426 lines
12 KiB
ReStructuredText
426 lines
12 KiB
ReStructuredText
.. _examples:
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Examples
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========
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Basics
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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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If playful naming turns you off, ``attrs`` comes with no-nonsense aliases:
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.. doctest::
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>>> @attr.attributes
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... class SeriousCoordinates(object):
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... x = attr.attr()
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... y = attr.attr()
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>>> SeriousCoordinates(1, 2)
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SeriousCoordinates(x=1, y=2)
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>>> attr.fields(Coordinates) == attr.fields(SeriousCoordinates)
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True
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For private attributes, ``attrs`` will strip the leading underscores for keyword arguments:
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.. doctest::
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>>> @attr.s
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... class C(object):
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... _x = attr.ib()
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>>> C(x=1)
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C(_x=1)
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An additional way (not unlike ``characteristic``) of defining attributes is supported too.
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This is useful in times when you want to enhance classes that are not yours (nice ``__repr__`` for Django models anyone?):
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.. doctest::
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>>> class SomethingFromSomeoneElse(object):
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... def __init__(self, x):
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... self.x = x
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>>> SomethingFromSomeoneElse = attr.s(these={"x": attr.ib()}, init=False)(SomethingFromSomeoneElse)
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>>> SomethingFromSomeoneElse(1)
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SomethingFromSomeoneElse(x=1)
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Or if you want to use properties:
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.. doctest::
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>>> @attr.s(these={"_x": attr.ib()})
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... class ReadOnlyXSquared(object):
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... @property
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... def x(self):
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... return self._x ** 2
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>>> rox = ReadOnlyXSquared(x=5)
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>>> rox
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ReadOnlyXSquared(_x=5)
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>>> rox.x
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25
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>>> rox.x = 6
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Traceback (most recent call last):
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...
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AttributeError: can't set attribute
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`Sub-classing <https://www.youtube.com/watch?v=3MNVP9-hglc>`_ is bad for you, but ``attrs`` will still do what you'd hope for:
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.. doctest::
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>>> @attr.s
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... class A(object):
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... a = attr.ib()
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... def get_a(self):
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... return self.a
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>>> @attr.s
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... class B(object):
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... b = attr.ib()
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>>> @attr.s
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... class C(B, A):
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... c = attr.ib()
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>>> i = C(1, 2, 3)
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>>> i
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C(a=1, b=2, c=3)
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>>> i == C(1, 2, 3)
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True
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>>> i.get_a()
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1
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The order of the attributes is defined by the `MRO <https://www.python.org/download/releases/2.3/mro/>`_.
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In Python 3, classes defined within other classes are `detected <https://www.python.org/dev/peps/pep-3155/>`_ and reflected in the ``__repr__``.
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In Python 2 though, it's impossible.
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Therefore ``@attr.s`` comes with the ``repr_ns`` option to set it manually:
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.. doctest::
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>>> @attr.s
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... class C(object):
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... @attr.s(repr_ns="C")
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... class D(object):
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... pass
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>>> C.D()
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C.D()
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``repr_ns`` works on both Python 2 and 3.
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On Python 3 is overrides the implicit detection.
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Converting to Dictionaries
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--------------------------
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When you have a class with data, it often is very convenient to transform that class into a :class:`dict` (for example if you want to serialize it to JSON):
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.. doctest::
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>>> attr.asdict(Coordinates(x=1, y=2))
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{'y': 2, 'x': 1}
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Some fields cannot or should not be transformed.
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For that, :func:`attr.asdict` offers a callback that decides whether an attribute should be included:
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.. doctest::
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>>> @attr.s
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... class UserList(object):
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... users = attr.ib()
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>>> @attr.s
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... class User(object):
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... email = attr.ib()
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... password = attr.ib()
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>>> attr.asdict(UserList([User("jane@doe.invalid", "s33kred"),
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... User("joe@doe.invalid", "p4ssw0rd")]),
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... filter=lambda attr, value: attr.name != "password")
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{'users': [{'email': 'jane@doe.invalid'}, {'email': 'joe@doe.invalid'}]}
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For the common case where you want to :func:`include <attr.filters.include>` or :func:`exclude <attr.filters.exclude>` certain types or attributes, ``attrs`` ships with a few helpers:
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.. doctest::
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>>> @attr.s
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... class User(object):
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... login = attr.ib()
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... password = attr.ib()
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... id = attr.ib()
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>>> attr.asdict(User("jane", "s33kred", 42), filter=attr.filters.exclude(User.password, int))
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{'login': 'jane'}
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>>> @attr.s
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... class C(object):
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... x = attr.ib()
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... y = attr.ib()
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... z = attr.ib()
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>>> attr.asdict(C("foo", "2", 3), filter=attr.filters.include(int, C.x))
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{'x': 'foo', 'z': 3}
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Defaults
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--------
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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=attr.Factory(collections.deque))
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... debug = attr.ib(default=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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Validators
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----------
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Although your initializers should be as dumb as possible, it can come handy to do some kind of validation on the arguments.
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That's when :func:`attr.ib`\ ’s ``validator`` argument comes into play.
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A validator is simply a callable that takes three arguments:
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#. The *instance* that's being validated.
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#. The *attribute* that it's validating
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#. and finally the *value* that is passed for it.
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If the value does not pass the validator's standards, it just raises an appropriate exception.
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Since the validator runs *after* the instance is initialized, you can refer to other attributes while validating :
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.. doctest::
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>>> def x_smaller_than_y(instance, attribute, value):
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... if value >= instance.y:
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... raise ValueError("'x' has to be smaller than 'y'!")
