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Merge pull request #209 from Jongy/dict-loopup-performance
Add "dict lookup performance" section
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README.md
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README.md
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@ -92,6 +92,7 @@ So, here we go...
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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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+ [▶ `dict` lookup performance](#-dict-lookup-performance)
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+ [▶ Minor Ones *](#-minor-ones-)
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- [Contributing](#contributing)
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- [Acknowledgements](#acknowledgements)
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@ -3348,6 +3349,37 @@ Let's increase the number of iterations by a factor of 10.
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---
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### ▶ `dict` lookup performance
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```py
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>>> some_dict = {str(i): 1 for i in range(1_000_000)}
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>>> %timeit some_dict['5']
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28.6 ns ± 0.115 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
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>>> some_dict[1] = 1
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>>> %timeit some_dict['5']
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37.2 ns ± 0.265 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
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# why did it become much slower?
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```
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#### 💡 Explanation:
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+ CPython has a generic dictionary lookup function that handles all types of keys (`str`, `int`, any object ...), and a specialized one for the common case of dictionaries composed of `str`-only keys.
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+ The specialized function (named `lookdict_unicode` in CPython's sources) knows all existing keys (including the looked-up key) are strings, and uses the faster & simpler string comparison to compare keys, instead of calling the `__eq__` method.
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+ The first time a `dict` instance is accessed with a non-`str` key, it's modified so future lookups use the generic function.
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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 - attempting a failed lookup has the same effect:
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```py
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>>> some_dict = {str(i): 1 for i in range(1_000_000)}
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>>> %timeit some_dict['5']
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28.5 ns ± 0.142 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
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>>> some_dict[1]
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Traceback (most recent call last):
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File "<stdin>", line 1, in <module>
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KeyError: 1
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>>> %timeit some_dict['5']
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38.5 ns ± 0.0913 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
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```
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---
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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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