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(Python) Compelling a class to implement particular methods

I know the inheritance in class. Every methods which a superclass has is inherited to its subclass. So unless needed particularly, the subclass doesn't have to implement the inherited methods again.
But I want to make sure a subclass to re-implement all the methods which a superclass has. The point is that all classes in the same group(such as classes inherit the same superclass) have to implement the same methods individually. The classes need some structure that designate what methods have to be implemented.
What am I supposed to do?

In object oriented programming, this is achieved through the use of an interface. You'd have to import the python interface module, then have your class inherit from this module. Here is a guide with some practical examples: https://www.geeksforgeeks.org/python-interface-module/#:~:text=In%20python%2C%20interface%20is%20defined%20using%20python%20class,interfaces%20are%20implemented%20using%20implementer%20decorator%20on%20class.

In C++ you would be looking to make pure virtual methods on your base class, prompting a compile-time error on a subclass not implementing the method, this is typically the way interfaces are defined.
Python provides the abstractmethod decorator (see here:
https://docs.python.org/3/library/abc.html#abc.abstractmethod) for similar ends. Unlike C++ you will be able to run your program with subclasses which haven't implemented the base class abstractmethod, but the subclass will throw a TypeError exception at instantiation.
There's an example here: https://stackoverflow.com/a/26458670/5932855

Use abstract base classes in Python. Create a class definition that extends (subclasses) from ABC. This is an abstract class, similar to interfaces in other languages such as Java for example, which additionally can implement concrete methods if so desired. To create a stub method, mark it as #abstractmethod (or a similar decorator for class and static methods) and add a comment block; no method definition is necessary. To create a concrete method, simply add logic under the method as you normally would to any class. Then you can have your concrete subclasses extend from this abstract class, and the interpreter will by default require you to implement all declared abstract methods.

why we use polymorphism and abstract classes in python

I first learned polymorphism in c++, in c++ we had types for every variable. So we used polymorphism to get a single pointer which can point to different type objects, and we could use them very nice.
But I don't get polymorphism and abstract classes in python. Here every variable can be everything. It could be an iterator, a list, a singe variable or a function. Every thing. So what makes a programmer to use an abstract class or use polymorphism here?
In c++ we used inheritance in many ways. But in python, it is just used to use another classes method or attribute. Am I right? what's the matter?

You don't understand what polymorphism is (OO polymorphic dispatch I mean). Polymorphism is the ability to have objects of different types understanding the same message, so you can use those objects the same way without worrying about their concrete type.
C++ actually uses the same concept (class) to denote two slightly different semantics: the abstract type (interface) which is the set of messages an object of this type understand) and the concrete type (implementation) which defines how this type reacts to those messages.
Java clearly distinguishes between abstract type (interface) and concrete type (class).
Python, being dynamically typed, relies mostly on "duck typing" (if it walks like a duck and quack like duck, then it's a duck - or at least it's "kind-of-a-duck" enough). You'll often find terms like "file-like" or "dict-like" in Python docs, meaning "anything that has the same interface as a file (or dict)", and quite a few "interfaces" are (or at least have long been) more or less implicit.
The issue with those implicit interfaces is that they are seldom fully documented, and one sometimes have to get to a function source code to find out exactly what the object passed needs to support. That's one of the reasons why the abc module was introduced in python 2 and improved in python 3: as a way to better document those implicit interfaces by creating an abstract base type that clearly defines the interface.
Another reason for abstract base classes (whether using the abc module or not) is to provide a common base implementation for a set of concrete subclasses. This is specially useful for frameworks, ie Django's models.Model (ORM) or forms.Form (user input collection and validation) classes - in both cases, just defining the database or form fields is enough to have something working.
Inheritance in C++ suffers from the same issue as classes: it serves both as defining the interface and implementation. This adds to the confusion... Java had the good idea (IMHO) to have separate abstract type from implementation, but failed to go all the way and restrict typing to interfaces - you can use either classes or interfaces for type declaration, so it still doesn't make the distinction clear.
In Python, since we don't have static typing, inheritance is mostly about implementation reuse indeed. The abc module allows you to register totally unrelated classes (no inheritance relationship) as also being subtypes of a defined abstract base case, but the point here is mostly to document that your class implements the same interface (and that it's not an accident...).

Create a class dynamically without instantiating it - no metaclasses?

