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wtfpython-pypi/content.md
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<p align="center"><img src="/images/logo.png" alt=""></p>
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<h1 align="center"> What the f*ck Python! 🐍 </h1>
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<p align="center"> An interesting collection of surprising snippets and lesser-known Python features.</p>
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<h1 align="center">What the f*ck Python! 🐍</h1>
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<p align="center">An interesting collection of surprising snippets and lesser-known Python features.</p>
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[![WTFPL 2.0][license-image]][license-url]
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Translations: [Chinese 中文](https://github.com/leisurelicht/wtfpython-cn)
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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.
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@ -13,7 +14,7 @@ While some of the examples you see below may not be WTFs in the truest sense, bu
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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:
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If you're a returning reader, you can learn about the new modifications [here](https://github.com/satwikkansal/wtfpython/releases/).
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PS: If you're a returning reader, you can learn about the new modifications [here](https://github.com/satwikkansal/wtfpython/releases/).
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So, here we go...
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@ -77,7 +78,7 @@ So, here we go...
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- [▶ Yes, it exists!](#-yes-it-exists)
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- [▶ Inpinity *](#-inpinity-)
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- [▶ Mangling time! *](#-mangling-time-)
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- [Section: Miscallaneous](#section-miscallaneous)
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- [Section: Miscellaneous](#section-miscellaneous)
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- [▶ `+=` is faster](#--is-faster)
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- [▶ Let's make a giant string!](#-lets-make-a-giant-string)
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- [▶ Explicit typecast of strings](#-explicit-typecast-of-strings)
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@ -200,10 +201,10 @@ Makes sense, right?
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+ 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:
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* All length 0 and length 1 strings are interned.
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* Strings are interned at compile time (`'wtf'` will be interned but `''.join(['w', 't', 'f']` will not be interned)
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* Strings that are not composed of ASCII letters, digits or underscores, are not interned. This explains why `'wtf!'` was not interned due to `!`.
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* 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)
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<img src="/images/string-intern/string_intern.png" alt="">
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+ 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.
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+ 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`)
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+ 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.
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---
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@ -241,7 +242,7 @@ some_dict[5] = "Python"
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True
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```
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**Note:** Objects with different values may also have same hash (known as hash collision).
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* 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`.
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* 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`.
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* This StackOverflow [answer](https://stackoverflow.com/a/32211042/4354153) explains beautifully the rationale behind it.
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---
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@ -296,16 +297,24 @@ True
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* But why did the `is` operator evaluated to `False`? Let's see with this snippet.
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```py
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class WTF(object):
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def __init__(self): print("I ")
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def __del__(self): print("D ")
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def __init__(self): print("I")
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def __del__(self): print("D")
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```
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**Output:**
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```py
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>>> WTF() is WTF()
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I I D D
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I
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I
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D
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D
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False
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>>> id(WTF()) == id(WTF())
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I D I D
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I
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D
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I
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D
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True
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```
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As you may observe, the order in which the objects are destroyed is what made all the difference here.
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@ -530,7 +539,7 @@ And when the `board` is initialized by multiplying the `row`, this is what happe
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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).
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```py
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>>> board = [(['']*3)*3] # board = = [['']*3 for _ in range(3)]
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>>> board = [['']*3 for _ in range(3)]
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>>> board[0][0] = "X"
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>>> board
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[['X', '', ''], ['', '', ''], ['', '', '']]
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@ -547,7 +556,7 @@ for x in range(7):
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def some_func():
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return x
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funcs.append(some_func)
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results.append(some_func())
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results.append(some_func()) # note the function call here
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funcs_results = [func() for func in funcs]
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```
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@ -766,10 +775,10 @@ for item in mixed_list:
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**Output:**
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```py
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>>> booleans_found_so_far
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0
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>>> integers_found_so_far
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4
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>>> booleans_found_so_far
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0
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```
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2\.
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@ -830,7 +839,7 @@ class C(A):
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pass
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```
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**Ouptut:**
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**Output:**
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```py
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>>> A.x, B.x, C.x
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(1, 1, 1)
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@ -1148,7 +1157,7 @@ str
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>>> s = SomeClass('s')
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>>> some_dict[s] = 40
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>>> some_dict
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{'s': 40}
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{'s': 40, 's': 42}
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>>> keys = list(some_dict.keys())
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>>> type(keys[0]), type(keys[1])
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(__main__.SomeClass, str)
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@ -1697,7 +1706,7 @@ a += [5, 6, 7, 8]
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* The expression `a = a + [5,6,7,8]` generates a new list and sets `a`'s reference to that new list, leaving `b` unchanged.
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* 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.
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* 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.
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---
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@ -1895,7 +1904,7 @@ Sshh.. It's a super secret.
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#### 💡 Explanation:
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+ `antigravity` module is one of the few easter eggs released by Python developers.
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+ `import antigravity` opens up a web browser pointing to the [classic XKCD comic](http://xkcd.com/353/) about Python.
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+ 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/).
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+ 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/).
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---
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@ -2115,7 +2124,7 @@ AttributeError: 'Yo' object has no attribute '__honey'
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True
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```
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Why did `Yo()._Yo__honey` worked? Only Indian readers would understand.
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Why did `Yo()._Yo__honey` work? Only Indian readers would understand.
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#### 💡 Explanation:
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@ -2127,7 +2136,7 @@ Why did `Yo()._Yo__honey` worked? Only Indian readers would understand.
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---
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## Section: Miscallaneous
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## Section: Miscellaneous
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### ▶ `+=` is faster
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@ -2350,7 +2359,7 @@ The idea and design for this collection were initially inspired by Denys Dovhan'
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* https://stackoverflow.com/questions/530530/python-2-x-gotchas-and-landmines
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* https://stackoverflow.com/questions/1011431/common-pitfalls-in-python
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* https://www.python.org/doc/humor/
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* https://www.satwikkansal.xyz/archives/posts/python/My-Python-archives/
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* https://www.codementor.io/satwikkansal/python-practices-for-efficient-code-performance-memory-and-usability-aze6oiq65
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# 🎓 License
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@ -2365,12 +2374,12 @@ The idea and design for this collection were initially inspired by Denys Dovhan'
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If you have any wtfs, ideas or suggestions, please share.
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## Want to share wtfpython with friends?
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## Surprise your geeky pythonist friends?
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You can use these quick links for Twitter and Linkedin.
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You can use these quick links to recommend wtfpython to your friends,
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[Twitter](https://twitter.com/intent/tweet?url=https://github.com/satwikkansal/wtfpython&hastags=python,wtfpython) |
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[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.)
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[Twitter](https://twitter.com/intent/tweet?url=https://github.com/satwikkansal/wtfpython&hastags=python,wtfpython)
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| [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.)
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## Need a pdf version?
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