Python magicmethods

Python Magic Methods
the method of surrounded by double underscores

April 2013

dreampuf
http://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)

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()

stdlib: codecs.py

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()

Character

def __NAME__( *args, **kw):

Construction & Initialization

• __new__(cls, [...)
• __init__(self, [...)
• __del__(self)

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

if instance.equals(other_instance):
# do something

if instance == other_instance:
#do something

Comparison operators
__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

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_ordering
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 __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

Unary operators
__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)

Arithmetic 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

Reflected 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) bâ

Assignment 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

Pipe Module

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()

https://github.com/JulienPalard/Pipe

Type conversion
__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)

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/

Representing the Classes
__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)

Controlling Attribute Access

__getattr__(self, name) v.name

__setattr__(self, name, value) v.name = value

__delattr__(self, name) del v.name

__getattribute__(self, name) v.name

http://docs.python.org/2/reference/datamodel.html#object.__getattribute__


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

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__

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)

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)]

from sh import ifconfig
sh Module
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, wc
git.checkout("master")
print(ls("-l"))
longest_line = wc(__file__, "-L")

from sh import tail
# runs forever
for line in tail("-f", "/var/log/some_log_file.log", _iter=True):
print(line)

http://amoffat.github.io/sh/

sh Module

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)

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 ls
ls[0] = 4
reversed(ls)

Sequences
class 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 Classes
ABC 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
MutableSequence Sequence __getitem__, Inherited Sequence methods
count
__setitem__,__delitem__, and append, reverse, extend,
Set Sized, Iterable, Container __contains__, __iter__,__len__ __le__, __lt__,__iadd__
__len__,insert pop,remove, and
__eq__,
__ne__, __gt__, __ge__,
MutableSet Set __contains__, Inherited Set methods and
__and__, __or__,__sub__,
__iter__,__len__, add, clear, pop,
and , __ior__,
__xor__, removeisdisjoint
Mapping Sized, Iterable, Container __getitem__,
discard __contains__, keys, items,
__iand__,__ixor__, and __isub__
__iter__,__len__ values, get, __eq__, and
MutableMapping Mapping __getitem__, Inherited __ne__ methods
Mapping
__setitem__,__delitem__, and pop, popitem, clear, update,
MappingView Sized __iter__,__len__ __len__
andsetdefault
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


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


In [8]: entity = Entity(5, 0, 0)

In [9]: entity(1, 1)

In [10]: entity.x, entity.y
Out[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

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" module
from functools import partial
from 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" module
def 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):
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.meter
Out[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 defaultdict
except 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)

Pickling
import 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 '%st %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

Pickling
In [25]: s = Slate(123)
In [26]: s.change(4)

In [29]: s.print_changes()
Changelog for Slate object:
Tue Apr 16 22:03:02 2013 5
Tue Apr 16 22:03:01 2013 4
Tue 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 5
Tue Apr 16 22:03:01 2013 4
Tue Apr 16 22:02:50 2013 123

In [33]: s.value, ns.value
Out[33]: (6, None)

Why Pickling need reduce
pik> (I1 pik> (I0
...> S'abc' ...> I1
...> F2.0 ...> I2
...> t. ...> l.
(1, 'abc', 2.0) [0, 1, 2]
pik> (S'red'
...> I00
...> S'blue'
pik> (I1 ...> I01
...> (I2 ...> d.
...> I3 {'blue': True, 'red': False}
...> tt.
(1, (2, 3))

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.

Reference & Thanks
• 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 - Alexandre
http://peadrop.com/blog/2007/06/18/pickle-an-interesting-
stack-language/

• Flask, Django, Tornado, sh, Pipe, Gunicorn, Sqlalchemy

Python magicmethods

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