Merge pull request #241 from Jongy/methods
Add section on methods equality and identity
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Satwik Kansal
GitHub
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c954b71f67
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@ -21,7 +21,7 @@ Following are the wonderful people (in no specific order) who have contributed t
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| Ghost account | N/A | [#96](https://github.com/satwikkansal/wtfpython/issues/96)
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| koddo | [koddo](https://github.com/koddo) | [#80](https://github.com/satwikkansal/wtfpython/issues/80), [#73](https://github.com/satwikkansal/wtfpython/issues/73) |
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| jab | [jab](https://github.com/jab) | [#77](https://github.com/satwikkansal/wtfpython/issues/77) |
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| Jongy | [Jongy](https://github.com/Jongy) | [#208](https://github.com/satwikkansal/wtfpython/issues/208), [#210](https://github.com/satwikkansal/wtfpython/issues/210) |
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| 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) |
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| Diptangsu Goswami | [diptangsu](https://github.com/diptangsu) | [#193](https://github.com/satwikkansal/wtfpython/issues/193) |
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---
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@ -44,6 +44,7 @@ So, here we go...
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+ [▶ The sticky output function](#-the-sticky-output-function)
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+ [▶ The chicken-egg problem *](#-the-chicken-egg-problem-)
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+ [▶ Subclass relationships](#-subclass-relationships)
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+ [▶ Methods equality and identity](#-methods-equality-and-identity)
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+ [▶ All-true-ation *](#-all-true-ation-)
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+ [▶ The surprising comma](#-the-surprising-comma)
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+ [▶ Strings and the backslashes](#-strings-and-the-backslashes)
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@ -51,7 +52,6 @@ So, here we go...
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+ [▶ Half triple-quoted strings](#-half-triple-quoted-strings)
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+ [▶ What's wrong with booleans?](#-whats-wrong-with-booleans)
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+ [▶ Class attributes and instance attributes](#-class-attributes-and-instance-attributes)
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+ [▶ Non-reflexive class method *](#-non-reflexive-class-method-)
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+ [▶ yielding None](#-yielding-none)
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+ [▶ Yielding from... return! *](#-yielding-from-return-)
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+ [▶ Nan-reflexivity *](#-nan-reflexivity-)
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@ -1122,6 +1122,107 @@ The Subclass relationships were expected to be transitive, right? (i.e., if `A`
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---
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### ▶ Methods equality and identity
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<!-- Example ID: 94802911-48fe-4242-defa-728ae893fa32 --->
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1.
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```py
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class SomeClass:
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def method(self):
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pass
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@classmethod
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def classm(cls):
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pass
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@staticmethod
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def staticm():
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pass
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```
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**Output:**
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```py
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>>> print(SomeClass.method is SomeClass.method)
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True
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>>> print(SomeClass.classm is SomeClass.classm)
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False
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>>> print(SomeClass.classm == SomeClass.classm)
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True
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>>> print(SomeClass.staticm is SomeClass.staticm)
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True
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```
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Accessing `classm` twice, we get an equal object, but not the *same* one? Let's see what happens
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with instances of `SomeClass`:
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2.
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```py
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o1 = SomeClass()
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o2 = SomeClass()
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```
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**Output:**
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```py
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>>> print(o1.method == o2.method)
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False
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>>> print(o1.method == o1.method)
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True
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>>> print(o1.method is o1.method)
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False
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>>> print(o1.classm is o1.classm)
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False
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>>> print(o1.classm == o1.classm == o2.classm == SomeClass.classm)
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True
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>>> print(o1.staticm is o1.staticm is o2.staticm is SomeClass.staticm)
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True
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```
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Accessing` classm` or `method` twice, creates equal but not *same* objects for the same instance of `SomeClass`.
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#### 💡 Explanation
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* Functions are [descriptors](https://docs.python.org/3/howto/descriptor.html). Whenever a function is accessed as an
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attribute, the descriptor is invoked, creating a method object which "binds" the function with the object owning the
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attribute. If called, the method calls the function, implicitly passing the bound object as the first argument
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(this is how we get `self` as the first argument, despite not passing it explicitly).
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```py
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>>> o1.method
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<bound method SomeClass.method of <__main__.SomeClass object at ...>>
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```
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* Accessing the attribute multiple times creates a method object every time! Therefore `o1.method is o1.method` is
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never truthy. Accessing functions as class attributes (as opposed to instance) does not create methods, however; so
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`SomeClass.method is SomeClass.method` is truthy.
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```py
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>>> SomeClass.method
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<function SomeClass.method at ...>
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```
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* `classmethod` transforms functions into class methods. Class methods are descriptors that, when accessed, create
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a method object which binds the *class* (type) of the object, instead of the object itself.
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```py
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>>> o1.classm
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<bound method SomeClass.classm of <class '__main__.SomeClass'>>
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```
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* Unlike functions, `classmethod`s will create a method also when accessed as class attributes (in which case they
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bind the class, not to the type of it). So `SomeClass.classm is SomeClass.classm` is falsy.
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```py
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>>> SomeClass.classm
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<bound method SomeClass.classm of <class '__main__.SomeClass'>>
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```
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* A method object compares equal when both the functions are equal, and the bound objects are the same. So
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`o1.method == o1.method` is truthy, although not the same object in memory.
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* `staticmethod` transforms functions into a "no-op" descriptor, which returns the function as-is. No method
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objects are ever created, so comparison with `is` is truthy.
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```py
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>>> o1.staticm
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<function SomeClass.staticm at ...>
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>>> SomeClass.staticm
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<function SomeClass.staticm at ...>
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```
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* Having to create new "method" objects every time Python calls instance methods and having to modify the arguments
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every time in order to insert `self` affected performance badly.
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CPython 3.7 [solved it](https://bugs.python.org/issue26110) by introducing new opcodes that deal with calling methods
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without creating the temporary method objects. This is used only when the accessed function is actually called, so the
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snippets here are not affected, and still generate methods :)
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### ▶ All-true-ation *
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<!-- Example ID: dfe6d845-e452-48fe-a2da-0ed3869a8042 -->
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@ -1451,49 +1552,6 @@ True
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---
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### ▶ Non-reflexive class method *
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<!-- Example ID: 3649771a-f733-413c-8060-3f9f167b83fd -->
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```py
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class SomeClass:
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def instance_method(self):
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pass
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@classmethod
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def class_method(cls):
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pass
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```
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**Output:**
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```py
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>>> SomeClass.instance_method is SomeClass.instance_method
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True
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>>> SomeClass.class_method is SomeClass.class_method
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False
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>>> id(SomeClass.class_method) == id(SomeClass.class_method)
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True
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```
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#### 💡 Explanation:
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- The reason `SomeClass.class_method is SomeClass.class_method` is `False` is due to the `@classmethod` decorator.
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```py
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>>> SomeClass.instance_method
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<function __main__.SomeClass.instance_method(self)>
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>>> SomeClass.class_method
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<bound method SomeClass.class_method of <class '__main__.SomeClass'>
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```
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A new bound method every time `SomeClass.class_method` is accessed.
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- `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).
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---
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### ▶ yielding None
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<!-- Example ID: 5a40c241-2c30-40d0-8ba9-cf7e097b3b53 --->
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```py
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