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[Pypi]
An OrderedSet is a mutable data structure that is a hybrid of a list and a
set.
It remembers the order of its entries, and every entry has an index number
that
can be looked up.
## Installation
`ordered_set` is available on PyPI and packaged as a wheel. You can list it
as a dependency of your project, in whatever form that takes.
To install it into your current Python environment:
pip install ordered-set
To install the code for development, after checking out the repository:
pip install flit
flit install
## Usage examples
An OrderedSet is created and used like a set:
>>> from ordered_set import OrderedSet
>>> letters = OrderedSet('abracadabra')
>>> letters
OrderedSet(['a', 'b', 'r', 'c', 'd'])
>>> 'r' in letters
True
It is efficient to find the index of an entry in an OrderedSet, or find an
entry by its index. To help with this use case, the `.add()` method returns
the index of the added item, whether it was already in the set or not.
>>> letters.index('r')
2
>>> letters[2]
'r'
>>> letters.add('r')
2
>>> letters.add('x')
5
OrderedSets implement the union (`|`), intersection (`&`), and difference
(`-`)
operators like sets do.
>>> letters |= OrderedSet('shazam')
>>> letters
OrderedSet(['a', 'b', 'r', 'c', 'd', 'x', 's', 'h', 'z', 'm'])
>>> letters & set('aeiou')
OrderedSet(['a'])
>>> letters -= 'abcd'
>>> letters
OrderedSet(['r', 'x', 's', 'h', 'z', 'm'])
The `__getitem__()` and `index()` methods have been extended to accept any
iterable except a string, returning a list, to perform NumPy-like "fancy
indexing".
>>> letters = OrderedSet('abracadabra')
>>> letters[[0, 2, 3]]
['a', 'r', 'c']
>>> letters.index(['a', 'r', 'c'])
[0, 2, 3]
OrderedSet implements `__getstate__` and `__setstate__` so it can be
pickled,
and implements the abstract base classes `collections.MutableSet` and
`collections.Sequence`.
OrderedSet can be used as a generic collection type, similar to the
collections
in the `typing` module like List, Dict, and Set. For example, you can
annotate
a variable as having the type `OrderedSet[str]` or `OrderedSet[Tuple[int,
str]]`.
## OrderedSet in data science applications
An OrderedSet can be used as a bi-directional mapping between a sparse
vocabulary and dense index numbers. As of version 3.1, it accepts NumPy
arrays
of index numbers as well as lists.
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