Update content.md
This commit is contained in:
Satwik Kansal
Satwik
parent
810dbe1582
commit
f8a7d38b70
|
|
@ -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?
|
||||
|
||||
You can use these quick links for Twitter and Linkedin.
|
||||
You can use these quick links to recommend wtfpython to your friends,
|
||||
|
||||
[Twitter](https://twitter.com/intent/tweet?url=https://github.com/satwikkansal/wtfpython&hastags=python,wtfpython) |
|
||||
[Linkedin](https://www.linkedin.com/shareArticle?url=https://github.com/satwikkansal&title=What%20the%20f*ck%20Python!&summary=An%20interesting%20collection%20of%20subtle%20and%20tricky%20Python%20snippets.)
|
||||
[Twitter](https://twitter.com/intent/tweet?url=https://github.com/satwikkansal/wtfpython&hastags=python,wtfpython)
|
||||
| [Linkedin](https://www.linkedin.com/shareArticle?url=https://github.com/satwikkansal&title=What%20the%20f*ck%20Python!&summary=An%20interesting%20collection%20of%20subtle%20and%20tricky%20Python%20snippets.)
|
||||
|
||||
## Need a pdf version?
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue