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 !