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Add non reflexive class method and the matmul operator examples

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Satwik 2019-10-30 01:03:07 +05:30
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@ -1321,6 +1321,48 @@ True
---
### ▶ Non-reflexive class methods
<!-- 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 everytime `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
@ -2843,7 +2885,7 @@ True
#### 💡 Explanation:
* `this` module in Python is an easter egg for The Zen Of Python ([PEP 20](https://www.python.org/dev/peps/pep-0020)).
* And if you think that's already interesting enough, check out the implementation of [this.py](https://hg.python.org/cpython/file/c3896275c0f6/Lib/this.py). Interestingly, the code for the Zen violates itself (and that's probably the only place where this happens).
* And if you think that's already interesting enough, check out the implementation of [this.py](https://hg.python.org/cpython/file/c3896275c0f6/Lib/this.py). Interestingly, **the code for the Zen violates itself** (and that's probably the only place where this happens).
* Regarding the statement `love is not True or False; love is love`, ironic but it's self-explanatory (if not, please see the examples related to `is` and `is not` operators).
---
@ -2888,7 +2930,7 @@ Try block executed successfully...
```
#### 💡 Explanation:
- The `else` clause after a loop is executed only when there's no explicit `break` after all the iterations.
- The `else` clause after a loop is executed only when there's no explicit `break` after all the iterations. You can think of it as a "nobreak" clause.
- `else` clause after try block is also called "completion clause" as reaching the `else` clause in a `try` statement means that the try block actually completed successfully.
---
@ -3041,11 +3083,10 @@ AttributeError: 'A' object has no attribute '__variable'
* So, to access `__honey` attribute in first snippet, we had to append `_Yo` to the front which would prevent conflicts with the same name attribute defined in any other class.
* But then why didn't it work in the second snippet? Because name mangling excludes the names ending with double underscores.
* The third snippet was also a consequence of name mangling. The name `__variable` in the statement `return __variable` was mangled to `_A__variable` which also happens to be the name of the variable we declared in outer scope.
* Also, if the mangled name is longer than 255 characters truncation will happen.
---
---
## Section: Miscellaneous
@ -3205,9 +3246,8 @@ nan
<!-- Example ID: f885cb82-f1e4-4daa-9ff3-972b14cb1324 --->
* `join()` is a string operation instead of list operation. (sort of counter-intuitive at first usage)
**💡 Explanation:**
If `join()` is a method on a string then it can operate on any iterable (list, tuple, iterators). If it were a method on a list, it'd have to be implemented separately by every type. Also, it doesn't make much sense to put a string-specific method on a generic `list` object API.
**💡 Explanation:** If `join()` is a method on a string then it can operate on any iterable (list, tuple, iterators). If it were a method on a list, it'd have to be implemented separately by every type. Also, it doesn't make much sense to put a string-specific method on a generic `list` object API.
* Few weird looking but semantically correct statements:
+ `[] = ()` is a semantically correct statement (unpacking an empty `tuple` into an empty `list`)
+ `'a'[0][0][0][0][0]` is also a semantically correct statement as strings are [sequences](https://docs.python.org/3/glossary.html#term-sequence)(iterables supporting element access using integer indices) in Python.
@ -3242,22 +3282,47 @@ nan
>>> a
>>> 44
```
**💡 Explanation:**
This prank comes from [Raymond Hettinger's tweet](https://twitter.com/raymondh/status/1131103570856632321?lang=en). The space invader operator is actually just a malformatted `a -= (-1)`. Which is equivalent to `a = a - (- 1)`. Similar for the `a += (+ 1)` case.
**💡 Explanation:** This prank comes from [Raymond Hettinger's tweet](https://twitter.com/raymondh/status/1131103570856632321?lang=en). The space invader operator is actually just a malformatted `a -= (-1)`. Which is equivalent to `a = a - (- 1)`. Similar for the `a += (+ 1)` case.
* Python has an undocumented [converse implication](https://en.wikipedia.org/wiki/Converse_implication) operator.
```py
>>> False ** False == True
True
>>> False ** True == False
True
>>> True ** False == True
True
>>> True ** True == True
True
```
**💡 Explanation:** If you replace `False` and `True` by 0 and 1 and do the maths, the truth table is equivalent to converse implication operator. ([Source](https://github.com/cosmologicon/pywat/blob/master/explanation.md#the-undocumented-converse-implication-operator))
* Since we are talking operators, there's also `@` operator for matrix multiplication (don't worry, this time it's for real).
```py
>>> import numpy as np
>>> np.array([2, 2, 2]) @ np.array([7, 8, 8])
46
```
**💡 Explanation:** The `@` operator was added in Python 3.5 keeping scientific community in mind. Any object can overload `__matmul__` magic method to define behavior for this operator.
* Python uses 2 bytes for local variable storage in functions. In theory, this means that only 65536 variables can be defined in a function. However, python has a handy solution built in that can be used to store more than 2^16 variable names. The following code demonstrates what happens in the stack when more than 65536 local variables are defined (Warning: This code prints around 2^18 lines of text, so be prepared!):
```py
import dis
exec("""
def f():
""" + """
""".join(["X" + str(x) + "=" + str(x) for x in range(65539)]))
f()
""".join(["X" + str(x) + "=" + str(x) for x in range(65539)]))
f()
print(dis.dis(f))
```
```
* Multiple Python threads won't run your *Python code* concurrently (yes you heard it right!). It may seem intuitive to spawn several threads and let them execute your Python code concurrently, but, because of the [Global Interpreter Lock](https://wiki.python.org/moin/GlobalInterpreterLock) in Python, all you're doing is making your threads execute on the same core turn by turn. Python threads are good for IO-bound tasks, but to achieve actual parallelization in Python for CPU-bound tasks, you might want to use the Python [multiprocessing](https://docs.python.org/2/library/multiprocessing.html) module.
* Sometimes the `print` method might not print values immediately. For example,
@ -3281,21 +3346,6 @@ nan
* `int('١٢٣٤٥٦٧٨٩')` returns `123456789` in Python 3. In Python, Decimal characters include digit characters, and all characters that can be used to form decimal-radix numbers, e.g. U+0660, ARABIC-INDIC DIGIT ZERO. Here's an [interesting story](http://chris.improbable.org/2014/8/25/adventures-in-unicode-digits/) related to this behavior of Python.
* Python has an undocumented [converse implication](https://en.wikipedia.org/wiki/Converse_implication) operator.
```py
>>> False ** False == True
True
>>> False ** True == False
True
>>> True ** False == True
True
>>> True ** True == True
True
```
If you replace `False` and `True` by 0 and 1 and do the maths, the truth table is equivalent to converse implication operator. ([Source](https://github.com/cosmologicon/pywat/blob/master/explanation.md#the-undocumented-converse-implication-operator)).
* `'abc'.count('') == 4`. Here's an approximate implementation of `count` method, which would make the things more clear
```py
def count(s, sub):