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@ -1,9 +1,10 @@
<p align="center"><img src="/images/logo.png" alt=""></p>
<h1 align="center"> What the f*ck Python! 🐍 </h1>
<p align="center"> An interesting collection of surprising snippets and lesser-known Python features.</p>
<h1 align="center">What the f*ck Python! 🐍</h1>
<p align="center">An interesting collection of surprising snippets and lesser-known Python features.</p>
[![WTFPL 2.0][license-image]][license-url]
Translations: [Chinese 中文](https://github.com/leisurelicht/wtfpython-cn)
Python, being a beautifully designed high-level and interpreter-based programming language, provides us with many features for the programmer's comfort. But sometimes, the outcomes of a Python snippet may not seem obvious to a regular user at first sight.
@ -13,7 +14,7 @@ While some of the examples you see below may not be WTFs in the truest sense, bu
If you're an experienced Python programmer, you can take it as a challenge to get most of them right in first attempt. You may be already familiar with some of these examples, and I might be able to revive sweet old memories of yours being bitten by these gotchas :sweat_smile:
If you're a returning reader, you can learn about the new modifications [here](https://github.com/satwikkansal/wtfpython/releases/).
PS: If you're a returning reader, you can learn about the new modifications [here](https://github.com/satwikkansal/wtfpython/releases/).
So, here we go...
@ -77,7 +78,7 @@ So, here we go...
- [▶ Yes, it exists!](#-yes-it-exists)
- [▶ Inpinity *](#-inpinity-)
- [▶ Mangling time! *](#-mangling-time-)
- [Section: Miscallaneous](#section-miscallaneous)
- [Section: Miscellaneous](#section-miscellaneous)
- [▶ `+=` is faster](#--is-faster)
- [▶ Let's make a giant string!](#-lets-make-a-giant-string)
- [▶ Explicit typecast of strings](#-explicit-typecast-of-strings)
@ -97,22 +98,22 @@ All the examples are structured like below:
> ### ▶ Some fancy Title *
> The asterisk at the end of the title indicates the example was not present in the first release and has been recently added.
>
>
> ```py
> # Setting up the code.
> # Preparation for the magic...
> ```
>
>
> **Output (Python version):**
> ```py
> >>> triggering_statement
> Probably unexpected output
> ```
> (Optional): One line describing the unexpected output.
>
>
>
>
> #### 💡 Explanation:
>
>
> * Brief explanation of what's happening and why is it happening.
> ```py
> Setting up examples for clarification (if necessary)
@ -200,10 +201,10 @@ Makes sense, right?
+ In the snippets above, strings are implicitly interned. The decision of when to implicitly intern a string is implementation dependent. There are some facts that can be used to guess if a string will be interned or not:
* All length 0 and length 1 strings are interned.
* Strings are interned at compile time (`'wtf'` will be interned but `''.join(['w', 't', 'f']` will not be interned)
* Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`.
* Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`. Cpython implementation of this rule can be found [here](https://github.com/python/cpython/blob/3.6/Objects/codeobject.c#L19)
<img src="/images/string-intern/string_intern.png" alt="">
+ When `a` and `b` are set to `"wtf!"` in the same line, the Python interpreter creates a new object, then references the second variable at the same time. If you do it on separate lines, it doesn't "know" that there's already `wtf!` as an object (because `"wtf!"` is not implicitly interned as per the facts mentioned above). It's a compiler optimization and specifically applies to the interactive environment.
+ Constant folding is a technique for [peephole optimization](https://en.wikipedia.org/wiki/Peephole_optimization) in Python. This means the expression `'a'*20` is replaced by `'aaaaaaaaaaaaaaaaaaaa'` during compilation to reduce few clock cycles during runtime. But since the python bytecode generated after compilation is stored in `.pyc` files, the strings greater than length of 20 are discarded for peephole optimization (Why? Imagine the size of `.pyc` file generated as a result of the expression `'a'*10**10`)
+ Constant folding is a technique for [peephole optimization](https://en.wikipedia.org/wiki/Peephole_optimization) in Python. This means the expression `'a'*20` is replaced by `'aaaaaaaaaaaaaaaaaaaa'` during compilation to reduce few clock cycles during runtime. Constant folding only occurs for strings having length less than 20. (Why? Imagine the size of `.pyc` file generated as a result of the expression `'a'*10**10`). [Here's](https://github.com/python/cpython/blob/3.6/Python/peephole.c#L288) the implementation source for the same.
---
@ -241,7 +242,7 @@ some_dict[5] = "Python"
True
```
**Note:** Objects with different values may also have same hash (known as hash collision).
* When the statement `some_dict[5] = "Python"` is executed, the existing value "JavaScript" is overwritten with "Python" because Python recongnizes `5` and `5.0` as the same keys of the dictionary `some_dict`.
* When the statement `some_dict[5] = "Python"` is executed, the existing value "JavaScript" is overwritten with "Python" because Python recognizes `5` and `5.0` as the same keys of the dictionary `some_dict`.
* This StackOverflow [answer](https://stackoverflow.com/a/32211042/4354153) explains beautifully the rationale behind it.
---
@ -296,16 +297,24 @@ True
* But why did the `is` operator evaluated to `False`? Let's see with this snippet.
