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      Reflection

      Python Tutorial

      This Python tutorial has been written for the beginners to help them understand the basic to advanced concepts of Python Programming Language. After completing this tutorial, you will find yourself at a great level of expertise in Python, from where you can take yourself to the next levels to become a world class Software Engineer.

      Reflection

      In object-oriented programming, reflection refers to the ability to extract information about any object in use. You can get to know the type of object, is it a subclass of any other class, what are its attributes and much more. Python's standard library has a number of functions that reflect on different properties of an object. Reflection is also sometimes called introspect.
      Let us take a review of reflection functions.

      The type() Function

      We have used this function many times. It tells you which class does an object belong to.

      Example

      Following statements print the respective class of different built-in data type objects
      print (type(10))
      print (type(2.56))
      print (type(2+3j))
      print (type("Hello World"))
      print (type([1,2,3]))
      print (type({1:'one', 2:'two'}))
      Here, you will get the following output
      <class 'int'>
      <class 'float'>
      <class 'complex'>
      <class 'str'>
      <class 'list'>
      <class 'dict'>
      Let us verify the type of an object of a user-defined class
      class test:
         pass
         
      obj = test()
      print (type(obj))
      It will produce the following output
      <class '__main__.test'>

      The isinstance() Function

      This is another built-in function in Python which ascertains if an object is an instance of the given class

      Syntax

      isinstance(obj, class)
      This function always returns a Boolean value, true if the object is indeed belongs to the given class and false if not.

      Example

      Following statements return True
      print (isinstance(10, int))
      print (isinstance(2.56, float))
      print (isinstance(2+3j, complex))
      print (isinstance("Hello World", str))
      In contrast, these statements print False.
      print (isinstance([1,2,3], tuple))
      print (isinstance({1:'one', 2:'two'}, set))
      It will produce the following output 
      True
      True
      True
      True
      False
      False
      You can also perform check with a user defined class
      class test:
         pass
         
      obj = test()
      print (isinstance(obj, test))
      It will produce the following output
      True
      In Python, even the classes are objects. All classes are objects of object class. It can be verified by following code
      class test:
         pass
         
      print (isinstance(int, object))
      print (isinstance(str, object))
      print (isinstance(test, object))
      All the above print statements print True.

      The issubclass() Function

      This function checks whether a class is a subclass of another class. Pertains to classes, not their instances.
      As mentioned earlier, all Python classes are subclassed from object class. Hence, output of following print statements is True for all.
      class test:
         pass
         
      print (issubclass(int, object))
      print (issubclass(str, object))
      print (issubclass(test, object))
      It will produce the following output
      True
      True
      True

      The callable() Function

      An object is callable if it invokes a certain process. A Python function, which performs a certain process, is a callable object. Hence callable(function) returns True. Any function, built-in, user defined or a method is callable. Objects of built-in data types such as int, str, etc., are not callable.

      Example

      def test():
         pass
         
      print (callable("Hello"))
      print (callable(abs))
      print (callable(list.clear([1,2])))
      print (callable(test))
      A string object is not callable. But abs is a function which is callable. The pop method of list is callable, but clear() is actually call to the function and not a function object, hence not a callable
      It will produce the following output
      False
      True
      True
      False
      True
      A class instance is callable if it has a __call__() method. In the example below, the test class includes __call__() method. Hence, its object can be used as if we are calling function. Hence, object of a class with __call__() function is a callable.
      class test:
         def __init__(self):
            pass
         def __call__(self):
            print ("Hello")
            
      obj = test()
      obj()
      print ("obj is callable?", callable(obj))
      It will produce the following output
      Hello
      obj is callable? True

      The getattr() Function

      The getattr() built-in function retrieves the value of the named attribute of object.

      Example

      class test:
         def __init__(self):
            self.name = "Manav"
            
      obj = test()
      print (getattr(obj, "name"))
      It will produce the following output
      Manav

      The setattr() Function

      The setattr() built-in function adds a new attribute to the object and assigns it a value. It can also change the value of an existing attribute.
      In the example below, the object of test class has a single attribute − name. We use setattr to add age attribute and to modify the value of name attribute.
      class test:
         def __init__(self):
            self.name = "Manav"
            
      obj = test()
      setattr(obj, "age", 20)
      setattr(obj, "name", "Madhav")
      print (obj.name, obj.age)
      It will produce the following output
      Madhav 20

      The hasattr() Function

      This built-in function returns True if the given attribute is available to the object argument, and false if not. We use the same test class and check if it has a certain attribute or not.
      class test:
         def __init__(self):
            self.name = "Manav"
            
      obj = test()
      print (hasattr(obj, "age"))
      print (hasattr(obj, "name"))
      It will produce the following output
      False
      True

      The dir() Function

      If his built in function called without an argument, return the names in the current scope. Fpr any object as argument, it returns a list the attributes of the given object, and of attributes reachable from it.
      • For a module object − the function returns the module's attributes.
      • For a class object − the function returns its attributes, and recursively the attributes of its bases.
      • For any other object − its attributes, its class's attributes, and recursively the attributes of its class's base classes.

      Example

      print ("dir(int):", dir(int))
      It will produce the following output
      dir(int): ['__abs__', '__add__', '__and__', '__bool__', '__ceil__', '__class__', '__delattr__', '__dir__', '__divmod__', '__doc__', '__eq__', '__float__', '__floor__', '__floordiv__', '__format__', '__ge__', '__getattribute__', '__getnewargs__', '__getstate__', '__gt__', '__hash__', '__index__', '__init__', '__init_subclass__', '__int__', '__invert__', '__le__', '__lshift__', '__lt__', '__mod__', '__mul__', '__ne__', '__neg__', '__new__', '__or__', '__pos__', '__pow__', '__radd__', '__rand__', '__rdivmod__', '__reduce__', '__reduce_ex__', '__repr__', '__rfloordiv__', '__rlshift__', '__rmod__', '__rmul__', '__ror__', '__round__', '__rpow__', '__rrshift__', '__rshift__', '__rsub__', '__rtruediv__', '__rxor__', '__setattr__', '__sizeof__', '__str__', '__sub__', '__subclasshook__', '__truediv__', '__trunc__', '__xor__', 'as_integer_ratio', 'bit_count', 'bit_length', 'conjugate', 'denominator', 'from_bytes', 'imag', 'numerator', 'real', 'to_bytes']

      Example

      print ("dir(dict):", dir(dict))
      It will produce the following output
      dir(dict): ['__class__', '__class_getitem__', '__contains__', '__delattr__', '__delitem__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getitem__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__ior__', '__iter__', '__le__', '__len__', '__lt__', '__ne__', '__new__', '__or__', '__reduce__', '__reduce_ex__', '__repr__', '__reversed__', '__ror__', '__setattr__', '__setitem__', '__sizeof__', '__str__', '__subclasshook__', 'clear', 'copy', 'fromkeys', 'get', 'items', 'keys', 'pop', 'popitem', 'setdefault', 'update', 'values']

      Example

      class test:
         def __init__(self):
            self.name = "Manav"
      
      obj = test()
      print ("dir(obj):", dir(obj))
      It will produce the following output
      dir(obj): ['__class__', '__delattr__', '__dict__', '__dir__', '__doc__', '__eq__', '__format__', '__ge__', '__getattribute__', '__getstate__', '__gt__', '__hash__', '__init__', '__init_subclass__', '__le__', '__lt__', '__module__', '__ne__', '__new__', '__reduce__', '__reduce_ex__', '__repr__', '__setattr__', '__sizeof__', '__str__', '__subclasshook__', '__weakref__', 'name']

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