Conditionally disable caching decorator based on instance variable

Question

I want to cache certain methods of a class - but only if a read_only flag is set on the instance of the class.

So, in the object below, I want the get() method to be cacheable, but only if self.readonly is True.

I can't conditionally use the decorator, because it's set when the class is defined, not when it's instantiated.

from beaker.cache import cache_regions, cache_region
cache_regions.update({
    'long_term':{
        'expire':86400,
        'type':'dbm',
        'data_dir':'/tmp',
        'key_length': 32,
    }
})

class Foo(object):
  def __init__(self, read_only=True):
    self.read_only = read_only

  @cache_region('long_term')
  def get(self, arg):
    return arg + 1

Answer 1

You can use a decorator to call the right (cached or not) function checking the desired attribute:

def conditional(decorator):

    def conditional_decorator(fn):
        dec = decorator(fn)
        def wrapper(self, *args, **kw):
             if self.read_only:
                 return dec(self, *args, **kw)
             return fn(self, *args, **kw)
        return wrapper

    return conditional_decorator

Use like this:

@conditional(cache_region('long_term'))
def get(self, arg):
    return arg + 1