A decorator is a function that takes another function and returns a newer, prettier version of that function. Here are three examples of Python decorators for people who know how to use them, but forget how to write them. If you make it past the examples, there is further discussion of some of the awesome features of Python that are utilized in the code below. Also, these specific examples are for Django views, but you can generalize it to any function by pulling out the request argument.

This one allows you to pass arguments into the decorator for some additional customization. It needs to wrap everything in an additional function creating a closure in order to make this possible. A You usually want the decorated function to take the same arguments as the original function, and B Django views always start with a request, but they may have any number of additional arguments.

You can easily pass these through to the view function, and potentially read or modify them inside your decorator too. The functools. This is why my friend Harold likes to refer to it as an interior decorator.

If you read through the source code for wraps in functools, then you saw that it uses the partial function. It lets you create a new function from an existing function with some of the arguments predefined. Comments 1. Did you find this helpful or fun? Please donate!Before learning about the property decorator, let's understand what is a decorator.

In Python, the function is a first-order object. It means that it can be passed as an argument to another function.

python custom decorator

It is also possible to define a function inside another function. Such a function is called a nested function. Moreover, a function can return another function. A decorator is a function that receives another function as argument.

The behaviour of the argument function is extended by the decorator without actually modifying it. In this way, Python includes decorator functions. Also, we can define our own decorator function to extend the behaviour of a function without modifying it. Now, let's take a simple example to demonstrate how to define a custom decorator function and its' usage. Consider the following simple function. Now, let's define a decorator function which modifies the output of the above display function by prepending 'Output:' to the result of the display function.

You can now decorate this function to extend its behaviour by passing it to the decorator. As you can see, 'Output' is prepended with the result of the display function. The displaydecorator function is used to modify the behaviour of the display function without modifying it.

We can specify displaydecorator to the display function to denote that the display function is decorated with the displaydecorator function, as shown below. After applying the decorator displaydecorator in the above example, we can directly call the display function to get the extended behaviour, as shown below.

The following code uses the built-in property decorator to define the name property in the person class. The above person class includes two methods with the same name namebut with a different number of parameters. This is called method overloading. The name self function is marked with the property decorator which indicates that the name self method is a getter method and the name of the property is the method name only, in this case name.I predict that in time it will be seen as one of the more powerful features in the language.

The problem is that all the introductions to decorators that I have seen have been rather confusing, so I will try to rectify that here. First, you need to understand that the word "decorator" was used with some trepidation, because there was concern that it would be completely confused with the Decorator pattern from the Design Patterns book.

At one point other terms were considered for the feature, but "decorator" seems to be the one that sticks. Indeed, you can use Python decorators to implement the Decorator pattern, but that's an extremely limited use of it.

Python decorators, I think, are best equated to macros. The macro has a long history, but most people will probably have had experience with C preprocessor macros. The problems with C macros were 1 they were in a different language not C and 2 the behavior was sometimes bizarre, and often inconsistent with the behavior of the rest of C. Both Java and C have added annotationswhich allow you to do some things to elements of the language.

Both of these have the problems that 1 to do what you want, you sometimes have to jump through some enormous and untenable hoops, which follows from 2 these annotation features have their hands tied by the bondage-and-discipline or as Martin Fowler gently puts it: "Directing" nature of those languages. Many other languages have incorporated macros, but without knowing much about it I will go out on a limb and say that Python decorators are similar to Lisp macros in power and possibility.

I think it's safe to say that the goal of macros in a language is to provide a way to modify elements of the language. That's what decorators do in Python -- they modify functions, and in the case of class decoratorsentire classes. This is why they usually provide a simpler alternative to metaclasses. The major failings of most language's self-modification approaches are that they are too restrictive and that they require a different language I'm going to say that Java annotations with all the hoops you must jump through to produce an interesting annotation comprises a "different language".

Python falls into Fowler's category of "enabling" languages, so if you want to do modifications, why create a different or restricted language? Why not just use Python itself? And that's what Python decorators do. Decorators allow you to inject or modify code in functions or classes. Except that it's both much simpler and as a result much more powerful. For example, suppose you'd like to do something at the entry and exit points of a function such as perform some kind of security, tracing, locking, etc.

