In Python, create a decorator that dynamically accepts arguments Create a decorator

background

If you define and use your own decorator in Python, you may want to be able to pass arguments to the decorator later.

Where to use the original decorator

@my_decorator
def hoge():
	# ...

If you implement a general decorator, you'll want to be able to pass arguments to the decorator later. It will be necessary to rewrite all the decorators that were used like ↑ as ↓.

When setting arguments when using a decorator

@my_decorator(arg1='value1')
def hoge():
	# ...

Even if you don't need any arguments, you need to put parentheses if the decorator implementation changes.

When no argument is set when using the decorator

@my_decorator()
def hoge():
	# ...

Rewriting all the parts that use the decorator is bothersome to modify and verify, so I wanted it to work without the parentheses if no arguments were needed.

Purpose

I want to create a decorator (my_decorator) that can dynamically use arguments as shown below. By the way, I would like to implement it as a decorator for decorators so that such functions can be used universally.

python

#With no arguments
@my_decorator
def hoge():
	pass

#With arguments
@my_decorator(arg1='value1')
def fuga():
	pass

Implementation

Implement a decorator that dynamically interprets the arguments to apply to the decorator, as shown below.

decorators.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
import functools


def dynamic_args(func0):
    def wrapper(*args, **kwargs):
        if len(args) != 0 and callable(args[0]):
            #When a function is passed as the first argument:Treat as a decorator with no arguments
            func = args[0]
            return functools.wraps(func)(func0(func))
        else:
            #Other:Treat as a decorator with arguments
            def _wrapper(func):
                return functools.wraps(func)(func0(func, *args, **kwargs))
            return _wrapper
    return wrapper

Usage image

If you prepare a decorator like the one above, you can achieve your purpose simply by defining your own decorator as follows.

python

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from decorators import dynamic_args


#Apply the decorator created above to your decorator
@dynamic_args
def my_decorator(func, arg1='default_value1', arg2='default_value2'):
    def wrapper(*args, **kwargs):
    	#Defined arguments(arg1, arg2)Write the process you want to actually do using
    	# ...
    return wrapper

Usage sample

As a sample, create a decorator named my_decorator that puts parentheses before and after the return value (string).

sample.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-
from decorators import dynamic_args


#####################
#Your own decorator definition#
#####################
@dynamic_args
def my_decorator(func, before='「', after='」'):
    def wrapper(*args, **kwargs):
        return "{} {} {}".format(before, func(*args, **kwargs), after)
    return wrapper


##########################
#Example of using a decorator with no arguments#
##########################
@my_decorator
def func_without_args():
    return "Argumentless decorator"

print(func_without_args()) # output:"Decorator with no arguments"


##########################
#Example of using a decorator with arguments#
##########################
@my_decorator(before='『', after='』')
def func_with_args():
    return "Decorator with arguments"

print(func_with_args()) # output:"Decorator with arguments"


####################################
#An example of applying a decorator to a predefined function#
####################################
def func_without_decorator():
    return "No decorator"


print(my_decorator(func_without_decorator)()) # output:"No decorator"
print(my_decorator(before="『", after="』")(func_without_decorator)()) # output:"No decorator"

reference

Python Tips: I want to pass arguments to the decorator-Life with Python http://www.lifewithpython.com/2016/09/python-decorator-with-arguments.html

Python Decorator Expressions (2)-When decorators have arguments and you want to apply multiple decorators | Immediately forget your brain notes for that http://jutememo.blogspot.jp/2008/10/python-2.html

python - AttributeError: 'str' object has no attribute 'module' - Stack Overflow http://stackoverflow.com/questions/18493879/attributeerror-str-object-has-no-attribute-module