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Closes #14298: update section about dict implementation.

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Georg Brandl 2012-03-14 07:50:17 +01:00
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Doc/faq/design.rst
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@ -526,14 +526,16 @@ far) under most circumstances, and the implementation is simpler.
Dictionaries work by computing a hash code for each key stored in the dictionary
using the :func:`hash` built-in function. The hash code varies widely depending
on the key; for example, "Python" hashes to -539294296 while "python", a string
that differs by a single bit, hashes to 1142331976. The hash code is then used
to calculate a location in an internal array where the value will be stored.
Assuming that you're storing keys that all have different hash values, this
means that dictionaries take constant time -- O(1), in computer science notation
-- to retrieve a key. It also means that no sorted order of the keys is
maintained, and traversing the array as the ``.keys()`` and ``.items()`` do will
output the dictionary's content in some arbitrary jumbled order.
on the key and a per-process seed; for example, "Python" could hash to
-539294296 while "python", a string that differs by a single bit, could hash
to 1142331976. The hash code is then used to calculate a location in an
internal array where the value will be stored. Assuming that you're storing
keys that all have different hash values, this means that dictionaries take
constant time -- O(1), in computer science notation -- to retrieve a key. It
also means that no sorted order of the keys is maintained, and traversing the
array as the ``.keys()`` and ``.items()`` do will output the dictionary's
content in some arbitrary jumbled order that can change with every invocation of
a program.
Why must dictionary keys be immutable?