Date post: | 03-Sep-2014 |
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Upload: | dreampuf |
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Python Magic Methodsthe method of surrounded by double underscores
April 2013
dreampufhttp://huangx.in
Agenda
• What is magic ?
• How ?
• Why ?
What is magic ?http://www.flickr.com/photos/cayusa/2962437091/sizes/l/in/photostream/
class Parser(object): def __init__(self, mesg_class, cfg, source): self.mesg_class = mesg_class self.cfg = cfg if hasattr(source, "recv"): self.unreader = SocketUnreader(source) else: self.unreader = IterUnreader(source) self.mesg = None
# request counter (for keepalive connetions) self.req_count = 0
def __iter__(self): return self
def __next__(self): # Stop if HTTP dictates a stop. if self.mesg and self.mesg.should_close(): raise StopIteration()
# Discard any unread body of the previous message if self.mesg: data = self.mesg.body.read(8192) while data: data = self.mesg.body.read(8192)
# Parse the next request self.req_count += 1 self.mesg = self.mesg_class(self.cfg, self.unreader, self.req_count) if not self.mesg: raise StopIteration() return self.mesg
next = __next__
class WSGIApplication(web.Application): def __call__(self, environ, start_response): handler = web.Application.__call__(self, HTTPRequest(environ)) assert handler._finished reason = handler._reason status = str(handler._status_code) + " " + reason headers = list(handler._headers.get_all()) if hasattr(handler, "_new_cookie"): for cookie in handler._new_cookie.values(): headers.append(("Set-Cookie", cookie.OutputString(None))) start_response(status, [(native_str(k), native_str(v)) for (k, v) in headers]) return handler._write_buffer
class WSGIHandler(base.BaseHandler): initLock = Lock() request_class = WSGIRequest
def __call__(self, environ, start_response): # Set up middleware if needed. We couldn't do this earlier, because # settings weren't available. if self._request_middleware is None: with self.initLock: try: # Check that middleware is still uninitialised. if self._request_middleware is None: self.load_middleware() except: # Unload whatever middleware we got self._request_middleware = None raise
set_script_prefix(base.get_script_name(environ)) signals.request_started.send(sender=self.__class__) try: request = self.request_class(environ) except UnicodeDecodeError: logger.warning('Bad Request (UnicodeDecodeError)', exc_info=sys.exc_info(), extra={ 'status_code': 400, } ) response = http.HttpResponseBadRequest() else: response = self.get_response(request)
class WSGIHandler(base.BaseHandler): initLock = Lock() request_class = WSGIRequest
def __call__(self, environ, start_response): # Set up middleware if needed. We couldn't do this earlier, because # settings weren't available. if self._request_middleware is None: with self.initLock: try: # Check that middleware is still uninitialised. if self._request_middleware is None: self.load_middleware() except: # Unload whatever middleware we got self._request_middleware = None raise
set_script_prefix(base.get_script_name(environ)) signals.request_started.send(sender=self.__class__) try: request = self.request_class(environ) except UnicodeDecodeError: logger.warning('Bad Request (UnicodeDecodeError)', exc_info=sys.exc_info(), extra={ 'status_code': 400, } ) response = http.HttpResponseBadRequest() else: response = self.get_response(request)
def handle_request(self, listener, req, client, addr): environ = {} resp = None self.cfg.pre_request(self, req) resp, environ = wsgi.create(req, client, addr, listener.getsockname(), self.cfg) respiter = self.wsgi(environ, resp.start_response) try: if isinstance(respiter, environ['wsgi.file_wrapper']): resp.write_file(respiter) else: for item in respiter: resp.write(item) resp.close() finally: if hasattr(respiter, "close"): respiter.close()
class StreamReader(Codec): #... def __iter__(self): return self
def __getattr__(self, name, getattr=getattr): return getattr(self.stream, name)
def __enter__(self): return self
def __exit__(self, type, value, tb): self.stream.close()
stdlib: codecs.py
Character
def __NAME__( *args, **kw):
How ?
• __new__(cls, [...)
• __init__(self, [...)
