mirror of
https://github.com/wassname/catalyst.git
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Eddie Hebert
76e14eda2f
Add a cache interface which supports expirable entries with a changeable
backend for the cache into which they are entered.
The default cache is a `dict` but could swapped for
`cachetools.LRUCache` or any other cache which supports `__get__`,
`__set__`, and `__del__`.
So that consumers can change the use of `CachedObjects` stored in a
cache from:
```
self._cache = {}
...
try:
obj = self._cache[key]
try:
return obj.unwrap(dt)
except Expired:
pass
except KeyError:
pass
...
self._cache[key] = CachedObject(value, new_expiration)
```
to:
```
self._cache = ExpiringCache(LRUCache(maxsize=6))
...
try:
return self._cache.get(key, dt)
except KeyError:
# Get fresh value
...
self._cache.set(key, value, new_expiration)
```
115 lines
2.9 KiB
Python
115 lines
2.9 KiB
Python
"""
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Cached object with an expiration date.
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"""
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from collections import namedtuple
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class Expired(Exception):
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pass
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class CachedObject(namedtuple("_CachedObject", "value expires")):
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"""
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A simple struct for maintaining a cached object with an expiration date.
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Parameters
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----------
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value : object
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The object to cache.
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expires : datetime-like
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Expiration date of `value`. The cache is considered invalid for dates
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**strictly greater** than `expires`.
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Methods
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-------
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get(self, dt)
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Get the cached object.
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Usage
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-----
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>>> from pandas import Timestamp, Timedelta
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>>> expires = Timestamp('2014', tz='UTC')
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>>> obj = CachedObject(1, expires)
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>>> obj.unwrap(expires - