mirror of https://github.com/jab/bidict.git
50 lines
1.2 KiB
PHP
50 lines
1.2 KiB
PHP
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Equivalence vs. Identity
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^^^^^^^^^^^^^^^^^^^^^^^^
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Consider the following::
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>>> d = dict([(1, 'int'), (1.0, 'float')])
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How many items do you expect *d* to contain?
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If you expected *d* to be *{1: 'int', 1.0: 'float'}*,
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the actual result might surprise you::
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>>> d
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{1: 'float'}
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And similarly,
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>>> dict([(1, 'int'), (1.0, 'float'), (1+0j, 'complex'), (True, 'bool')])
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{1: 'bool'}
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>>> set([True, 1+0j, 1.0, 1]) # doctest: +SKIP
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{True}
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>>> 1.0 in {True}
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True
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(Note that ``1 == 1.0 == 1+0j == True``.)
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This illustrates that a mapping cannot contain two items
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with equivalent but distinct keys
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(and likewise a set cannot contain two equivalent but distinct elements).
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If an object being looked up in a set or mapping
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is equal to a contained object,
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the contained object will be found,
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even if it is distinct.
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With bidict,
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since values function as keys in the inverse mapping,
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this behavior occurs on both sides::
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>>> from bidict import bidict
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>>> b = bidict({1: 1})
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>>> b.inv[True]
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1
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>>> b[2] = True
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Traceback (most recent call last):
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...
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ValueDuplicationError: 1
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>>> b.put(True, 'true')
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Traceback (most recent call last):
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...
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KeyDuplicationError: 1
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