Python class definition syntax
If you really want to do that thing, then you must somehow «fix» the classes after the parent class was defined. Solution 3: I there are some attributes in the constructor of a class, they can come either from parent class (super class) or a child class .
Python class definition syntax
Is there a difference between
I just realized that a couple of my classes are defined as the former and they work just fine. Do the empty parenthesis make any difference?
While it might not be syntactically incorrect to use the empty parentheses in a class definition, parentheses after a class definition are used to indicate inheritance, e.g:
In Python, the preferred syntax for a class declaration without any base classes is simply:
Don’t use parentheses unless you are subclassing other classes.
The docs on the matter should give you a better understanding of how to declare and use classes in Python.
The latter is a syntax error on older versions of Python. In Python 2.x you should derive from object whenever possible though, since several useful features are only available with new-style classes (deriving from object is optional in Python 3.x, since new-style classes are the default there).
A class definition is a bit different from a function/method definition.
The parentheses in class definitions are for defining from which class you inherit. You don’t write def in front of it, and when you inherit from ‘object’ which is the default you don’t need the parentheses for the definition.
Function/method definitions always take parentheses, even if you don’t define parameters. If you don’t use them, you’ll get a SyntaxError.
Later, after the definition of a class/function/method in the code, just writing the name will point you to the class/function/method.
If you want to call or access any of these, you’ll need (), [], . or whatever.
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How to find a definition of a class field?
In a constructor of a class I see that some self variables are used without being initialized. For example:
def __init__(self, x): self.x = x + self.y In the above example, as you can see, self.y is used without being initialized. My assumption is that the value of this field is coming from the super class.
Hover, in the super class I also do not see a definition of self.y . So, where can it come from?
I need to add that this field is not defined as a «class attribute» as well. So, I have something like this:
class MyClass(SomeBaseClass): def __init__(self, x, y): self.aaa = self.bbb + self.ccc # some other code For the check I did the following:
class MyClass(SomeBaseClass): def __init__(self, x, y): print(self.__dict__) # and, as a result, I see a dictionary with keys and values. So, my assumptions is that these values are coming from the base class. However, if I go to the base class, and add prints to the constructor
def __init__(self, x, y, z): print('some additional print in the beginning') # some code here print('print at the end of the constructor') then I do not see these prints (as if the constructor of the base class is not executed).
if self.y is not assigned in the __init__ before, then this only can be a class-attribute (possibly inherited). self usally indicates the instance of a class, but instance-attributes are first init ialised in the init() . If it's not there, it's not an instance-attribute , at this point.
class test : y = 7 # scope: class def __init__( self ): self.x = self.y + 1 # scope: instance a = test() a.x # 8 # scope: instance a.y # 7 # scope: instance, pulled from class test.y # 7 # scope: class For your studys: In Python, it’s all about the attributes
( the best explanation about the difference between class and instance attributes, that I found so far )
If you override init , the init methods of the base classes are not executed. You have to explicitly call super(). init () in your init to do that. If you want to find out which classes are used to find instance attributes do: MyClass.mro() . The list you will get is searched from left to right by __getattribute__ as far as I know.
I there are some self.* attributes in the constructor of a class, they can come either from parent class (super class) or a child class .
If super().__init__() was not called from the constructor of the considered class, then definition done in the constructor of the parent class are not implemented yet. So, the attribute cannot come from the parent class.
What can also happen is that some child class makes some definitions in its constructor (for example some attributes has been introduced) and then a constructor of its parent class is called (which means that constructor of your class is called). In this case your class will see the attributes that are defined in the child class. They will be seen as its own attributes.
class your_class(): def __init__(self): print self.x # How can we print self.x if it does not exist? class child_class(): def __init__(self, x): self.x = x super().__init__() # here is constructor of the parent class (your_class) is called and this constructor will see self.x defined here. Python OOPs Concepts, Class Definition Syntax: class ClassName: # Statement-1 . . . # Statement-N Example: Creating an empty Class in Python . Python # Python3 program to # demonstrate defining # a class . class Dog: pass. In the above example, we have created a class named dog using the class keyword. Objects. …
Conditional Class Creation (Python)
From the tutorial: "A class definition is an executable statement."