• __del__(self)
Construction & Initialization
from os.path import join
class FileObject(object): '''Wrapper for file objects to make sure the file gets closed on deletion.'''
def __init__(self, filepath='~', filename='sample.txt'): # open a file filename in filepath in read and write mode self.file = open(join(filepath, filename), 'r+') # super(FileObject, self).__init__(*)
def __del__(self): self.file.close() del self.file
class MinervaOpenAPI(API): HOST = OPENAPI_HOST
@GET("tags/%(tag)s") def get_tag(tag): pass
@GET("tags/%(tag)s/feeds/filter:%(filter)s") def get_tag_feeds(tag, filter=str): pass
@GET("tags/%(tag)s/feeds/count/filter:%(filter)s") def get_tag_feeds_count(tag, filter=str): pass
@POST("tags/synonym_list") def get_synonym_list(taglist=str): pass
api = MinervaOpenAPI()api.get_tag("NBA")
class APIMeta(type): def __new__(cls, clsname, bases, dct): for name, method in dct.iteritems(): if not isinstance(method, types.FunctionType) or not hasattr(method, "url"): continue args, varags, varkw, defaults = inspect.getargspec(method) kw_len = 0 if defaults is None else len(defaults) required = args[:len(args)-kw_len] optional_type = dict(zip(args[kw_len:], defaults)) if kw_len > 0 else {}
dct[name] = APIMethod(name=method.func_name, url=HOST + method.url, method=method.method, dsc=method.__doc__, required=required, optional_type=optional_type)
return super(APIMeta, cls).__new__(cls, clsname, bases, dct)
class API(object): HOST = "http://localhost:8080/" __metaclass__ = APIMeta
Operators with Custom Classes
if instance.equals(other_instance): # do something
if instance == other_instance: #do something
__cmp__(self, other) cmp(a, b)
__eq__(self, other) a == b
__ne__(self, other) a != b
__lt__(self, other) a < b
__gt__(self, other) a > b
__le__(self, other) a <= b
__ge__(self, other) a >= b
Comparison operators
class Word(str): '''Class for words, defining comparison based on word length.'''
def __new__(cls, word): # Note that we have to use __new__. This is because str is an immutable # type, so we have to initialize it early (at creation) if ' ' in word: print "Value contains spaces. Truncating to first space." word = word[:word.index(' ')] # Word is now all chars before first space return str.__new__(cls, word)
def __gt__(self, other): return len(self) > len(other) def __lt__(self, other): return len(self) < len(other) def __ge__(self, other): return len(self) >= len(other) def __le__(self, other): return len(self) <= len(other) def __eq__(self, other): return super(str, self).__eq__(other)
@total_orderingclass Word(str): '''Class for words, defining comparison based on word length.'''
def __new__(cls, word): # Note that we have to use __new__. This is because str is an immutable # type, so we have to initialize it early (at creation) if ' ' in word: print "Value contains spaces. Truncating to first space." word = word[:word.index(' ')] # Word is now all chars before first space return str.__new__(cls, word)
def __le__(self, other): return len(self) <= len(other) def __eq__(self, other): return super(str, self).__eq__(other)
http://docs.python.org/2/library/functools.html#functools.total_ordering
__pos__(self) +v
__neg__(self) -v
__abs__(self) abs(v)
__invert__(self) ~v
__round__(self, n) round(v)
__floor__(self) math.floor(v)
__ceil__(self) math.ceil(v)
__trunc__(self) math.trunc(v)
Unary operators
__add__(self, other) a+b
__sub__(self, other) a-b
__mul__(self, other) a*b
__floordiv__(self, other) a // b
__div__(self, other) a / b
__truediv__(self, other) a / b (from __furture__ import division)
__mod__(self, other) a % b
__divmod__(self, other) divmod(a, b)
__pow__ a ** b
__lshift__(self, other) a << b
__rshift__(self, other) a >> b
__and__(self, other) a & b
__or__(self, other) a | b
__xor__(self, other) a ^ b
Arithmetic operators
__radd__(self, other) b+a
__rsub__(self, other) b-a
__rmul__(self, other) b*a
__rfloordiv__(self, other) b // a
__rdiv__(self, other) b / a
__rtruediv__(self, other) b / a (from __furture__ import division)
__rmod__(self, other) b % a
__rdivmod__(self, other) divmod(b, a)
__rpow__ b ** a
__rlshift__(self, other) b << a
__rrshift__(self, other) b << a
__rand__(self, other) b & a
__ror__(self, other) b | a
__rxor__(self, other) b ^ a
Reflected arithmetic operators
__iadd__(self, other) a+=b
__isub__(self, other) a-=b
__imul__(self, other) a*=b
__ifloordiv__(self, other) a //= b
__idiv__(self, other) a /= b
__itruediv__(self, other) a/=b (from __furture__ import division)
__imod_(self, other) a %= b
divmod(a, b)