```py
class WTF(object):
def __init__(self): print("I ")
def __del__(self): print("D ")
def __init__(self): print("I")
def __del__(self): print("D")
```
**Output:**
```py
>>> WTF() is WTF()
I I D D
I
I
D
D
False
>>> id(WTF()) == id(WTF())
I D I D
I
D
I
D
True
```
As you may observe, the order in which the objects are destroyed is what made all the difference here.
@ -530,7 +539,7 @@ And when the `board` is initialized by multiplying the `row`, this is what happe
We can avoid this scenario here by not using `row` variable to generate `board`. (Asked in [this](https://github.com/satwikkansal/wtfpython/issues/68) issue).
```py
>>> board = [(['']*3)*3] # board = = [['']*3 for _ in range(3)]
>>> board = [['']*3 for _ in range(3)]
>>> board[0][0] = "X"
>>> board
[['X', '', ''], ['', '', ''], ['', '', '']]
@ -547,7 +556,7 @@ for x in range(7):
def some_func():
return x
funcs.append(some_func)
results.append(some_func())
results.append(some_func()) # note the function call here
funcs_results = [func() for func in funcs]
```
@ -766,10 +775,10 @@ for item in mixed_list:
**Output:**
```py
>>> booleans_found_so_far
0
>>> integers_found_so_far
4
>>> booleans_found_so_far
0
```
2\.
@ -830,7 +839,7 @@ class C(A):
pass
```
**Ouptut:**
**Output:**
```py
>>> A.x, B.x, C.x
(1, 1, 1)
@ -1135,7 +1144,7 @@ str
and type(other) is SomeClass
and super().__eq__(other)
)
# When we define a custom __eq__, Python stops automatically inheriting the
# __hash__ method, so we need to define it as well
__hash__ = str.__hash__
@ -1148,7 +1157,7 @@ str
>>> s = SomeClass('s')
>>> some_dict[s] = 40
>>> some_dict
{'s': 40}
{'s': 40, 's': 42}
>>> keys = list(some_dict.keys())
>>> type(keys[0]), type(keys[1])
(__main__.SomeClass, str)
@ -1313,7 +1322,7 @@ Shouldn't that be 100?
> First, tabs are replaced (from left to right) by one to eight spaces such that the total number of characters up to and including the replacement is a multiple of eight <...>
* So the "tab" at the last line of `square` function is replaced with eight spaces, and it gets into the loop.
* Python 3 is kind enough to throw an error for such cases automatically.
**Output (Python 3.x):**
```py
TabError: inconsistent use of tabs and spaces in indentation
@ -1697,7 +1706,7 @@ a += [5, 6, 7, 8]
* The expression `a = a + [5,6,7,8]` generates a new list and sets `a`'s reference to that new list, leaving `b` unchanged.
* The expression `a + =[5,6,7,8]` is actually mapped to an "extend" function that operates on the list such that `a` and `b` still point to the same list that has been modified in-place.
* The expression `a += [5,6,7,8]` is actually mapped to an "extend" function that operates on the list such that `a` and `b` still point to the same list that has been modified in-place.
---
@ -1731,7 +1740,7 @@ UnboundLocalError: local variable 'a' referenced before assignment
a += 1
return a
```
**Output:**
```py
>>> another_func()
@ -1895,7 +1904,7 @@ Sshh.. It's a super secret.
#### 💡 Explanation:
+ `antigravity` module is one of the few easter eggs released by Python developers.
+ `import antigravity` opens up a web browser pointing to the [classic XKCD comic](http://xkcd.com/353/) about Python.
+ Well, there's more to it. There's **another easter egg inside the easter egg**. If look at the [code](https://github.com/python/cpython/blob/master/Lib/antigravity.py#L7-L17), there's a function defined that purports to implement the [XKCD's geohashing algorithm](https://xkcd.com/426/).
+ Well, there's more to it. There's **another easter egg inside the easter egg**. If you look at the [code](https://github.com/python/cpython/blob/master/Lib/antigravity.py#L7-L17), there's a function defined that purports to implement the [XKCD's geohashing algorithm](https://xkcd.com/426/).
---
@ -2115,7 +2124,7 @@ AttributeError: 'Yo' object has no attribute '__honey'
True
```
Why did `Yo()._Yo__honey` worked? Only Indian readers would understand.
Why did `Yo()._Yo__honey` work? Only Indian readers would understand.
#### 💡 Explanation:
@ -2127,7 +2136,7 @@ Why did `Yo()._Yo__honey` worked? Only Indian readers would understand.
---
## Section: Miscallaneous
## Section: Miscellaneous
### ▶ `+=` is faster
@ -2272,7 +2281,7 @@ nan
### ▶ Minor Ones
* `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.
@ -2350,7 +2359,7 @@ The idea and design for this collection were initially inspired by Denys Dovhan'
* https://stackoverflow.com/questions/530530/python-2-x-gotchas-and-landmines
* https://stackoverflow.com/questions/1011431/common-pitfalls-in-python
* https://www.python.org/doc/humor/
* https://www.satwikkansal.xyz/archives/posts/python/My-Python-archives/
* https://www.codementor.io/satwikkansal/python-practices-for-efficient-code-performance-memory-and-usability-aze6oiq65
# 🎓 License
@ -2365,12 +2374,12 @@ The idea and design for this collection were initially inspired by Denys Dovhan'
If you have any wtfs, ideas or suggestions, please share.
## Want to share wtfpython with friends?
## Surprise your geeky pythonist friends?
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