With decorators, it looks like this:. The indicates the application of the decorator. A function decorator is applied to a function definition by placing it on the line before that function definition begins.

For example:. When the compiler passes over this code, aFunction is compiled and the resulting function object is passed to the myDecorator code, which does something to produce a function-like object that is then substituted for the original aFunction. What does the myDecorator code look like? Well, most introductory examples show this as a function, but I've found that it's easier to start understanding decorators by using classes as decoration mechanisms instead of functions.

In addition, it's more powerful. The only constraint upon the object returned by the decorator is that it can be used as a function -- which basically means it must be callable. What should the decorator do? Well, it can do anything but usually you expect the original function code to be used at some point.Learn Data Science by completing interactive coding challenges and watching videos by expert instructors.

Start Now! Decorators allow you to make simple modifications to callable objects like functionsmethods, or classes. We shall deal with functions for this tutorial. The syntax. As you may have seen, a decorator is just another function which takes a functions and returns one.

For example you could do this:. Let's say you want to multiply the output by a variable amount.

You could define the decorator and use it as follows:. You can do anything you want with the old function, even completely ignore it! Advanced decorators can also manipulate the doc string and argument number. Make a decorator factory which returns a decorator that decorates functions with one argument.

The factory should take one argument, a type, and then returns a decorator that makes function should check if the input is the correct type. If it is wrong, it should print "Bad Type" In reality, it should raise an error, but error raising isn't in this tutorial. Look at the tutorial code and expected output to see what it is if you are confused I know I would be. This site is generously supported by DataCamp. Join over a million other learners and get started learning Python for data science today!

Decorators Decorators allow you to make simple modifications to callable objects like functionsmethods, or classes. Exercise Make a decorator factory which returns a decorator that decorates functions with one argument. Previous Tutorial Next Tutorial. Read our Terms of Use and Privacy Policy.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

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And how can I use this decorator across various applications in my project? If you really want to write your own decorator then there's a lot of good documentation on the net. And well, to re- use the decorator just put your decorator in a module on your path and you can import it from any other module.

Played around with the various links above and couldn't get them working and then came across this really simple one which I adapted. Using wraps is better than manually overriding like doing wrap. Amongst other things, it ensures your wrapper function gets the same name as the wrapped function. This function has the benefit of being reusable in other places, as a drop in replacement for user. It could for instance be exposed as a template tag. To use anywhere, make a python package and import it from there.

As long as its on your sys. Here is a slightly different implementation, which allows additional parameters in order to specify which page to redirect to when validation fails, and which message to display to the end user:.

A guide to Python's function decorators

Learn more. How to write a custom decorator in django? Ask Question. Asked 9 years ago. Active 1 year, 6 months ago. Viewed 57k times. Active Oldest Votes. So, what about creating a group "premiumusers" and adding your users to that group?

Then you can use the snippet as it is and just pass in the name of your group. Its not your fault it happens to the best of us ; ] I owe you an awful lot of time. Vadorequest PhoebeB PhoebeB 7, 7 7 gold badges 49 49 silver badges 68 68 bronze badges. This should be the accepted answer, thumbs up! Any clue to solve it? Would have to see your code, but most likely you didn't pass request to your function you are decorating.

Thanks to arie, the answer helped a long way, but it doesn't work for me. Spacedman Spacedman This amazing feature appeared in the language almost apologetically and with concern that it might not be that useful. I predict that in time it will be seen as one of the more powerful features in the language.

The problem is that all the introductions to decorators that I have seen have been rather confusing, so I will try to rectify that here. Python decorators, I think, are best equated to macros. The macro has a long history, but most people will probably have had experience with C preprocessor macros.

The problems with C macros were 1 they were in a different language not C and 2 the behavior was sometimes bizarre, and often inconsistent with the behavior of the rest of C. Both Java and C have added annotationswhich allow you to do some things to elements of the language.