__ipow__ a **= b
__ilshift__(self, other) a <<= b
__irshift__(self, other) a >>= b
__iand__(self, other) a &= b
__ior__(self, other) a |= b
__ixor__(self, other) a ^= b
Assignment operators
sum(select(where(take_while(fib(), lambda x: x < 1000000) lambda x: x % 2), lambda x: x * x))
fib() | take_while(lambda x: x < 1000000) \ | where(lambda x: x % 2) \ | select(lambda x: x * x) \ | sum()
Pipe Module
https://github.com/JulienPalard/Pipe
__int__(self) int(v)
__long__(self) long(v)
__float__(self) float(v)
__complex__(self) complex(v)
__oct__(self) oct(v)
__hex__(self) hex(v)
__index__(self) slice(v)
__trunc__(self) math.trunc(v)
__coerce__(self, other) coerce(3, 3.0)
Type conversion
In [55]: slice(5)Out[55]: slice(None, 5, None)
In [56]: range(10)[slice(5)]Out[56]: [0, 1, 2, 3, 4]
In [57]: (5).__index__Out[57]: <method-wrapper '__index__' of int object at 0x7fcd2ac0ffd8>
http://www.python.org/dev/peps/pep-0357/
__str__(self) str(v)
__repr__(self) repr(v)
__unicode__(self) unicode(v)
__format__(self, formatstr) v.format(“123”)
__hash__(self) hash(v)
__nonzero__(self) bool(v)
__dir__(self) div(v)
__sizeof__(self) sys.getsizeof(v)
Representing the Classes
__getattr__(self, name) v.name
__setattr__(self, name, value) v.name = value
__delattr__(self, name) del v.name
__getattribute__(self, name) v.name
Controlling Attribute Access
http://docs.python.org/2/reference/datamodel.html#object.__getattribute__
Controlling Attribute Access
def __setattr__(self, name, value): self.name = value # since every time an attribute is assigned, __setattr__() is called, this # is recursion. # so this really means self.__setattr__('name', value). Since the method # keeps calling itself, the recursion goes on forever causing a crash
def __setattr__(self, name, value): self.__dict__[name] = value # assigning to the dict of names in the class # define custom behavior here
http://docs.python.org/2/reference/datamodel.html#object.__getattribute__
Controlling Attribute Access
class AccessCounter(object): '''A class that contains a value and implements an access counter. The counter increments each time the value is changed.'''
def __init__(self, val): super(AccessCounter, self).__setattr__('counter', 0) super(AccessCounter, self).__setattr__('value', val)
def __setattr__(self, name, value): if name == 'value': super(AccessCounter, self).__setattr__('counter', self.counter + 1) # Make this unconditional. # If you want to prevent other attributes to be set, raise AttributeError(name) super(AccessCounter, self).__setattr__(name, value)
def __delattr__(self, name): if name == 'value': super(AccessCounter, self).__setattr__('counter', self.counter + 1) super(AccessCounter, self).__delattr__(name)]
sh Modulefrom sh import ifconfig
print(ifconfig("wlan0"))"""wlan0 Link encap:Ethernet HWaddr 00:00:00:00:00:00 inet addr:192.168.1.100 Bcast:192.168.1.255 Mask:255.255.255.0 inet6 addr: ffff::ffff:ffff:ffff:fff/64 Scope:Link UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1 RX packets:0 errors:0 dropped:0 overruns:0 frame:0 TX packets:0 errors:0 dropped:0 overruns:0 carrier:0 collisions:0 txqueuelen:1000 RX bytes:0 (0 GB) TX bytes:0 (0 GB)"""
from sh import git, ls, wcgit.checkout("master")print(ls("-l"))longest_line = wc(__file__, "-L")
from sh import tail# runs foreverfor line in tail("-f", "/var/log/some_log_file.log", _iter=True): print(line)
http://amoffat.github.io/sh/
class SelfWrapper(ModuleType): def __init__(self, self_module, baked_args={}): self.self_module = self_module def __getattr__(self, name): return Command._create(name, **self.baked_args)
def __call__(self, **kwargs): return SelfWrapper(self.self_module, kwargs)
self = sys.modules[__name__]sys.modules[__name__] = SelfWrapper(self)
sh Module
Sequences
__len__(self) len(v)
__getitem__(self, key) v[key], v[start:stop:step]
__setitem__(self, key, value) v[key] = value
__delitem__(self, key) del v[key]
__iter__(self) iter(v)
__reversed__(self) reversed(v)
__contains__(self, item) item in v , item not in v
__missing__(self, key)
Sequences
ls = FunctionalList()ls.append(1)ls.append(2)ls.append(3)
print ls[0]print len(ls)print 5 in lsls[0] = 4reversed(ls)
Sequencesclass FunctionalList(object): '''A class wrapping a list with some extra functional magic, like head, tail, init, last, drop, and take.'''
