Merge pull request satwikkansal#241 from Jongy/methods

satwikkansal · web-flow · commit c954b71f67ab · 2020-11-08T11:03:34.000+05:30
Add section on methods equality and identity
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
@@ -21,7 +21,7 @@ Following are the wonderful people (in no specific order) who have contributed t
 | Ghost account | N/A | [#96](https://github.com/satwikkansal/wtfpython/issues/96)
 | koddo | [koddo](https://github.com/koddo) | [#80](https://github.com/satwikkansal/wtfpython/issues/80), [#73](https://github.com/satwikkansal/wtfpython/issues/73) |
 | jab | [jab](https://github.com/jab) | [#77](https://github.com/satwikkansal/wtfpython/issues/77) |
-| Jongy | [Jongy](https://github.com/Jongy) | [#208](https://github.com/satwikkansal/wtfpython/issues/208), [#210](https://github.com/satwikkansal/wtfpython/issues/210) |
+| Jongy | [Jongy](https://github.com/Jongy) | [#208](https://github.com/satwikkansal/wtfpython/issues/208), [#210](https://github.com/satwikkansal/wtfpython/issues/210), [#233](https://github.com/satwikkansal/wtfpython/issues/233) |
 | Diptangsu Goswami | [diptangsu](https://github.com/diptangsu) | [#193](https://github.com/satwikkansal/wtfpython/issues/193) |
 
 ---
diff --git a/README.md b/README.md
@@ -44,14 +44,14 @@ So, here we go...
     + [▶ The sticky output function](#-the-sticky-output-function)
     + [▶ The chicken-egg problem *](#-the-chicken-egg-problem-)
     + [▶ Subclass relationships](#-subclass-relationships)
+    + [▶ Methods equality and identity](#-methods-equality-and-identity)
     + [▶ All-true-ation *](#-all-true-ation-)
     + [▶ The surprising comma](#-the-surprising-comma)
     + [▶ Strings and the backslashes](#-strings-and-the-backslashes)
     + [▶ not knot!](#-not-knot)
     + [▶ Half triple-quoted strings](#-half-triple-quoted-strings)
     + [▶ What's wrong with booleans?](#-whats-wrong-with-booleans)
     + [▶ Class attributes and instance attributes](#-class-attributes-and-instance-attributes)
-    + [▶ Non-reflexive class method *](#-non-reflexive-class-method-)
     + [▶ yielding None](#-yielding-none)
     + [▶ Yielding from... return! *](#-yielding-from-return-)
     + [▶ Nan-reflexivity *](#-nan-reflexivity-)
@@ -1122,6 +1122,107 @@ The Subclass relationships were expected to be transitive, right? (i.e., if `A`
 