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>>> @attr.s
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... class C(object):
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... x = attr.ib(validator=x_smaller_than_y)
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... y = attr.ib()
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>>> C(x=3, y=4)
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C(x=3, y=4)
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>>> C(x=4, y=3)
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Traceback (most recent call last):
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...
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ValueError: 'x' has to be smaller than 'y'!
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``attrs`` won't intercept your changes to those attributes but you can always call :func:`attr.validate` on any instance to verify, that it's still valid:
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.. doctest::
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>>> i = C(4, 5)
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>>> i.x = 5 # works, no magic here
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>>> attr.validate(i)
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Traceback (most recent call last):
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...
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ValueError: 'x' has to be smaller than 'y'!
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``attrs`` ships with a bunch of validators, make sure to :ref:`check them out <api_validators>` before writing your own:
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.. doctest::
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>>> @attr.s
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... class C(object):
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... x = attr.ib(validator=attr.validators.instance_of(int))
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>>> C(42)
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C(x=42)
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>>> C("42")
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Traceback (most recent call last):
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...
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TypeError: ("'x' must be <type 'int'> (got '42' that is a <type 'str'>).", Attribute(name='x', default=NOTHING, factory=NOTHING, validator=<instance_of validator for type <type 'int'>>), <type 'int'>, '42')
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If you like `zope.interface <http://docs.zope.org/zope.interface/api.html#zope-interface-interface-specification>`_, ``attrs`` also comes with a :func:`attr.validators.provides` validator:
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.. doctest::
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>>> import zope.interface
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>>> class IFoo(zope.interface.Interface):
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... def f():
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... """A function called f."""
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>>> @attr.s
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... class C(object):
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... x = attr.ib(validator=attr.validators.provides(IFoo))
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>>> C(x=object())
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Traceback (most recent call last):
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...
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TypeError: ("'x' must provide <InterfaceClass __builtin__.IFoo> which <object object at 0x10bafaaf0> doesn't.", Attribute(name='x', default=NOTHING, factory=NOTHING, validator=<provides validator for interface <InterfaceClass __builtin__.IFoo>>), <InterfaceClass __builtin__.IFoo>, <object object at 0x10bafaaf0>)
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>>> @zope.interface.implementer(IFoo)
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... @attr.s
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... class Foo(object):
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... def f(self):
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... print("hello, world")
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>>> C(Foo())
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C(x=Foo())
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You can also disable them globally:
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>>> attr.set_run_validators(False)
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>>> C(42)
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C(x=42)
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>>> attr.set_run_validators(True)
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>>> C(42)
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Traceback (most recent call last):
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...
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TypeError: ("'x' must provide <InterfaceClass __builtin__.IFoo> which 42 doesn't.", Attribute(name='x', default=NOTHING, validator=<provides validator for interface <InterfaceClass __builtin__.IFoo>>, repr=True, cmp=True, hash=True, init=True), <InterfaceClass __builtin__.IFoo>, 42)
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Conversion
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----------
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Attributes can have a ``convert`` function specified, which will be called with the attribute's passed-in value to get a new value to use.
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This can be useful for doing type-conversions on values that you don't want to force your callers to do.
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.. doctest::
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>>> @attr.s
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... class C(object):
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... x = attr.ib(convert=int)
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>>> o = C("1")
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>>> o.x
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1
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Converters are run *before* validators, so you can use validators to check the final form of the value.
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.. doctest::
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>>> def validate_x(instance, attribute, value):
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... if value < 0:
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... raise ValueError("x must be be at least 0.")
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>>> @attr.s
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... class C(object):
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... x = attr.ib(convert=int, validator=validate_x)
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>>> o = C("0")
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>>> o.x
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0
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>>> C("-1")
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Traceback (most recent call last):
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...
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ValueError: x must be be at least 0.
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Other Goodies
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-------------
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Do you like Rich Hickey?
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I'm glad to report that Clojure's core feature is part of ``attrs``: `assoc <https://clojuredocs.org/clojure.core/assoc>`_!
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I guess that means Clojure can be shut down now, sorry Rich!
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.. doctest::
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>>> @attr.s
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... class C(object):
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... x = attr.ib()
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... y = attr.ib()
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>>> i1 = C(1, 2)
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>>> i1
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C(x=1, y=2)
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>>> i2 = attr.assoc(i1, y=3)
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>>> i2
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C(x=1, y=3)
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>>> i1 == i2
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False
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Sometimes you may want to create a class programmatically.
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``attrs`` won't let you down:
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.. doctest::
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>>> @attr.s
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... class C1(object):
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... x = attr.ib()
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... y = attr.ib()
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>>> C2 = attr.make_class("C2", ["x", "y"])
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>>> attr.fields(C1) == attr.fields(C2)
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True
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You can still have power over the attributes if you pass a dictionary of name: ``attr.ib`` mappings and can pass arguments to ``@attr.s``:
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.. doctest::
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>>> C = attr.make_class("C", {"x": attr.ib(default=42),
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... "y": attr.ib(default=attr.Factory(list))},
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... repr=False)
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>>> i = C()
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>>> i # no repr added!
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<attr._make.C object at ...>
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>>> i.x
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42
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>>> i.y
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[]
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Finally, you can exclude single attributes from certain methods:
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.. doctest::
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>>> @attr.s
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... class C(object):
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... user = attr.ib()
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... password = attr.ib(repr=False)
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>>> C("me", "s3kr3t")
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C(user='me')
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