def __init__(self, values=None): if values is None: self.values = [] else: self.values = values
def __len__(self): return len(self.values)
def __getitem__(self, key): # if key is of invalid type or value, the list values will raise the error return self.values[key]
def __setitem__(self, key, value): self.values[key] = value
def __delitem__(self, key): del self.values[key]
def __iter__(self): return iter(self.values)
def __reversed__(self): return FunctionalList(reversed(self.values))
def append(self, value): self.values.append(value) # get last element return self.values[-1]
class Query(object): def slice(self, start, stop): """apply LIMIT/OFFSET to the ``Query`` based on a " "range and return the newly resulting ``Query``.""" pass
def __getitem__(self, item): if isinstance(item, slice): start, stop, step = util.decode_slice(item)
if (isinstance(start, int) and start < 0) \ or (isinstance(stop, int) and stop < 0): return list(self)[item]
res = self.slice(start, stop) return list(res)[None:None:item.step] else: if item == -1: return list(self)[-1] else: return list(self[item:item + 1])[0]
def __iter__(self): context = self._compile_context() context.statement.use_labels = True if self._autoflush and not self._populate_existing: self.session._autoflush() return self._execute_and_instances(context)
Collections Abstract Base ClassesABC Inherits from Abstract Methods Mixin Methods
Container __contains__
Hashable __hash__
Iterable __iter__
Iterator Iterable next __iter__
Sized __len__
Callable __call__
Sequence Sized, Iterable, Container __getitem__, __len__ __contains__, __iter__, __reversed__, index, and
countMutableSequence Sequence __getitem__, __setitem__,__delitem__,
__len__,insert
Inherited Sequence methods and append, reverse, extend, pop,remove, and __iadd__Set Sized, Iterable, Container __contains__, __iter__,__len__ __le__, __lt__, __eq__,
__ne__, __gt__, __ge__, __and__, __or__,__sub__,
__xor__, and isdisjoint
MutableSet Set __contains__, __iter__,__len__, add,
discard
Inherited Set methods and clear, pop, remove, __ior__,
__iand__,__ixor__, and __isub__Mapping Sized, Iterable, Container __getitem__, __iter__,__len__
__contains__, keys, items, values, get, __eq__, and
__ne__MutableMapping Mapping __getitem__, __setitem__,__delitem__,
__iter__,__len__
Inherited Mapping methods and pop, popitem, clear, update,
andsetdefaultMappingView Sized __len__
ItemsView MappingView, Set __contains__, __iter__
KeysView MappingView, Set __contains__, __iter__
ValuesView MappingView __contains__, __iter__
class ListBasedSet(collections.Set): def __init__(self, iterable): self.elements = lst = [] for value in iterable: if value not in lst: lst.append(value) def __iter__(self): return iter(self.elements) def __contains__(self, value): return value in self.elements def __len__(self): return len(self.elements)
s1 = ListBasedSet('abcdef')s2 = ListBasedSet('defghi')overlap = s1 & s2
Callable objects
• __call__(self, [args...])
