand what the exact problems each of the functions are
        trying to solve.  On the other hand the introduced error handling mechanism
        causes unnecessary overhead (and sometimes even redundant checkings in seemingly
        unrelated places), that is, the checks are running regardless of the correctness
        of the input data which may already have been checked.
        
        This is where contract programming comes in!  If we can make sure, that the
        top-level component which is using the other two components can guarentee the
        correctness of the inputs, it is completely unnecessary to introduce the above
        shown error handling:
        
        ```python
        from pcd import contract
        
        @contract(post=lambda r: isinstance(r, dict))
        def get_user_input():
            input = {}
            while True:
                try:
                    key, value = raw_input('<key>,<value> or <return>: ').split(',')
                    input[key] = value
                except ValueError:
                    break
            return process_input(input)
        
        @contract(pre=lambda: isinstance(input, dict),
                  post=lambda r: isinstance(r, dict))
        def process_input(input):
            cleaned = {}
            for key, value in input.items():
                cleaned[clean_value(key)] = clean_value(value)
            return cleaned
        
        @contract(pre=lambda: isinstance(value, str) or
                              isinstance(value, unicode))
        def clean_value(value):
            return value.strip()
        ```
        
        The result is much cleaner, easier to read and understand, and best of all at
        the same time it is also conditionally there, and can be removed without
        touching the code again.  So, after heavily testing the program with the
        contracts enabled, the application can be optimised greatly by stripping the
        decorators out.  During the development phase, if another component wants to use
        an already *contracted* one then that component has to respect its contracts,
        which is exactly what the decorator ensures.
        
        But that's not all!  Contract programming can be used with *classes*.  As a
        matter of fact, it can be used very efficiently to take care of data integrity,
        which is one of the most important aspects of using user defined types.  Of
        course one can use the above explained contracts on selected methods of a class,
        but `pcd` offers an even better by implementing *invariants*.
        
        By using invariants, one is automatically generating contracts that are called
        after the `__init__`, before the `__del__`, and before and after every public
        method and every `property` of a class.  They are inherited by subclasses, and
        they can be combined with manually defined contracts as well.
        
        So, let's look at an example:
        
        ```python
        class Triangle:
        
            def __init__(self, a, b, c):
                if a <= 0 or b <= 0 or c <=0:
                    raise ValueError('All sides need to be greater than 0')
                elif a + b <= c or a + c <= b or b + c <= a:
                    raise ValueError(
                        'The sum of any two sides has to be greater than the third one')
                self.a = a
                self.b = b
                self.c = c
        
            @property
            def area(self):
                pass  # Here goes the implementation of Heron's Formula
        ```
        
        For a seasoned developer it is obvious how dangerious it can be to define (and
        then use) *public* side names.  Because eventhough we made sure during the
        initialisation, that all sides have valid sizes &mdash; therefore we can safely
        call the `area` method on it &mdash; unfortunately we cannot guarantee the data
        integrity, as anyone can assign `-1` freely to any of the sides.  Therefore one
        is tempted to rewrite the above as follows:
        
        ```python
        class Triangle:
        
            def __init__(self, a, b, c):
                self._validate(a, b, c)
                self._a = a
                self._b = b
                self._c = c
        
            @staticmethod
            def _validate(a, b, c):
                if a <= 0 or b <= 0 or c <=0:
                    raise ValueError('All sides need to be greater than 0')
                elif a + b <= c or a + c <= b or b + c <= a:
                    raise ValueError(
                        'Sum of any two sides have to be greather than the third one')
        
            @property
            def a(self):
                return self._a
            @a.setter
            def a(self, value):
                self._validate(value, self._b, self._c)
                self._a = value
        
            @property
            def b(self):
                return self._b
            @b.setter
            def b(self, value):
                self._validate(self._a, value, self._c)
                self._b = value
        
            @property
            def c(self):
                return self._c
            @a.setter
            def c(self, value):
                self._validate(self._a, self._b, value)
                self._c = value
        
            @property
            def area(self):
                pass  # Here goes the implementation of Heron's Formula
        ```
        
        Although this approach is pretty solid (if fellow developers are not overriding
        variables marked as private by convention) it is very verbose, and the error
        checking is spread across the entire class, not to mention, that it is also
        mixed with the logic of the program.  And not only that, but if `Triangle` is
        only used internally, that is, the values are not coming from user inputs, but
        from a trusted source, the checks are running redundantly anyway, creating quite
        an overhead on every assignment.
        
        Let's see how this example would look like with *invariants*:
        
        ```python
        class Triangle:
        
            __metaclass__ = Invariant
            __conditions  = (lambda: self.a > 0,
                             lambda: self.b > 0,
                             lambda: self.c > 0,
                             lambda: self.a + self.b > self.c,
                             lambda: self.a + self.c > self.b,
                             lambda: self.b + self.c > self.a)
        
            def __init__(self, a, b, c):
                self.a = a
                self.b = b
                self.c = c
        
            @property
            def area(self):
                pass  # Here goes the implementation of Heron's Formula
        ```
        
        All the conditions defined for `Triangle` will run after the call of the
        `__init__` method, and before and after the `area` method.  Which means, we can
        accidentally override a side to an invalid value, but next time we want to use
        that value in a method, it will be checked immediately.  If that is considered
        to be too much of a risk, one can also define the getters and setters for each
        of the values and the validators will be generated for those as well.  And guess
        what?  Yup, the *invariants* can be turned on or off anytime without changing
        the code.  And even more!  They are inherited as well!
        
