I seem to be writing the following boilerplate/pattern quite
frequently to avoid hitting the database until absolutely necessary,
and to only do it at most once:
class Foo(object):
@property
def expensive(self) :
if not hasattr(self, '_expensiv'):
self._expensive = <insert expensive db call here>
return self._expensive
it's a bit verbose, and it violates the DRY (Don't Repeat Yourself)
principle -- and it has on at least one occasion resulted in really
slow code that produces the correct results (did you spot the typo
above?).
It would have been nice to be able to write
class Foo(object):
@property
def expensive(self) :
self.expensive = <insert expensive db call here>
return self.expensive
but apparently I "can't set [that] attribute" :-(
I'm contemplating using a decorator to hide the first pattern:
def memprop(fn):
def _fn(self): # properties only take self
memname = '_' + fn.__name__
if not hasattr(self, memname):
setattr(self, memname, fn(self))
return getattr(self, memname)
return property(fget=_ fn, doc=fn.__doc__)
which means I can very simply write
class Foo(object):
@memprop
def expensive(self) :
return <insert expensive db call here>
I'm a bit hesitant to start planting home-grown memprop-s all over the
code-base though, so I'm wondering... does this seem like a reasonable
thing to do? Am I re-inventing the wheel? Is there a better way to
approach this problem?
-- bjorn 4 3025
thebjorn <Bj************ ************@gm ail.comwrote:
It would have been nice to be able to write
class Foo(object):
@property
def expensive(self) :
self.expensive = <insert expensive db call here>
return self.expensive
but apparently I "can't set [that] attribute" :-(
You can set and access it directly in __dict__. How about something along
these lines:
def cachedproperty( f):
name = f.__name__
def getter(self):
try:
return self.__dict__[name]
except KeyError:
res = self.__dict__[name] = f(self)
return res
return property(getter )
class Foo(object):
@cachedproperty
def expensive(self) :
print "expensive called"
return 42
>>f = Foo() f.expensive
expensive called
42
>>f.expensive
42
It is still calling the getter every time though, so not as fast as a plain
attribute lookup.
On Dec 20, 5:02 pm, thebjorn <BjornSteinarFj eldPetter...@gm ail.com>
wrote:
I seem to be writing the following boilerplate/pattern quite
frequently to avoid hitting the database until absolutely necessary ...
I use the following module:
$ cat cache.py
class cached(property ):
'Convert a method into a cached attribute'
def __init__(self, method):
private = '_' + method.__name__
def fget(s):
try:
return getattr(s, private)
except AttributeError:
value = method(s)
setattr(s, private, value)
return value
def fdel(s):
del s.__dict__[private]
super(cached, self).__init__( fget, fdel=fdel)
@staticmethod
def reset(self):
cls = self.__class__
for name in dir(cls):
attr = getattr(cls, name)
if isinstance(attr , cached):
delattr(self, name)
if __name__ == '__main__': # a simple test
import itertools
counter = itertools.count ()
class Test(object):
@cached
def foo(self):
return counter.next()
reset = cached.reset
p = Test()
print p.foo
print p.foo
p.reset()
print p.foo
print p.foo
p.reset()
print p.foo
Michele Simionato
On Dec 20, 5:43 pm, Michele Simionato <michele.simion ...@gmail.com>
wrote:
On Dec 20, 5:02 pm, thebjorn <BjornSteinarFj eldPetter...@gm ail.com>
wrote:
I seem to be writing the following boilerplate/pattern quite
frequently to avoid hitting the database until absolutely necessary ...
I use the following module:
[...]
I love it! much better name too ;-)
I changed your testcase to include a second cached property (the
naming is in honor of my late professor in Optimization of Functional
Languages class: "...any implementation that calls bomb_moscow is per
definition wrong, even if the program produces the correct result..."
-- it was a while ago ;-)
if __name__ == '__main__': # a simple test
import itertools
counter = itertools.count ()
class Test(object):
@cached
def foo(self):
return counter.next()
@cached
def bomb_moscow(sel f):
print 'fire missiles'
return counter.next()
reset = cached.reset
it didn't start WWIII, but I had to protect attribute deletion to get
it to run:
def fdel(s):
if private in s.__dict__:
del s.__dict__[private]
I'm a bit ambivalent about the reset functionality. While it's a
wonderful demonstration of a staticmethod, the very few times I've
felt the need to "freshen-up" the object, I've always felt it was best
to create it again from scratch. Do you have many uses of it in your
code?
-- bjorn
On Dec 20, 6:40 pm, thebjorn
I'm a bit ambivalent about the reset functionality. While it's a
wonderful demonstration of a staticmethod, the very few times I've
felt the need to "freshen-up" the object, I've always felt it was best
to create it again from scratch. Do you have many uses of it in your
code?
My use case is for web applications where the configuration
parameters are stored in a database. If you change them,
you can re-read them by resetting the configuration object
(you may have a reset button in the administrator Web user
interface) without restarting the application.
Michele Simionato This thread has been closed and replies have been disabled. Please start a new discussion. Similar topics |
by: Chris Reedy |
last post by:
I would like to memoize (I think I've got that right) a collection of
functions for a project I'm working.
<Aside for those not familiar with memoizing functions:>
Given:
def foo(a,b,c):
return <result of large ugly computation>
|
by: Michael Hohn |
last post by:
Hi,
under python 2.2, the pickle/unpickle sequence incorrectly restores
a larger data structure I have.
Under Python 2.3, these structures now give an explicit exception from
Pickle.memoize():
assert id(obj) not in self.memo
I'm shrinking the offending data structure down to find the problem
|
by: km |
last post by:
Hi all,
was going thru the new features introduced into python2.4 version.
i was stuck with 'decorators' - can someone explain me the need of such a thing called decorators ?
tia
KM
|
by: George Ter-Saakov |
last post by:
What is HttpContext.Cache for ?
