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gh-77753: Add hash examples for set/dict #92549

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20 changes: 20 additions & 0 deletions Doc/library/stdtypes.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -4181,6 +4181,12 @@ another set. The :class:`frozenset` type is immutable and :term:`hashable` ---
its contents cannot be altered after it is created; it can therefore be used as
a dictionary key or as an element of another set.

Note that as long as the objects have the same hash values, they are seen as
equivalent by sets::
Comment on lines +4184 to +4185
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It is not true.

>>> hash(1) == hash(2**61)
True
>>> {1, 2**61}
{1, 2305843009213693952}


>>> {0, True, False, 1}
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Could you please add some of the types Terry listed in the issue too? I was frankly surprised at them (especially the Decimal and Fractions with the same hash value).
Maybe this would help:

Suggested change
>>> {0, True, False, 1}
>>> {0, True, False, 1, 0.0, 1.0}

{0, True}

Non-empty sets (not frozensets) can be created by placing a comma-separated list
of elements within braces, for example: ``{'jack', 'sjoerd'}``, in addition to the
:class:`set` constructor.
Expand Down Expand Up @@ -4383,6 +4389,20 @@ then they can be used interchangeably to index the same dictionary entry. (Note
however, that since computers store floating-point numbers as approximations it
is usually unwise to use them as dictionary keys.)

This also means if two keys have the same hash values, they will map to the same
entry in the dictionary::
Comment on lines +4392 to +4393
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Same as above.


>>> {0: 0, False: 0}
{0: 0}

However, note that when a key with the same hash as an existing key is inserted
into a dictionary, it replaces the value but not the key::

>>> d = {False: 'false'}
>>> d[0] = 0
>>> d
{False: 0}
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Suggested change
{False: 0}
{False: 0}
>>> {1: 'a', True: 'b'}
{1: 'b'}


.. class:: dict(**kwargs)
dict(mapping, **kwargs)
dict(iterable, **kwargs)
Expand Down