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Python: Objects

Every object has
An object's identity can be queried with id(obj).
The Identities of two object can be compared with the is operator.
Each object belongs to a module.
The same object might be bound to any number of variables (or more technically: entries in symbol tables)

Every object has a type

Every object has type. This type determines the abilities of an object and by extension what an object represents.
Because in Python a type is also an object, the type of a type is the type type.
The built-in function type(obj) returns the type-object of obj. 
def printTypeOfType(obj):

    type_of_obj  = type(obj        )
    type_of_type = type(type_of_obj)

    print(f'obj          = {obj         }')
    print(f'type_of_obj  = {type_of_obj }')
    print(f'type_of_type = {type_of_type}')
    print('')

printTypeOfType( 42          )
#
# obj          = 42
# type_of_obj  = <class 'int'>
# type_of_type = <class 'type'>

printTypeOfType( 99.9        )
#
# obj          = 99.9
# type_of_obj  = <class 'float'>
# type_of_type = <class 'type'>

printTypeOfType('Hello world')
#
# obj          = Hello world
# type_of_obj  = <class 'str'>
# type_of_type = <class 'type'>

printTypeOfType(printTypeOfType)
#
# obj          = <function printTypeOfType at 0x000001F329B07F70>
# type_of_obj  = <class 'function'>
# type_of_type = <class 'type'>
Github repository about-Python, path: /objects/objects-have-a-type.py

Serialization (persistation)

Python objects might be serialized (persisted) with the pickle, shelve or json module.

Mutable vs immutable objects

Python distinguishes between mutable and immutable objects.

Internal C structures and mechanisms

CPython stores a Python object internally in a PyObject or PyVarObject (if variable size) struct (both defined in Include/object.h)
All objects except type objects are always allocated on the heap, never on the stack.
The characteristics of a given type are stored in a PyTypeObject struct.

__dir__

If obj is an object that has a method named __dir__, this method will be called by dir(obj).
This is typically used in conjuction with __getattr__() and/or __getattribute__().

See also

Evaluating objects in boolean contexts, such as if or while statements.
Ccallable objects somewhat behave like functions.
Awaitable objects
An object that stores references to other objects are containers.
CPython: Include/cpython/object.h and Include/object.h.
A descriptor is an object that defines one of the dunders __get__, __set__ and __delete__.
A str representation of the object obj is returned by the built-in function str(obj).
The __repr__() method, if defined, returns a represenation of the object as a string.
The method inspect(obj) of the Rich package creates a nice report on any Python object.

Index

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