Is the following recommended in a script?
my_switch = False if my_switch: class Hello: def __init__(self): self.greeting = "Hello!" else: class Hello: def __init__(self): self.greeting = "Salut!" class Hello: def __init__(self): self.greeting = "Hello!" class Salut: def __init__(self): self.greeting = "Salut!" if my_switch: Hello = Salut (note that your code needs lower-case Class keywords. )
If you like it better, you could put each class definition in a separate .py file and just import the one you want. Something like the following:
if my_switch: from hello_en import Hello else: from hello_fr import Hello h = Hello() Yes, the code is valid. but why not just run the code to find out?
You can also do this with function definitions.
You can also put conditions around the "declarations" inside the class, since those are executed as part of the class construction:
class Hello: if my_switch: igreeting = "Hello!" else: igreeting = "Salut!" def __init__(self, greeting): self.greeting = self.igreeting (igreeting here is a class variable, greeting a member variable)
class Hello: if my_switch: greeting = "Hello!" else: greeting = "Salut!" . will usually give the same effect.
How To Construct Classes and Define Objects in Python 3, Introduction. Python is an object-oriented programming language. Object-oriented programming (OOP) focuses on creating reusable patterns of code, in contrast to procedural programming, which focuses on explicit sequenced instructions. When working on complex programs in particular, object-oriented …
How to initialize/define child class with use class scope not class method scope?
How to initialize child class with use class scope? How to pass parent abstract class scope to child class?
I can write this code but with every call of getChild I will create one class but want avoid:
class Parent(object): # abstract class! @staticmethod def getParentName(): raise NotImplementedError() @classmethod def getChild(cls): # solid class class Child(object): @staticmethod def getChildName(): return 'Child of ' + cls.getParentName() return Child class SomeParent(Parent): @staticmethod def getParentName(): return 'Solid Parent' print SomeParent.getChild().getChildName() # == 'Child of Solid Parent' How to convert this code above into defining child class in parent scope (consider that parent is abstract so we cannot use Parent2.getParentName() since it will be overridden?
class Parent2(object): # abstract class! @staticmethod def getParentName() raise NotImplementedError() class Child2(object): # solid class # what code here to do the same like Child. pass class SomeParent2(Parent): # final class @staticmethod def getParentName() return 'Solid Parent2' SomeParent2.getChildClass().getChildName() # == 'Child of Solid Parent2' Any help or hint will be welcome apart what is not constructive.
Python does not have class declarations . It has class definitions . When you define the Parent2 class, the indented code is executed . This means that any inner class defined there is created before the parent exists . Hence it is impossible to let Child2 know about Parent2 inside the class scope. Note that this is very different from other languages, such as Ruby that do allow references to the class inside the definition.
Also note that your two examples do two very different things. If you put the class definition inside a method that a new class will be created every time you call the method, while doing so at the class scope means that only one class will be created inside the parent scope.
Also I believe your design is broken. If Child is strictly related to Parent then you should either use inheritance, in which case you simply do self.getParentName() , without anything fancy, or you can use delegation.
If you really want to do that thing, then you must somehow "fix" the classes after the parent class was defined. In order to do this you can use a class decorator, or simply put the code explicitly after the parent class.
Python Classes, Python Classes/Objects. Python is an object oriented programming language. Almost everything in Python is an object, with its properties and methods. A Class is like an object constructor, or a "blueprint" for creating objects.
Python class definition syntax
I just realized that a couple of my classes are defined as the former and they work just fine. Do the empty parenthesis make any difference?
3 Answers 3
While it might not be syntactically incorrect to use the empty parentheses in a class definition, parentheses after a class definition are used to indicate inheritance, e.g:
In Python, the preferred syntax for a class declaration without any base classes is simply:
Don't use parentheses unless you are subclassing other classes.
The docs on the matter should give you a better understanding of how to declare and use classes in Python.
The latter is a syntax error on older versions of Python. In Python 2.x you should derive from object whenever possible though, since several useful features are only available with new-style classes (deriving from object is optional in Python 3.x, since new-style classes are the default there).
Was class A() ever a syntax error? The Python 1.4 docs say it's fine, and those are the oldest docs still available on python.org.
A class definition is a bit different from a function/method definition.
The parentheses in class definitions are for defining from which class you inherit. You don't write def in front of it, and when you inherit from 'object' which is the default you don't need the parentheses for the definition.
Function/method definitions always take parentheses, even if you don't define parameters. If you don't use them, you'll get a SyntaxError.
Later, after the definition of a class/function/method in the code, just writing the name will point you to the class/function/method.
If you want to call or access any of these, you'll need (), [], . or whatever.