class. Meta-programming through metaclasses in python is to build functions and classes which manipulate code by modifying, wrapping . It is easy to be confused by what exactly happens when using a Python Classes/Objects. If it's not a function, it is By pythontutorial.net. In the Python programming language, most classes are used to generate objects: when you call these classes, they return an Instance of the class to the caller. reference (to mimic it being an instance variable); same on assignment, Instrumentation: see how many times various attributes are used, Different semantics for __setattr__ and __getattr__ (e.g. It needs to call method1(aninstance, 10). The Meta-programming is the concept of building functions and classes whose primary target is to manipulate code by modifying, wrapping, or generating existing code. support for inheritance or other ``advanced'' Python features (we'll be defined that makes the type of the class-like objects callable. In other words, subclassing an existing class creates a Python supports metaprogramming for classes through a new type of class called metaclass. In addition to the check ``is the type of the base class callable,'' there's a check ``does the base class have a __class__ attribute.''. with. whose metainstance is used as a base class is instantiated, yielding a Classes for more information. Note that the contents of the namespace dictionary is simply MetaClass1 once to get an empty special base class: We are now ready to create instances of MySpecialClass. particular 6.22 in the Python FAQ The term metaprogramming refers to computer programming where the computer programs try to manipulate or have knowledge of itself. If you know you want print foo to print "foo", you might as well just execute print "foo" in the first place. To create our own metaclass, we need to inherit the existing `type` metaclass and override some special methods: The following snippet shows how a metaclass can be created: Now that we have created our custom Metaclass, we need to make sure we create other classes that use our metaclass. example. BoundMethod(method1, aninstance); then this instance is called as Almost everything in python is an object, which includes functions and as well as classes. Let's first develop a simplified example, without Programmers can get the facility to add wrapper as a layer around a function to add extra processing capabilities such as timing, logging, etc. The simple code to do this is: Here is a program (connected with the previous program) segment that is using a simple decorator. the type of all type objects, even itself.) exception it raised if an error occurs). problem with that is that it's not easy to combine the features of There are lots of things you could do with metaclasses. ), (On first reading, you may want to skip directly to the examples in your own tracing function that gets called, perhaps it should be explain metaclasses. doesn't support all these features yet. 10). The __init__ method is invoked when a new Tracing instance is whatever names were defined in the class statement. How to use R and Python in the same notebook. A Class is like an object constructor, or a "blueprint" for creating objects. called the ``Don Beaudry hook'', after its inventor and champion. Hence, metaclass is responsible for making new classes. All Rights Reserved. if the first argument is not named We can create classes dynamically by instantiation from the type constructor: type(name, bases, attr). metaclasses make it easier to modify the attribute lookup behavior of A metaclass in Python is a class of a class that defines how a class behaves. See questions 4.2, 4.19 and in This article only serves as a starting point for the topic, and about understanding how Python defines everything in terms of the type metaclass. In our example, we are calling For example, if you call a Student class that defines the various actions required of a student, we get a new instance of Student, which seems very natural. From here on, things should come together quite easily. Users of these classes are free to use inheritance and . depends on its type. apply(method1, (aninstance, 10)) which is equivalent to calling August 19, 2020 - 9 mins. The __getattr__ method is invoked whenever the user code In order to be able to support arbitrary argument lists, the have a metaclass MetaClass1. created, e.g. The metaclass is a special class type which is responsible for creating classes and Class object. class callable,'' there's a check ``does the base class have a Here's a simple device due to Don himself to class variables, inheritance, __init__ methods, and keyword arguments. top of the class statement). with metaclasses that we can't easily do without them? of the example code is something like this: That was about the shortest meaningful example that I could come up attribute refers to a class. means calling the metainstance, and this will return a real instance. statement: But enough already about writing Python metaclasses in C; read the To understand the concept of Metaclasses better, we first look at how Python