Easy caching

[jalanb3]

Evening all,

And thank you for your valuable reading skills.

The following pattern turned up in coding tonight.
It works, but I'm suspicious, it just seems too easy.

So any comments or constructive criticisms welcome ?

*** Start doctest format ***

>>class Cacher:

... def __init__(self):
... self.value_in_d atabase = 0
...
... def very_slow_datab ase_access(self ):
... return self.value_in_d atabase
...
... @property
... def show(self):
... result = self.very_slow_ database_access ()
... self.show = result
... return result
...

>>>

Create an instance

>>cacher = Cacher()

Someone writes to the database

>>cacher.value_ in_database = 9

Call show as a method, get the value

>>print cacher.show

9

show is now a cached string

>>print cacher.show

9

show is not a method so
if someone else writes to the database
we will not see the change

>>cacher.value_ in_database = 7
>>print cacher.show

9

To force another read from the database
delete the string attribute

>>del cacher.show

Next time we try to use the attribute
Python will find the method again

>>print cacher.show

7

***

--
Alan

[Peter Otten]

Re: Easy caching

jalanb3 wrote:

Evening all,
>
And thank you for your valuable reading skills.
>
The following pattern turned up in coding tonight.
It works, but I'm suspicious, it just seems too easy.
>
So any comments or constructive criticisms welcome ?
>
*** Start doctest format ***
>

>class Cacher:

... def __init__(self):
... self.value_in_d atabase = 0
...
... def very_slow_datab ase_access(self ):
... return self.value_in_d atabase
...
... @property
... def show(self):
... result = self.very_slow_ database_access ()
... self.show = result
... return result
...

The above only works because properties don't work with classic class. If
you make Cacher a newstyle class (have it inherit from object) you'll get
an AttributeError. Therefore I think it would be cleaner to either

use __getattr__() or

change the Cacher to something like

class Cacher(object):
def __init__(self):
self._show = None
# ...

@property
def show(self):
result = self._show
if result is None:
result = self._show = self.very_slow_ database_access ()
return result

Peter

[M.-A. Lemburg]

Re: Easy caching

On 2008-11-13 09:03, Peter Otten wrote:

jalanb3 wrote:
>

>Evening all,
>>
>And thank you for your valuable reading skills.
>>
>The following pattern turned up in coding tonight.
>It works, but I'm suspicious, it just seems too easy.
>>
>So any comments or constructive criticisms welcome ?
>>
>*** Start doctest format ***
>>

>>>>class Cacher:

>... def __init__(self):
>... self.value_in_d atabase = 0
>...
>... def very_slow_datab ase_access(self ):
>... return self.value_in_d atabase
>...
>... @property
>... def show(self):
>... result = self.very_slow_ database_access ()
>... self.show = result
>... return result
>...

>
The above only works because properties don't work with classic class. If
you make Cacher a newstyle class (have it inherit from object) you'll get
an AttributeError. Therefore I think it would be cleaner to either
>
use __getattr__() or
>
change the Cacher to something like
>
class Cacher(object):
def __init__(self):
self._show = None
# ...
>
@property
def show(self):
result = self._show
if result is None:
result = self._show = self.very_slow_ database_access ()
return result

You should also consider not using a property for this at all.

Attribute access is supposed to be fast and, if at all, only cause
AttributeErrors .

Your self.very_slow_ database_access () call can raise all sorts of
errors and likely take much too long for what a casual user would
expect from writing print (cache.show).

It's much better to write this as method which then uses an
instance attribute for caching the value read from the database.

--
Marc-Andre Lemburg
eGenix.com

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