 ---
 
+### ▶ Methods equality and identity
+<!-- Example ID: 94802911-48fe-4242-defa-728ae893fa32 --->
+
+1.
+```py
+class SomeClass:
+    def method(self):
+        pass
+
+    @classmethod
+    def classm(cls):
+        pass
+
+    @staticmethod
+    def staticm():
+        pass
+```
+
+**Output:**
+```py
+>>> print(SomeClass.method is SomeClass.method)
+True
+>>> print(SomeClass.classm is SomeClass.classm)
+False
+>>> print(SomeClass.classm == SomeClass.classm)
+True
+>>> print(SomeClass.staticm is SomeClass.staticm)
+True
+```
+
+Accessing `classm` twice, we get an equal object, but not the *same* one? Let's see what happens
+with instances of `SomeClass`:
+
+2.
+```py
+o1 = SomeClass()
+o2 = SomeClass()
+```
+
+**Output:**
+```py
+>>> print(o1.method == o2.method)
+False
+>>> print(o1.method == o1.method)
+True
+>>> print(o1.method is o1.method)
+False
+>>> print(o1.classm is o1.classm)
+False
+>>> print(o1.classm == o1.classm == o2.classm == SomeClass.classm)
+True
+>>> print(o1.staticm is o1.staticm is o2.staticm is SomeClass.staticm)
+True
+```
+
+Accessing` classm` or `method` twice, creates equal but not *same* objects for the same instance of `SomeClass`.
+
+#### 💡 Explanation
+* Functions are [descriptors](https://docs.python.org/3/howto/descriptor.html). Whenever a function is accessed as an
+attribute, the descriptor is invoked, creating a method object which "binds" the function with the object owning the
+attribute. If called, the method calls the function, implicitly passing the bound object as the first argument
+(this is how we get `self` as the first argument, despite not passing it explicitly).
+```py
+>>> o1.method
+<bound method SomeClass.method of <__main__.SomeClass object at ...>>
+```
+* Accessing the attribute multiple times creates a method object every time! Therefore `o1.method is o1.method` is
+never truthy. Accessing functions as class attributes (as opposed to instance) does not create methods, however; so
+`SomeClass.method is SomeClass.method` is truthy.
+```py
+>>> SomeClass.method
+<function SomeClass.method at ...>
+```
+* `classmethod` transforms functions into class methods. Class methods are descriptors that, when accessed, create
+a method object which binds the *class* (type) of the object, instead of the object itself.
+```py
+>>> o1.classm
+<bound method SomeClass.classm of <class '__main__.SomeClass'>>
+```
+* Unlike functions, `classmethod`s will create a method also when accessed as class attributes (in which case they
+bind the class, not to the type of it). So `SomeClass.classm is SomeClass.classm` is falsy.
+```py
+>>> SomeClass.classm
+<bound method SomeClass.classm of <class '__main__.SomeClass'>>
+```
+* A method object compares equal when both the functions are equal, and the bound objects are the same. So
+`o1.method == o1.method` is truthy, although not the same object in memory.
+* `staticmethod` transforms functions into a "no-op" descriptor, which returns the function as-is. No method
+objects are ever created, so comparison with `is` is truthy.
+```py
+>>> o1.staticm
+<function SomeClass.staticm at ...>
+>>> SomeClass.staticm
+<function SomeClass.staticm at ...>
+```
+* Having to create new "method" objects every time Python calls instance methods and having to modify the arguments
+every time in order to insert `self` affected performance badly.
+CPython 3.7 [solved it](https://bugs.python.org/issue26110) by introducing new opcodes that deal with calling methods
+without creating the temporary method objects. This is used only when the accessed function is actually called, so the
+snippets here are not affected, and still generate methods :)
+
 ### ▶ All-true-ation *
 
 <!-- Example ID: dfe6d845-e452-48fe-a2da-0ed3869a8042 -->
@@ -1451,49 +1552,6 @@ True
 
 ---
 
-### ▶ Non-reflexive class method *
-
-<!-- Example ID: 3649771a-f733-413c-8060-3f9f167b83fd -->
-
-```py
-class SomeClass:
-        def instance_method(self):
-                pass
-        
-        @classmethod
-        def class_method(cls):
-                pass
-```
-
-**Output:**
-
-```py
->>> SomeClass.instance_method is SomeClass.instance_method
-True
->>> SomeClass.class_method is SomeClass.class_method
-False
->>> id(SomeClass.class_method) == id(SomeClass.class_method)
-True
-```
-
-#### 💡 Explanation:
-
-- The reason `SomeClass.class_method is SomeClass.class_method` is `False` is due to the `@classmethod` decorator. 
-
-  ```py
-  >>> SomeClass.instance_method
-  <function __main__.SomeClass.instance_method(self)>
-  >>> SomeClass.class_method
-  <bound method SomeClass.class_method of <class '__main__.SomeClass'>
-  ```
-
-  A new bound method every time `SomeClass.class_method` is accessed.
-
--  `id(SomeClass.class_method) == id(SomeClass.class_method)` returned `True` because the second allocation of memory for `class_method` happened at the same location of first deallocation (See Deep Down, we're all the same example for more detailed explanation). 
-
----
-
-
 ### ▶ yielding None
 <!-- Example ID: 5a40c241-2c30-40d0-8ba9-cf7e097b3b53 --->
 ```py