        > For further info on contract programming, read this [Wikipedia][1] article.
        
        Install
        -------
        
        ```bash
        pip install pcd
        ```
        
        After the package has been installed import and use it:
        
        ```
        from pcd import contract
        ```
        
        For development, clone the [git repository][2] and install the requirements:
        
        ```bash
        pip install -r requirements_dev.txt
        ```
        
        To run the tests use `pytest`:
        
        ```bash
        pytest tests.py
        ```
        
        
        Documentation
        -------------
        
        The `pcd` module defines the following functions and classes:
        
        <pre><code><b>contract</b><i>(</i><b>pre</b>=[<i>callable</i> or <i>iterable of callables</i>],
                 <b>post</b>=[<i>callable</i> or <i>iterable of callables</i>],
                 <b>mut</b>=[<i>callable</i> or <i>iterable of callables</i>]<i>)</i></code></pre>
        
        The `pre` should contain all the *preconditions* of the decorated function.
        Each *callable* takes no argument, and can use the same argument names that are
        defined by the decorated function.  Every *callable* see all arguments.
        
        The `post` should contain all the *postconditions* of the decorated function.
        Each *callable* takes one argument which can be named freely.  This argument
        will contain the value returned by the decorated function.  Every *callable*
        see all of the arguments of the decorated functions as well.
        
        The `mut` should contain all the *postconditions* of the *mutable* arguments.
        This can be very useful in case the decorated function has side effects via its
        arguments.  Each *callable* takes no argument, and can use the same argument
        names that are defined by the decorated function.  The checks are called after
        the function returned.  Every *callable* see all arguments.
        
        If `__debug__` is `True` then `contract` has no effect.
        
        - - -
        
        <pre><code><b>Invariant</b><i>()</i></code></pre>
        
        The `Invariant` class can be used as a *metaclass*.  If that is happening, then
        the `__conditions` of that class will be automatically executed after the
        `__init__` method, before the `__del__` method, and before and after every
        public method and `property` invocations.  Each of the conditions can get access
        to the `self` variable.
        
        - - -
        
        Running the program in a *regular* fashion causes the `contract` and `Invariant`
        to kick in.  To remove the checks, run the program with optimisations:
        
        ```bash
        $ python -O sample.py
        ```
        
        - - -
        
        > **WARNING!** One should never change the arguments or the return value of the
        > decorated function inside the conditions of the `contract`, as those may be
        > mutable values, therefore removing the contracts will alter the behaviour of
        > the function and may lead to unexpected behaviour!  The same thing goes for
        > the invariant conditions, one shall never mutate anything inside these
        > callbacks!
        
        
        Performance
        -----------
        
        Invoking a function with or without the `contract` decorator by running python
        with the `-O` (optimisation) flag has asbolutely no performance penalty. The
        examples in the `perf.py` shows that both functions have the same amount
        of bytecode instructions and their execution times are the same as well.
        
        Running these functions with simple `assert`s instead while `__debug__` is
        `True` is course faster than any other execution due to the argument handling
        and injection that is done by the `contract` decorator.  However doing so makes
        it hard in most cases to check the return value and/or side effects of the
        decorated function, and `contract` is a convenient way of doing that.
        
        
        Testing
        -------
        
        Contract programming plays nicely with unit testing.  As a matter of fact it is
        highly recommended to test the contracts, and the generic behaviour of the code
        component as one would do anyway:
        
        ```python
        from pcd import contract
        from pytest import raises
        
        @contract(pre=lambda: len(name) > 0)
        def store_name(name):
            #
            # Normalise and store value ...
            #
        
            # Return the length of the actual value being stored
            return stored_length
        
        def test_store_name():
            # Check constraints of the contract
            if __debug__:
                with raises(AssertionError):
                    store_name('')
        
            # Check regular behaviour on correct data
            assert srore_name('hello') == 5
        ```
        
        
        License
        -------
        
        Copyright &copy; 2017 [Peter Varo][3]
        
        This program is free software: you can redistribute it and/or modify it under
        the terms of the GNU Lesser General Public License as published by the Free
        Software Foundation, either version 3 of the License, or (at your option) any
        later version.
        
        This program is distributed in the hope that it will be useful, but WITHOUT ANY
        WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A
        PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.
        
        You should have received a copy of the GNU Lesser General Public License along
        with this program.  If not, see http://www.gnu.org/licenses
        
        - - -
        
        Copyright &copy; 2017 [Peter Varo][3]
        
        The logo is licensed under a [Creative Commons Attribution-ShareAlike 4.0
        International License][6].
        
        [![license][7]][6]
        
        [0]: img/logo.png?raw=true "pcd"
        [1]: https://en.wikipedia.org/wiki/Design_by_contract
        [2]: https://gitlab.com/petervaro/pcd
        [3]: http://www.petervaro.com
        [4]: https://gitlab.com/petervaro/pcd/badges/master/pipeline.svg
        [5]: https://gitlab.com/petervaro/pcd/commits/master
        [6]: https://creativecommons.org/licenses/by-sa/4.0
        [7]: https://i.creativecommons.org/l/by-sa/4.0/80x15.png
        
Keywords: contract,design,testing
Platform: UNKNOWN