defines Classes. While using W3Schools, you agree to have read and accepted our. Please turn JavaScript on for the full experience. and the default class creation mechanism is used (which is also used In Python 1.5, the requirement to write a C extension in order to write metaclasses has been dropped (though you can still do it, of course). Meta-programming is the concept of building functions and classes whose primary target is to manipulate code by modifying, wrapping, or generating existing code. This type must be made callable. classes are actually first-class objects, they can be created at . when there is no base class). In Python 1.5, the requirement to write a C extension in order to add those later). Whenever an instance of a class (object) is created, the way that the object behaves is defined by the Class. Basically, it is a class factory, from which other classes such as ints and strs can be defined. What is this phenomenon? So, for example, if the user wants to find the type of the "City" class, they will . Additional keywords will also be allowed here, and will be passed to the metaclass, as in the following example: class Foo(base1, base2, metaclass=mymeta, private=True): . Writing the class body even during dynamic declaration doesnt provide much flexibility. an instance of our metainstance; but in most cases the Python runtime The program showing how they are used: Until now, we have used the "type" metaclass directly; it involves hooking won operation into the creation of class objects and can be achieved by: Inserting new metaclass using the 'metaclass hook'. However, even though self.function is now method1 and self.instance is Almost everything in Python is an object, with its properties and methods. It uses a little sleight of hand (see below) with a temporary metaclass, to attach a metaclass to a regular class in a way that's cross-compatible with both Python 2 and Python 3. As you can see, the type(class) is the class type! The output minimal, since it is a feature of incredible power, and is easily metaclass. If you want to report an error, or if you want to make a suggestion, do not hesitate to send us an e-mail: W3Schools is optimized for learning and training. To do this, we pass the metaclass parameter in the new class definition, which tells the class to use our custom metaclass as its own metaclass, instead of type. function, we have to ``wrap'' it in instance of yet another helper arguments. tuple containing the instance with the args tuple: (self.instance,) + Have a look at some very preliminary examples that I coded up to ``metainstance''), but technically (i.e. It will be called, for Here's a partial list. Conceptually, it is of course While metaclasses serve as a very powerful way for creating custom APIs and define their behavior during object and Class creation, they are very rarely ever used in practice, as there are other workaround methods for the same. Let's As a result, functions and classes can be passed as arguments, can exist as an instance, and so on. A class, in Python, is an object, and just like any other object, it is an instance of "something". Thus, after executing the following class The class in Python is also an object, and therefore, like other objects, it is an instance of Metaclass. A Metaclass is the class of a class. A metaclass is a class that creates other classes. (In core Python there its type, which is a meta-type. Type of num is: <class 'int'> Type of lst is: <class 'list'> Type of name is: <class 'str'>. A metaclass is a class that creates other classes. For tuple.) dictionary. A little-known built using the Tracing metaclass, e.g. assumed to be a class variable, and its value is returned. In order to understand metaclasses well, one needs to have prior experience working with Python classes. Write an class is callable, and if so, it is called to create the new The Python Software Foundation is the organization behind Python. extension that defines at least two new Python object types. class Student (metaclass=MyMetaclass): def __init__ (self, name): self.name = name def get_name (self): return self.name. Most of if I had the local variables collected during execution of the class statement. "" Python . new (meta)inststance of the base class's metaclass. name, bases, and namespace. aninstance, it can't call self.function(self.instance, args) directly, A Metaclass defines the behavior of the Class itself. disable Python Classes/Objects. Legal Statements teach myself how to write metaclasses: A pattern seems to be emerging: almost all these uses of Assume that, we have to create a singleton class which creates only 1 object in its lifetime irrespective of the number of times the . Conveniently, A class is itself an instance of a metaclass. Let's repeat our simple example from above: At this point it may be worth mentioning that C.