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CONTRIBUTORS.md
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CONTRIBUTORS.md
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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) |
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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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---
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**Translations**
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63
README.md
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63
README.md
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@ -93,6 +93,7 @@ So, here we go...
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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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+ [▶ Slowing down `dict` lookups *](#-slowing-down-dict-lookups)
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+ [▶ Bloating instance `dict`s *](#-bloating-instance-dicts-)
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+ [▶ Minor Ones *](#-minor-ones-)
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- [Contributing](#contributing)
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- [Acknowledgements](#acknowledgements)
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@ -3382,6 +3383,68 @@ Why are same lookups becoming slower?
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+ This process is not reversible for the particular `dict` instance, and the key doesn't even have to exist in the dictionary. That's why attempting a failed lookup has the same effect.
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### ▶ Bloating instance `dict`s *
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<!-- Example ID: fe706ab4-1615-c0ba-a078-76c98cbe3f48 --->
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```py
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import sys
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class SomeClass:
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def __init__(self):
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self.some_attr1 = 1
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self.some_attr2 = 2
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self.some_attr3 = 3
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self.some_attr4 = 4
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def dict_size(o):
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return sys.getsizeof(o.__dict__)
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```
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**Output:** (Python 3.8, other Python 3 versions may vary a little)
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```py
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>>> o1 = SomeClass()
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>>> o2 = SomeClass()
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>>> dict_size(o1)
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104
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>>> dict_size(o2)
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104
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>>> del o1.some_attr1
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>>> o3 = SomeClass()
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>>> dict_size(o3)
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232
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>>> dict_size(o1)
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232
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```
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Let's try again... In a new interpreter:
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```py
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>>> o1 = SomeClass()
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>>> o2 = SomeClass()
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>>> dict_size(o1)
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104 # as expected
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>>> o1.some_attr5 = 5
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>>> o1.some_attr6 = 6
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>>> dict_size(o1)
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360
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>>> dict_size(o2)
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272
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>>> o3 = SomeClass()
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>>> dict_size(o3)
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232
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```
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What makes those dictionaries become bloated? And why are newly created objects bloated as well?
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#### 💡 Explanation:
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+ CPython is able to reuse the same "keys" object in multiple dictionaries. This was added in [PEP 412](https://www.python.org/dev/peps/pep-0412/) with the motivation to reduce memory usage, specifically in dictionaries of instances - where keys (instance attributes) tend to be common to all instances.
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+ This optimization is entirely seamless for instance dictionaries, but it is disabled if certain assumptions are broken.
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+ Key-sharing dictionaries do not support deletion; if an instance attribute is deleted, the dictionary is "unshared", and key-sharing is disabled for all future instances of the same class.
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+ Additionaly, if the dictionary keys have be resized (because new keys are inserted), they are kept shared *only* if they are used by a exactly single dictionary (this allows adding many attributes in the `__init__` of the very first created instance, without causing an "unshare"). If multiple instances exist when a resize happens, key-sharing is disabled for all future instances of the same class: CPython can't tell if your instances are using the same set of attributes anymore, and decides to bail out on attempting to share their keys.
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+ A small tip, if you aim to lower your program's memory footprint: don't delete instance attributes, and make sure to initialize all attributes in your `__init__`!
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### ▶ Minor Ones *
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<!-- Example ID: f885cb82-f1e4-4daa-9ff3-972b14cb1324 --->
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* `join()` is a string operation instead of list operation. (sort of counter-intuitive at first usage)
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