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How to implement custom decorator in python?

python

Table of contents

What is decorator?
This is the example of function decorator:
Another example for function decorator:
Example for class decorator

What is decorator?

There are two types of decorator function decorator and class decorator the below digrams show how they work.

This is the example of function decorator:

python

import inspect
def auto_repr(cls):
    print(cls.__name__)
    members = vars(cls)
    for name, member in members.items():
        print(name, member)
    if "__repr__" in members:
        raise TypeError(f"{cls.__name__} already defines __repr__")
    if "__init__" not in members:
        raise TypeError(f"{cls.__name__} does not override __init__")
    sig = inspect.signature(cls.__init__) # sig is list of parameter of __init__ function
    parameter_names = list(sig.parameters)[1:]
    if not all(
        isinstance(members.get(name, None), property)
        for name in parameter_names
    ):
        raise TypeError(f"auto_repr does not apply for the {cls.__name__}")
    def synthesized_repr(self):
        return "{typename} : {list}".format(
            typename=self.__class__.__name__,
            list=", ".join(
                "{name}={value!r}".format(
                    name=name,
                    value=getattr(self, name)
                ) for name in parameter_names
            )
        )
    setattr(cls, "__repr__", synthesized_repr)
    return cls
@auto_repr
class SimpleList:
    def __init__(self, items):
        self._list = list(items)
    def add(self, item):
        self._list.append(item)
    def sort(self):
        self._list.sort()
    @property
    def items(self):
        return self._list

output

>> from main import *
SimpleList
__module__ main
__init__ <function SimpleList.__init__ at 0x101028c10>
add <function SimpleList.add at 0x101028ca0>
sort <function SimpleList.sort at 0x101028d30>
items <property object at 0x100dfdd00>
__dict__ <attribute '__dict__' of 'SimpleList' objects>
__weakref__ <attribute '__weakref__' of 'SimpleList' objects>
__doc__ None
>> s = SimpleList([1,2])
>> s
SimpleList : items=[1, 2]

Another example for function decorator:

This is example for class decorator factories

python

def post_condition(predicate):
    def function_decorator(f):
        @functools.wraps(f)
        def wrapper(self, *args, **kwargs):
            result = f(self, *args, **kwargs)
            if not predicate(self):
                raise RuntimeError(
                    f"Post-condition {predicate.__name__} not "
                    f"maintained for {self!r}"
                )
            return result
        return wrapper
    return function_decorator
def min_length(simple_list):
    return len(simple_list.items) >= 2
class SimpleList:
    @post_condition(min_length)
    def __init__(self, items):
        self._list = list(items)
    @post_condition(min_length)
    def add(self, item):
        self._list.append(item)
    def sort(self):
        self._list.sort()
    @property
    def items(self):
        return self._list
s1 = SimpleList([])
s2 = SimpleList([1,2])
print(s2)

Output

RuntimeError: Post-condition min_length not maintained for SimpleList : items=[]
SimpleList : items=[1, 2]

In this approach we should define decorators above each methods but there is another way, that we define a decorator above class and it is applied for every methods in that class.

python

def post_condition(predicate):
    def function_decorator(f):
        @functools.wraps(f)
        def wrapper(self, *args, **kwargs):
            result = f(self, *args, **kwargs)
            if not predicate(self):
                raise RuntimeError(
                    f"Post-condition {predicate.__name__} not "
                    f"maintained for {self!r}"
                )
            return result
        return wrapper
    return function_decorator
def invariant(predicate):
    function_decorator = post_condition(predicate)
    def class_decorator(cls):
        members = list(vars(cls).items())
        for name, member in members:
            if inspect.isfunction(member):
                decorated_member = function_decorator(member)
                setattr(cls, name, decorated_member)
        return cls
    return class_decorator
def min_length(simple_list):
    return len(simple_list.items) >= 2
@invariant(min_length)
class SimpleList:
    def __init__(self, items):
        self._list = list(items)
    def add(self, item):
        self._list.append(item)
    def sort(self):
        self._list.sort()
    @property
    def items(self):
        return self._list

Example for class decorator

For more detail for class decorator check this link

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