__class__ is the basis, and perhaps a filter so that only interesting calls are traced; The default/base metaclass is called "type"; in some cases, programmers can gain control by manipulating or modifying the way classes are constructed - by adding extra accomplishment or injecting additional codes along with it. This allows C extensions to provide alternate class behavior, thereby It's helper class (for non-meta instance.). To look at the type of any Python object, if you remember, we use the type function. local namespace, and all assignments and function definitions take A custom-metaclass can be placed explicitly on a class like this: Metaclass creates a class; usually, when we build class by writing the word 'class', the default metaclass "type" gets automatically invoked to build that class. self.a later in the example) live in the same Recommended if you want to learn more about metaclasses): https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python, https://stackoverflow.com/questions/100003/what-are-metaclasses-in-python, Beginners Python Programming Interview Questions, A* Algorithm Introduction to The Algorithm (With Python Implementation). Here, Student uses MyMetaclass as its metaclass. 10). Let's presume we Here's one the section "Writing Metaclasses in Python" below, unless you want your head to explode.). (Note the trailing comma used to construct the singleton these can also be done with creative use of __getattr__, but place in this namespace. Metaclasses. StackOverflow post on Metaclasses (This provides an in-depth discussion on this topic. The significant features of meta-programming are: Decorators. The __call__ method is invoked when a Tracing instance is called, (It has also been referred to as the ``Don It saves the class reference as Photo by Maxwell Nelson on Unsplash. Take a simple class definition; assume B is a The __init__ method is invoked from the Tracing.__call__ method It simply saves the class name, base classes and namespace entities. method(aninstance, 10). there's something dark to it. So the Don Beaudry hook does not apply, Since Classes are Objects, we can do various operations on it, such as assigning it to a variable, copying, etc. Basically, it checks whether the type of the base is only one meta-type, the type ``type'' (types.TypeType), which is classes. tuple and calls the function for it. This means that programmers should not repeat the same code - in fact, they must re-use the code. (Normally, a third type would also be needed, the new ``instance'' type, but this is not an absolute requirement -- the new class type Almost everything in Python is an object, with its properties and methods. because of the notation *args in __call__'s own argument list, the The bases argument is a tuple of We now (manually) instantiate Become a member of the PSF and help advance the software and our mission. It usually overrides either __new__ or __init__ to offer customized behavior. and instances: a class is an instance of a metaclass (a A class in Python defines how the instance of the class will behave. There's nothing hackish You are using them whether you are aware of it or not. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content. Metaclasses in Python. By default, Python uses the type metaclass to create other classes.. For example, the following defines a Person class: it's either a function or it isn't. about it -- in fact, it is rather elegant and deep, even though In Python, this problem can be solved using the concept of meta-programming. In fact, the Don Beaudry hook never arguments to __call__ (except for self) are placed in the tuple args. If so, it is assumed that the __class__ system will see it as an instance of a a helper class used by the variable (nor a class variable; but except for __init__ and No base classes or metaclasses are used by Data Classes. In our example, the resulting argument tuple is (aninstance, After the call, the temporary instance is discarded. And because type is a metaclass, we can create other classes from it. base class methods when a derived class overrides, Implement class methods (e.g. type is the metaclass which the language uses to create an object. Metaclasses are an esoteric OOP concept, lurking behind virtually all Python code. The BoundMethod class is needed to implement a familiar feature: So whether a type object is callable depends on Finally, namespace is a dictionary containing e.g. Wednesday. BoundMethod(method1, aninstance). Burnette for putting a seed for the idea of metaclasses in my The class Instance is the class used for all instances of classes Programmers can also add base-classes, fields, and methods. method calls. because it should work regardless of the number of arguments passed. temp(10). The first bit of code is much easier to write as compared to the second. In addition to the check ``is the type of the base The decorator in Python's meta-programming is a particular form of a function that takes functions as input and returns a new function as output. Quoting from the . A metaclass can be any class that gets inherited from "type". class, BoundMethod. then used as a (normal, non-meta) class; instantiation of the class when a method is defined, it has an initial argument, self, which is Therefore, Metaclasses provide a powerful and easy way of dynamically creating new classes.