Callable objects
• __call__(self, [args...])
class Entity(object): '''Class to represent an entity. Callable to update the entity's position.'''
def __init__(self, size, x, y): self.x, self.y = x, y self.size = size
def __call__(self, x, y): '''Change the position of the entity.''' self.x, self.y = x, y
# snip...
Callable objects
• __call__(self, [args...])
In [8]: entity = Entity(5, 0, 0)
In [9]: entity(1, 1)
In [10]: entity.x, entity.yOut[10]: (1, 1)
Context Managers
• __enter__(self)
• __exit__(self, exception_type, exception_value, traceback)
with open('foo.txt') as bar: # perform some action with bar
Context Managers
class Closer(object): '''A context manager to automatically close an object with a close method in a with statement.'''
def __init__(self, obj): self.obj = obj
def __enter__(self): return self.obj # bound to target
def __exit__(self, exception_type, exception_val, trace): try: self.obj.close() except AttributeError: # obj isn't closable print 'Not closable.' return True # exception handled successfully
from flask import g, request, session
@app.route("/verify/<email>/<verify_code>", methods=["GET"])def verify(email, verify_code=None): users = g.db.get("user", {}) user = users.get(email)
today = date.today()
for i in xrange(config.VERIFY_AVAILABILITY_DAY): t = today - timedelta(days=i) ciphertext = sha1("{0}{1}{2}".format(config.SALT, email, t.toordinal())).hexdigest() if ciphertext == verify_code: user["verified"] = True users[email] = user g.db["user"] = users g.db.sync() session["user"] = email break return render_template("success.html", user=user)
# "globlas" modulefrom functools import partialfrom werkzeug.local import LocalStack, LocalProxy
def _lookup_object(name): top = _request_ctx_stack.top if top is None: raise RuntimeError('working outside of request context') return getattr(top, name)
_request_ctx_stack = LocalStack()_app_ctx_stack = LocalStack()request = LocalProxy(partial(_lookup_object, 'request'))session = LocalProxy(partial(_lookup_object, 'session'))g = LocalProxy(partial(_lookup_object, 'g'))
# "app" moduledef wsgi_app(self, environ, start_response): with self.request_context(environ): try: response = self.full_dispatch_request() except Exception, e: response = self.make_response(self.handle_exception(e)) return response(environ, start_response)
class RequestContext(object): def __enter__(self): top = _request_ctx_stack.top app_ctx = _app_ctx_stack.top if app_ctx is None or app_ctx.app != self.app: app_ctx = self.app.app_context() app_ctx.push() self._implicit_app_ctx_stack.append(app_ctx) else: self._implicit_app_ctx_stack.append(None)
_request_ctx_stack.push(self) self.session = self.app.open_session(self.request) if self.session is None: self.session = self.app.make_null_session()
def __exit__(self, exc_type=None, exc=None, tb=None): app_ctx = self._implicit_app_ctx_stack.pop()
if not self._implicit_app_ctx_stack: if exc is None: exc = sys.exc_info()[1] self.app.do_teardown_request(exc)
def test_basic_url_generation(self): app = flask.Flask(__name__) app.config['SERVER_NAME'] = 'localhost' app.config['PREFERRED_URL_SCHEME'] = 'https'
@app.route('/') def index(): pass
with app.app_context(): rv = flask.url_for('index') self.assert_equal(rv, 'https://localhost/')
Descriptor Objects
• __get__(self, instance, owner)
• __set__(self, instance, value)
• __delete__(self, instance)
Descriptor Objects
In [19]: a = Distance()
In [20]: a.foot = 5
In [21]: a.meterOut[21]: 1.5240185320653499
class Meter(object): '''Descriptor for a meter.'''
def __init__(self, value=0.0): self.value = float(value) def __get__(self, instance, owner): return self.value def __set__(self, instance, value): self.value = float(value)
class Foot(object): '''Descriptor for a foot.'''
