What is Reflection?

In computer science, reflection is the ability of a computer program to examine, introspect, and modify its own structure and behaviour at runtime

Reflection vs Introspection?

Both are often used as synonymous, but a subtle difference exists:

Introspection refers to the ability of an object to examine itself.

Reflection includes the ability of an object to modify itself, not only to introspect.

Syntax

			dir([object])
		

Without arguments, return the list of names in the current local scope

If an argument is given, it attempt to return a list of valid attributes for that object

If the object has a method named __dir__(), this method will be called

dir() @python docs

Example

			class Person:
			  def __init__(self, name, age):
			    self.name = name
			    self.age = age

			maria = Person("Maria Popova", 25)

			print(dir(maria))
		

			['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'age', 'name']
		

We see Person object atributes, as well as these, inherited from the built-in object class

Syntax

			class type(object)
			class type(name, bases, dict)
		

With one argument, return the type of an object. The return value is a type object and generally the same object as returned by object.__class__.

With three arguments, return a new type object. This is essentially a dynamic form of the class statement.

type() @python docs

type(object) - example

			class Person:
			  def __init__(self, name, age):
			    self.name = name
			    self.age = age

			maria = Person("Maria Popova", 25)

			print(type(maria))
			# <class '__main__.Person'>
		

type(name, bases, dict) - example

Dynamically create Employee class

			Employee = type('Employee', (object,), dict())
			pesho = Employee()
			print(type(pesho))
			# <class '__main__.Employee'>
		

type(name, bases, dict) - example

Dynamically create Employee class, a subclass of Person

			Employee = type('Employee', (Person,), dict())
			pesho = Employee("Pesho", 30)
			print(type(pesho))
			# <class '__main__.Employee'>
		

Syntax

			id(object)
		

Return the “identity” of an object.

This is an integer which is guaranteed to be unique and constant for this object during its lifetime.

Two objects with non-overlapping lifetimes may have the same id() value.

In CPython, id() returns the address of the object in memory.

Example

			print(id(1))

			x = 1
			print(id(x))

			y=x
			print(id(y))

			#11065920
			#11065920
			#11065920
		

hasattr() - syntax

			hasattr(object, name)
		

Return True if the object has attribute with given name as string. False otherwise.

hasattr() - example

			class Person:
			  def __init__(self, name, age):
			    self.name = name
			    self.age = age


			maria = Person("Maria Popova", 25)

			print(hasattr(maria,"name"))
			print(hasattr(maria,"surname"))
			#True
			#False
		

getattr() - syntax

			getattr(object, name[, default])
		

Return the value of the named attribute of object

If the named attribute does not exist, default is returned if provided. Otherwise AttributeError is raised.

getattr() - example

			class Person:
			  def __init__(self, name, age):
			    self.name = name
			    self.age = age

			maria = Person("Maria Popova", 25)
			print(getattr(maria, "age"))
			# 25
		

setattr() - syntax

			setattr(object, name, value)
		

The arguments are an object, a string and an arbitrary value.

The string may name an existing attribute or a new attribute.

The function assigns the value to the attribute, provided the object allows it

setattr() - example

			class Person:
			  def __init__(self, name, age):
			    self.name = name
			    self.age = age

			maria = Person("Maria Popova", 25)

			setattr(maria, "surname", "Popova")
			print(getattr(maria, "surname"))
			# Popova
		

Overview

The inspect module provides several useful functions for:

type checking

getting source code

inspecting classes and functions

examining the interpreter stack

Docs: inspect — Inspect live objects

Example

Get a function name, within a function. Get the name of the object that called the function.

			import inspect

			def foo():
			  func_name = inspect.stack()[0][3]
			  caller_name = inspect.stack()[1][3]
			  print("I'm {}.\n{} called me!".format(func_name,caller_name))

			def bar(f):
			  f()

			bar(foo)
			# I'm foo.
			# bar called m