The usual HttpRequest lasts miliseconds.
Why would someone to cache anything?
We do have HttpContext.Items to keep any extra info during the request.
Can someone explain me the use of the HttpContext.Cache.
Thanks.
|
by: Keith Chadwick |
last post by:
Need some help on access the httpApplication cache.
I was informed to use the application cache over the old application object
and also to use a classes versus modules.
do I need to Imprt System.Web.HttpApplication into the class?
do I need to dim x as System.Web.Application then use
x.context.cache.item("myitem")=myobject
| |
by: Christopher |
last post by:
In one of our ASP.NET Pages, we are starting a new background thread that we
do not need to go and get any status on or use after the page finishes. The
thread merely does some background stuff on its own and finishes on its own,
no feedback back to the user (by design). Within that thread however, some
items need to be accessed and/or inserted into the Cache for possible
subsequent processing by another page. Many examples I see of...
|
by: thattommyhallll |
last post by:
im plugging away at the problems at
http://www.mathschallenge.net/index.php?section=project
im trying to use them as a motivator to get into advanced topics in
python.
one thing that Structure And Interpretation Of Computer Programs
teaches is that memoisation is good.
all of the memoize decorators at the python cookbook seem to make my
code slower.
is using a decorator a lazy and inefficient way of doing memoization?
can anyone point...
|
by: Harry Haller |
last post by:
What's wrong with this:
Error 3 'System.Web.Caching.Cache' is a 'type', which is not valid
in the given context
public List<AssetSummaryGetAssetSummary()
{
return
(List<AssetSummary>)System.Web.Caching.Cache("assetSummary");
}
|
by: Andrus |
last post by:
I need to create Dlinq entity property value caches for every entity and
every property so that
all entity caches can cleared if entity of this type is changed. Entity
class method uses this as:
string GetCustomerNameById( string customerId ) {
var cache = CacheManager.Get<Customer,string, string>("CustomerName");
string res;
if (cache.TryGetValue(customerId , out res))
return res;
|
by: marktang |
last post by:
ONU (Optical Network Unit) is one of the key components for providing high-speed Internet services. Its primary function is to act as an endpoint device located at the user's premises. However, people are often confused as to whether an ONU can Work As a Router. In this blog post, we’ll explore What is ONU, What Is Router, ONU & Router’s main usage, and What is the difference between ONU and Router. Let’s take a closer look !
Part I. Meaning of...
|
by: Hystou |
last post by:
Most computers default to English, but sometimes we require a different language, especially when relocating. Forgot to request a specific language before your computer shipped? No problem! You can effortlessly switch the default language on Windows 10 without reinstalling. I'll walk you through it.
First, let's disable language synchronization. With a Microsoft account, language settings sync across devices. To prevent any complications,...
| |
by: Oralloy |
last post by:
Hello folks,
I am unable to find appropriate documentation on the type promotion of bit-fields when using the generalised comparison operator "<=>".
The problem is that using the GNU compilers, it seems that the internal comparison operator "<=>" tries to promote arguments from unsigned to signed.
This is as boiled down as I can make it.
Here is my compilation command:
g++-12 -std=c++20 -Wnarrowing bit_field.cpp
Here is the code in...
|
by: jinu1996 |
last post by:
In today's digital age, having a compelling online presence is paramount for businesses aiming to thrive in a competitive landscape. At the heart of this digital strategy lies an intricately woven tapestry of website design and digital marketing. It's not merely about having a website; it's about crafting an immersive digital experience that captivates audiences and drives business growth.
The Art of Business Website Design
Your website is...
|
by: Hystou |
last post by:
Overview:
Windows 11 and 10 have less user interface control over operating system update behaviour than previous versions of Windows. In Windows 11 and 10, there is no way to turn off the Windows Update option using the Control Panel or Settings app; it automatically checks for updates and installs any it finds, whether you like it or not. For most users, this new feature is actually very convenient. If you want to control the update process,...
|
by: agi2029 |
last post by:
Let's talk about the concept of autonomous AI software engineers and no-code agents. These AIs are designed to manage the entire lifecycle of a software development project—planning, coding, testing, and deployment—without human intervention. Imagine an AI that can take a project description, break it down, write the code, debug it, and then launch it, all on its own....
Now, this would greatly impact the work of software developers. The idea...
|
by: isladogs |
last post by:
The next Access Europe User Group meeting will be on Wednesday 1 May 2024 starting at 18:00 UK time (6PM UTC+1) and finishing by 19:30 (7.30PM).
In this session, we are pleased to welcome a new presenter, Adolph Dupré who will be discussing some powerful techniques for using class modules.
He will explain when you may want to use classes instead of User Defined Types (UDT). For example, to manage the data in unbound forms.
Adolph will...
|
by: conductexam |
last post by:
I have .net C# application in which I am extracting data from word file and save it in database particularly. To store word all data as it is I am converting the whole word file firstly in HTML and then checking html paragraph one by one.
At the time of converting from word file to html my equations which are in the word document file was convert into image.
Globals.ThisAddIn.Application.ActiveDocument.Select();...
| |
by: bsmnconsultancy |
last post by:
In today's digital era, a well-designed website is crucial for businesses looking to succeed. Whether you're a small business owner or a large corporation in Toronto, having a strong online presence can significantly impact your brand's success. BSMN Consultancy, a leader in Website Development in Toronto offers valuable insights into creating effective websites that not only look great but also perform exceptionally well. In this comprehensive...
| |