Add section on methods equality and identity

Jongy · Jongy · commit f13b98e6d56d · 2020-11-05T01:11:18.000+02:00
Closes: satwikkansal#233
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
@@ -44,6 +44,7 @@ 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)
@@ -1122,6 +1123,84 @@ 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).
+* Accessing the attribute multiple times creates multiple method objects! Therefore `o1.method is o2.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.
+* `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.
+* 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.
+* 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.
+* Having to create new "method" objects every time Python calls instance methods 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 -->
