89 lines
2.5 KiB
ReStructuredText
89 lines
2.5 KiB
ReStructuredText
.. _examples:
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Examples
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========
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The simplest possible usage would be:
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.. doctest::
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>>> import attr
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>>> @attr.s
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... class Empty(object):
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... pass
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>>> Empty()
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Empty()
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>>> Empty() == Empty()
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True
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>>> Empty() is Empty()
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False
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So in other words: ``attrs`` useful even without actual attributes!
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But you'll usually want some data on your classes, so let's add some:
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.. doctest::
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>>> @attr.s
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... class Coordinates(object):
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... x = attr.ib()
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... y = attr.ib()
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These by default, all features are added, so you have immediately a fully functional data class with a nice ``repr`` string and comparison methods.
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.. doctest::
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>>> c1 = Coordinates(1, 2)
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>>> c1
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Coordinates(x=1, y=2)
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>>> c2 = Coordinates(x=2, y=1)
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>>> c2
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Coordinates(x=2, y=1)
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>>> c1 == c2
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False
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As shown, the generated ``__init__`` method allows both for positional and keyword arguments.
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Sometimes you want to have default values for your initializer.
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And sometimes you even want mutable objects as default values (ever used accidentally ``def f(arg=[])``?).
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``attrs`` has you covered in both cases:
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.. doctest::
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>>> import collections
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>>> @attr.s
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... class Connection(object):
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... socket = attr.ib()
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... @classmethod
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... def connect(cl, db_string):
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... # connect somehow to db_string
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... return cl(socket=42)
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>>> @attr.s
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... class ConnectionPool(object):
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... db_string = attr.ib()
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... pool = attr.ib(default_factory=collections.deque)
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... debug = attr.ib(default_value=False)
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... def get_connection(self):
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... try:
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... return self.pool.pop()
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... except IndexError:
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... if self.debug:
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... print "New connection!"
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... return Connection.connect(self.db_string)
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... def free_connection(self, conn):
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... if self.debug:
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... print "Connection returned!"
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... self.pool.appendleft(conn)
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...
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>>> cp = ConnectionPool("postgres://localhost")
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>>> cp
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ConnectionPool(db_string='postgres://localhost', pool=deque([]), debug=False)
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>>> conn = cp.get_connection()
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>>> conn
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Connection(socket=42)
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>>> cp.free_connection(conn)
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>>> cp
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ConnectionPool(db_string='postgres://localhost', pool=deque([Connection(socket=42)]), debug=False)
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More information on why class methods for constructing objects are awesome can be found in this insightful `blog post <http://as.ynchrono.us/2014/12/asynchronous-object-initialization.html>`_.
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