Python Programming/Reflection

Context Managers Metaclasses A Python script can find out about the type, class, attributes and methods of an object. This is referred to as reflection or introspection. See also Metaclasses. Reflection-enabling functions include type(), isinstance(), callable(), dir(), and getattr(). Type The type method enables to find out about the type of an object. The following tests return True: type(3) is int type(3.0) is float type(10**10) is long # Python 2 type(1 + 1j) is complex type('Hello') is str type([1, 2]) is list type([1, [2, 'Hello']]) is list type({'city': 'Paris'}) is dict type((1,2)) is tuple type(set()) is set type(frozenset()) is frozenset # -------- type(3).__name__ == "int" type('Hello').__name__ == "str" # -------- import types, re, Tkinter # For the following examples type(re) is types.ModuleType type(re.sub) is types.FunctionType type(Tkinter.Frame) is types.ClassType type(Tkinter.Frame).__name__ == "classobj" type(Tkinter.Frame()).__name__ == "instance" type(re.compile('myregex')).__name__ == "SRE_Pattern" type(type(3)) is types.TypeType The type function disregards class inheritance: type(3) is object yields False while isinstance(3, object) yields True. Links: 2. Built-in Functions # type, python.org 8.15. types — Names for built-in types, python.org Isinstance Determines whether an object is an instance of a type or class. The following tests return True: isinstance(3, int) isinstance([1, 2], list) isinstance(3, object) isinstance([1, 2], object) import Tkinter; isinstance(Tkinter.Frame(), Tkinter.Frame) import Tkinter; Tkinter.Frame().__class__.__name__ == "Frame"Note that isinstance provides a weaker condition than a comparison using type. Function isinstance and a user-defined class: class Plant: pass # Dummy class class Tree(Plant): pass # Dummy class derived from Plant tree = Tree() # A new instance of Tree class print(isinstance(tree, Tree)) # True print(isinstance(tree, Plant)) # True print(isinstance(tree, object)) # True print(type(tree) is Tree) # True print(type(tree).__name__ == "instance") # False print(tree.__class__.__name__ == "Tree") # True Links: Built-in Functions # isinstance, python.org isinstance() considered harmful, canonical.org Issubclass Determines whether a class is a subclass of another class. Pertains to classes, not their instances. class Plant: pass # Dummy class class Tree(Plant): pass # Dummy class derived from Plant tree = Tree() # A new instance of Tree class print(issubclass(Tree, Plant)) # True print(issubclass(Tree, object)) # False in Python 2 print(issubclass(int, object)) # True print(issubclass(bool, int)) # True print(issubclass(int, int)) # True print(issubclass(tree, Plant)) # Error - tree is not a class Links: Built-in Functions # issubclass, python.org Duck typing Duck typing provides an indirect means of reflection. It is a technique consisting in using an object as if it was of the requested type, while catching exceptions resulting from the object not supporting some of the features of the class or type. Links: Glossary # duck-typing, python.org Callable For an object, determines whether it can be called. A class can be made callable by providing a __call__() method. Examples: callable(2) Returns False. Ditto for callable("Hello") and callable([1, 2]). callable([1,2].pop) Returns True, as pop without "()" returns a function object. callable([1,2].pop()) Returns False, as [1,2].pop() returns 2 rather than a function object. Links: Built-in Functions # callable, python.org Dir Returns the list of names of attributes of an object, which includes methods. Is somewhat heuristic and possibly incomplete, as per python.org. Examples: dir(3) dir("Hello") dir([1, 2]) import re; dir(re) Lists names of functions and other objects available in the re module for regular expressions. Links: Built-in Functions # dir, python.org Getattr Returns the value of an attribute of an object, given the attribute name passed as a string. An example: getattr(3, "imag") The list of attributes of an object can be obtained using #Dir. Links: Built-in Functions # getattr, python.org Keywords A list of Python keywords can be obtained from Python: import keyword pykeywords = keyword.kwlist print(keyword.iskeyword("if")) # True print(keyword.iskeyword("True")) # False Links: 32.6. keyword — Testing for Python keywords, python.org Built-ins A list of Python built-in objects and functions can be obtained from Python: print(dir(__builtins__)) # Output the list print(type(__builtins__.list)) # = <type 'type'> print(type(__builtins__.open)) # = <type 'builtin_function_or_method'> print(list is __builtins__.list) # True print(open is __builtins__.open) # True Links: 28.3. __builtin__ — Built-in objects, python.org Built-in Functions # dir, python.org External links 2. Built-in Functions, docs.python.org How to determine the variable type in Python?, stackoverflow.com Differences between isinstance() and type() in python, stackoverflow.com W:Reflection (computer_programming)#Python, Wikipedia W:Type introspection#Python, Wikipedia Context Managers Metaclasses