7.1. Decorators About¶

7.1.1. Rationale¶

Since Python 2.4: PEP 318 -- Decorators for Functions and Methods
Decorator is an object, which takes another object as it's argument
Decorators can:
- Do things before call
- Do things after call
- Modify arguments
- Modify returned value
- Avoid calling
- Modify globals
- Add or change metadata

7.1.2. Syntax¶

func is a pointer to function which is being decorated
args arbitrary number of positional arguments
kwargs arbitrary number of keyword arguments
By calling func(*args, **kwargs) you actually run original (wrapped) function with it's original arguments

def mydecorator(func):
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper


@mydecorator
def myfunction(*args, **kwargs):
    pass


myfunction()

7.1.3. Names¶

def mydecorator(func):
    def wrapper(*args, **kwargs):
        return func(*args, **kwargs)
    return wrapper

def mydecorator(fn):
    def wrap(*a, **b):
        return fn(*a, **b)
    return wrap

def mydecorator(fn):
    def _(*a, **b):
        return fn(*a, **b)
    return _

def mydecorator(fn):
    return lambda *a, **kw: fn(*a, **kw)

7.1.4. Types of decorators¶

By type:

Function decorators

Class decorators

By decorated object:

Decorating function

Decorating class

Decorating method

By wrapper type:

Wrapper function

Wrapper class

Wrapper method

By number of arguments:

Without arguments

With arguments

7.1.5. Decorator Types¶

Function decorators
Class decorators
In this example:
- obj is a decorated object
- doesn't matter, whether is a function, class or method

def mydecorator(obj):
    ...

class MyDecorator:
    def __init__(self, obj):
        ...

7.1.6. Wrapper Type¶

Wrapper function
Wrapper class
Wrapper method
In this example:
- obj is a decorated object
- doesn't matter, whether is a function, class or method
If obj and Wrapper are classes, Wrapper can inherit from obj (to extend it)

def mydecorator(obj):
    def wrapper(*args, **kwargs)
        ...
    return wrapper

def mydecorator(obj):
    class Wrapper:
        def __init__(*args, **kwargs)
            ...
    return Wrapper

class MyDecorator:
    def __init__(self, obj):
        ...

    def __call__(*args, **kwargs):
        ...

7.1.7. Decorated Object¶

Decorating function (by convention func or fn)
Decorating class (by convention cls)
Decorating method (by convention mth, meth or method)

def mydecorator(func):
    ...

def mydecorator(cls):
    ...

def mydecorator(mth):
    ...

class MyDecorator:
    def __init__(self, func):
        ...

class MyDecorator:
    def __init__(self, cls):
        ...

class MyDecorator:
    def __init__(self, mth):
        ...

7.1.8. Usage¶

@mydecorator
def myfunction(*args, **kwargs):
    ...

class MyClass:
    @mydecorator
    def mymethod(self, *args, **kwargs):
        ...

@mydecorator
class MyClass:
    ...

@MyDecorator
def myfunction(*args, **kwargs):
    ...

class MyClass:
    @MyDecorator
    def mymethod(self, *args, **kwargs):
        ...

@MyDecorator
class MyClass:
    ...

7.1.9. Arguments¶

Without arguments
With arguments

@mydecorator
def myfunction(*args, **kwargs):
    ...

@mydecorator(a, b)
def myfunction(*args, **kwargs):
    ...

@MyClass
def myfunction(*args, **kwargs):
    ...

@MyClass(a, b)
def myfunction(*args, **kwargs):
    ...