Many other languages have incorporated macros, but without knowing much about it I will go out on a limb and say that Python decorators are similar to Lisp macros in power and possibility. This is why they usually provide a simpler alternative to metaclasses.

Why not just use Python itself? Decorators allow you to inject or modify code in functions or classes. With decorators, it looks like this:. The indicates the application of the decorator. A function decorator is applied to a function definition by placing it on the line before that function definition begins.

For example:. When the compiler passes over this code, aFunction is compiled and the resulting function object is passed to the myDecorator code, which does something to produce a function-like object that is then substituted for the original aFunction.

What does the myDecorator code look like? The only constraint upon the object returned by the decorator is that it can be used as a function — which basically means it must be callable. What should the decorator do? Well, it can do anything but usually you expect the original function code to be used at some point. This is not required, however:. Note also that the decorator constructor receives the function object being decorated. Indeed, before decorators were added you had to do something much less elegant to achieve the same thing:.

What you need to know about Decorators

With the addition of the decoration operator, you now get the same result by saying:. The reason I think decorators will have such a big impact is because this little bit of syntax sugar changes the way you think about programming. The constructor stores the argument, which is the function object.

The only constraint on the result of a decorator is that it be callable, so it can properly replace the decorated function. But a function object is also callable, so we can rewrite the previous example using a function instead of a class, like this:. The output of the line print func1. If this is a problem you can change the name of the decorator function before you return it:. The information you can dynamically get about functions, and the modifications you can make to those functions, are quite powerful in Python.

The output is:.

python custom decorator

The return value of the decorator function must be a function used to wrap the function to be decorated. That is, Python will take the returned function and call it at decoration time, passing the function to be decorated. Python 3 Patterns, Recipes and Idioms latest. Contributors ToDo List The remainder are from context, from the book. Decorators vs. With decorators, it looks like this: entryExit def func1 : print "inside func1 " entryExit def func2 : print "inside func2 ". For example: myDecorator def aFunction : print "inside aFunction".

@property Decorator

Entering func1 inside func1 Exited func1 Entering func2 inside func2 Exited func2.Python decorators are a very useful tool in python and used to enhance the functionality and behavior of the functions and classes. If we want to modify the behavior of the existing function without modifying the original function then we can use decorators to alter its behavior and we can also wrap another function.

The function is used as an argument in the decorator and then we can call it using a wrapper function and it returns wrapper function as a result. So we are modifying or enhancing the behavior of the function without modifying the original function.

We use symbol to denote decorator is defined and python will call decorator first before calling the original function. Decorators works on four concepts:. In the above program, we have defined a function printMessage that prints a message. We have defined a decorator function named as a decorator that is taking a function as an input argument. Now we have also defined a wrapper function that will print our desired message and then it will call our original function.

We have written our message and then we have returned the wrapper function. Now we have to use to call decorator function before our original function. When python reads symbol it first calls decorator function then original function.

Now we want to check the name of the function which is executing. The decorator never shows the information on the original function like function name, parameter list, and docstrings. In the above program, we have passed two-parameter x and y in the wrapper function and we are performing division between them.

python custom decorator

We are checking if the value of the y is 0 or not. If the value is 0 then we are returning the custom message, and if the value is not zero then the if condition will be ignored and the operation will be carried out in the original function without any error.

So we can say that we are catching error using a decorator otherwise python must have given error. We have created a constructor and passed a self and name parameter. We have created a method name as printMessage passed self keyword and print our message. Self refers to the instance of the class. We have created an object obj of the class and then call our function using the class object. Decorator is a very useful feature and it provides us the option to improve the function of the existing function without modifying it.

For using decorator we use symbol and decorator name. It takes a function as an input argument and returns a function. This is a guide to Decorator in Python. Here we discuss the Introduction and syntax of decorator in python along with different examples and its code implementation. You may also have a look at the following articles to learn more —. Forgot Password? Skip to primary navigation Skip to main content Skip to primary sidebar Skip to footer Decorator in Python. Popular Course in this category.

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