Python - Reflection

In object-oriented programming,reflectionrefers to the ability to extract information about any object in use. You can get to know the type of object, whether is it a subclass of any other class, what are itsattributes, and much more. Python’s standard library has several functions that reflect on different properties of an object.Reflectionis also sometimes calledintrospect.

Following is the list of reflection functions in Python −

• type() Function
• isinstance() Function
• issubclass() Function
• callable() Function
• getattr() Function
• setattr() Function
• hasattr() Function
• dir() Function

The type() Function

We have used this function many times. It tells you which class an object belongs 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 followingoutput−

<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 followingoutput−

<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 aBoolean 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))

It will produce the followingoutput−

True
True
True
True

In contrast, these statements print False.

print (isinstance([1,2,3], tuple))
print (isinstance({1:'one', 2:'two'}, set))

It will produce the followingoutput−

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 followingoutput−

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 followingoutput−

True
True
True

The callable() Function

An object is callable if it invokes a certain process. APython function, which performs a certain process, is a callable object. Hence callable(function) returns True. Any function, built-in, user-defined, ormethodis callable. Objects of built-indata typessuch 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))

Astringobject is not callable. But abs is a function which is callable. The pop method of list is callable, butclear()is actually call to the function and not a function object, hence not a callable

It will produce the followingoutput−

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 followingoutput−

Hello
obj is callable? True

The getattr() Function

Thegetattr()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 followingoutput−

Manav

The setattr() Function

Thesetattr()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 usesetattr()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 followingoutput−

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 followingoutput−

False
True

The dir() Function

If this built-in function is called without an argument, return the names in the current scope. For any object as an argument, it returns a list of the attributes of the given object and 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 followingoutput−

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 followingoutput−

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 followingoutput−

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']