def __get__(self, instance, owner): return instance.meter * 3.2808 def __set__(self, instance, value): instance.meter = float(value) / 3.2808
class Distance(object): '''Class to represent distance holding two descriptors for feet and meters.''' meter = Meter() foot = Foot()
Copying
__copy__(self) copy.copy(v)
__deepcopy__(self, memodict={}) copy.deepcopy(v)
http://www.peterbe.com/plog/must__deepcopy__
>>> b = copy.deepcopy(a)Traceback (most recent call last): File "<stdin>", line 1, in <module> File "/usr/local/Cellar/python/2.7.2/lib/python2.7/copy.py", line 172, in deepcopy copier = getattr(x, "__deepcopy__", None) File "<stdin>", line 3, in __getattr__KeyError: '__deepcopy__'
try: from collections import defaultdictexcept ImportError: class defaultdict(dict):
def __init__(self, default_factory=None, *a, **kw): if (default_factory is not None and not hasattr(default_factory, '__call__')): raise TypeError('first argument must be callable') dict.__init__(self, *a, **kw) self.default_factory = default_factory
def __getitem__(self, key): try: return dict.__getitem__(self, key) except KeyError: return self.__missing__(key)
def __copy__(self): return type(self)(self.default_factory, self)
def __deepcopy__(self, memo): import copy return type(self)(self.default_factory, copy.deepcopy(self.items()))
http://www.peterbe.com/plog/must__deepcopy__
Pickling
__getinitargs__(self)
__getnewargs__(self)
__getstate__(self) pickle.dump(pkl_file, self)
__setstate__(self, state) data = pickle.load(pkl_file)
__reduce__(self)
__reduce_ex__(self)
Picklingimport time
class Slate: '''Class to store a string and a changelog, and forget its value when pickled.'''
def __init__(self, value): self.value = value self.last_change = time.asctime() self.history = {}
def change(self, new_value): # Change the value. Commit last value to history self.history[self.last_change] = self.value self.value = new_value self.last_change = time.asctime()
def print_changes(self): print 'Changelog for Slate object:' for k, v in self.history.items(): print '%s\t %s' % (k, v)
def __getstate__(self): # Deliberately do not return self.value or self.last_change. # We want to have a "blank slate" when we unpickle. return self.history
def __setstate__(self, state): # Make self.history = state and last_change and value undefined self.history = state self.value, self.last_change = None, None
PicklingIn [25]: s = Slate(123)In [26]: s.change(4)In [27]: s.change(5)In [28]: s.change(6)
In [29]: s.print_changes()Changelog for Slate object:Tue Apr 16 22:03:02 2013 5Tue Apr 16 22:03:01 2013 4Tue Apr 16 22:02:50 2013 123
In [30]: pickled_s = pickle.dumps(s)
In [31]: ns = pickle.loads(pickled_s)
In [32]: ns.print_changes()Changelog for Slate object:Tue Apr 16 22:03:02 2013 5Tue Apr 16 22:03:01 2013 4Tue Apr 16 22:02:50 2013 123
In [33]: s.value, ns.valueOut[33]: (6, None)
Why Pickling need reducepik> (I1...> S'abc'...> F2.0...> t.(1, 'abc', 2.0)
pik> (I1...> (I2...> I3...> tt.(1, (2, 3))
pik> (I0...> I1...> I2...> l.[0, 1, 2]pik> (S'red'...> I00...> S'blue'...> I01...> d.{'blue': True, 'red': False}
pik> c__builtin__...> set...> ((S'a'...> S'a'...> S'b'...> ltR.set(['a', 'b'])
http://aisk.me/blog/2013/02/01/170/
Conclusion• Feature-Rich
• Clear, Simple.. (Zen of python)
• Interface
Why to use?Beautiful is better than ugly.Explicit is better than implicit.Simple is better than complex.Complex is better than complicated.Flat is better than nested.Sparse is better than dense.Readability counts.Special cases aren't special enough to break the rules.Although practicality beats purity.Errors should never pass silently.Unless explicitly silenced.In the face of ambiguity, refuse the temptation to guess.
Why not ?
• http://docs.python.org/2/reference/datamodel.html
• A Guide to Python's Magic Methods - Rafe Kettler http://www.rafekettler.com/magicmethods.html
• Pickle: An interesting stack language - Alexandrehttp://peadrop.com/blog/2007/06/18/pickle-an-interesting-stack-language/
• Flask, Django, Tornado, sh, Pipe, Gunicorn, Sqlalchemy
Reference & Thanks
Q & A
Thank you !