Using WTForms form definition classes as an example:
class RegistrationForm(Form):
username = StringField('Username', [validators.Length(min=4, max=25)])
email = StringField('Email Address', [validators.Length(min=6, max=35)])
accept_rules = BooleanField('I accept the site rules', [validators.InputRequired()])
and looking at the source of the library it seems WTForms allows a user to define a very simple class of a customised form structure (as above), which in turn then gets used to construct a new field class which is not instantiated when the class is generated.
I've read a number of tutorials about class factories and metaclasses and the general consensus is to avoid metaclasses and use things like class decorators instead. The problem is the tutorials either start importing extra libraries eg: import six, mix explanations of different Python versions together, use overly complex examples or advise not to use metaclasses at all.
Please can somebody provide a very simple explanation (for Python 3) of how to use a simple class definition (like the WTForms example above) along with metaclasses to customise a brand new construction of a class without actually instantiating the class when it's constructed.
Edit: Apologies for finding it difficult to explain what my end goal is but as I have gone through tutorials it has been unclear as to whether class decorators, metaclasses, magic methods (call, new, init) or a combination of such are what I needed to achieve what I visualised, or if what I was visualising was the wrong way of doing things. Unfortunately it seems impossible to judge if my goal was wrong without being able to understand the mechanisms needed to achieve it. I've realised metaclasses are the way to go and just need pointing in the right direction for a very simple metaclass example done the Python 3.x way.

You can create classes dynamically - with no custom metaclasses and no decorators with what looks to the programmer as simple function call.
Just make a call to Python's builtin type with three parameters:
the name of the class, a tuple with its bases, and a mapping object with its namespace (i.e. a dictionary containing the attributes and methods you would ordinarily define on the class body).
def __init__(self):
...
namespace = {
'__init__': init,
'name': 'default name'
}
MyClass = type("MyClass", (object,), namespace)
You loose some features that are only possible due to the compiler doing a couple special things during building functions declared within a class body - mostly the ability to use paramterless super and name mangling of attributes starting with __, but that is it.
That said it should be noted this is not with "no metaclasses". "type" is itself a metaclass - the default Python metaclass for all objects - and is calling a metaclass that create a class. There is no other way to create a class. A "class decorator" is just a method that can makes changes to a class object after it is created.
Any function or method that yields a new, dynamic class, will have inside it to, at some point, call type or other metaclass. In the same mood, a "metaclass" does not create dynamic classes by itself - it needs do be either used in a class body declaration, or called with (at least) the same parameters used for calling type.
As for the recommendations for "class decorators" instead of metaclasses, I am not sure is that true (beyond the fact there is no way a "class decorator" can create classes dynamically by itself): their main drawback is that there is no ordinary way for subclasses of decorated classes to have the parent's class decorators applied to themselves automatically, while metaclasses are inherited.
In Python 3.6 you have the __init_subclass__ protocol which, yes, can avoid a lot of the traditional uses for a metaclass (but still, it won't "create classes dynamically" - calling type does that).

How do I suggest methods to override in Python?

Let's say I define a base class that provides some methods that will likely be overridden. It's not required though, as the base class provides naive default implementations. How can I highlight those methods best?
In C++, I would just use virtual but Python is a dynamic language where methods can always be overridden without marking them. I am rather looking for a hint here. Ideally, this is faster to see than a textual explanation in the docstring and easier to understand to others than a custom decorator.

Good real-world uses of metaclasses (e.g. in Python)

I'm learning about metaclasses in Python. I think it is a very powerful technique, and I'm looking for good uses for them. I'd like some feedback of good useful real-world examples of using metaclasses. I'm not looking for example code on how to write a metaclass (there are plenty examples of useless metaclasses out there), but real examples where you have applied the technique and it was really the appropriate solution. The rule is: no theoretical possibilities, but metaclasses at work in a real application.
I'll start with the one example I know:
Django models, for declarative programming, where the base class Model uses a metaclass to fill the model objects of useful ORM functionality from the attribute definitions.
Looking forward to your contributions.

In Python 2.6 and 3.1, the Python standard library provides an abc.ABCMeta, a meta-class for Abstract Base Classes ("ABCs"). Classes that use the meta-class can use #abstractmethod and #abstractproperty to define abstract methods and properties. The meta-class will ensure that derived classes override the abstract methods and properties.
Also, classes that implement the ABC without actually inheriting from it can register as implementing the interface, so that issubclass and isinstance will work.
For example, the collections module defines the Sequence ABC. It also calls Sequence.register(tuple) to register the built-in tuple type as a Sequence, even though tuple does not actually inherit from Sequence.

The Python implementation of Protocol Buffers uses metaclasses to generate the Python bindings that represent your data format. From the tutorial:
The important line in each class is __metaclass__ = reflection.GeneratedProtocolMessageType. While the details of how Python metaclasses work is beyond the scope of this tutorial, you can think of them as like a template for creating classes. At load time, the GeneratedProtocolMessageType metaclass uses the specified descriptors to create all the Python methods you need to work with each message type and adds them to the relevant classes. You can then use the fully-populated classes in your code.

FormEncode validators and Turbogears / Tosca widgets.
You might also be interested in class decorators: they can be written with the latest releases, and cover many use cases that were previously handled with metaclasses.

SQLalchemy also uses them for declarative database models.
Sorry my answer isn't very different from your example, but if you're looking for example code, I found declarative to be pretty readable.

The only time I used a metaclass so far was to write a deprecation warning mechanism. It was something along the following lines - syntax may be very approximative, but code will illustrate my point more easily than a complicated sentence :
class New(object):
pass
class Old(object):
def __new__(self):
deprecation_warning("Old class is no more supported, use New class instead")
return New()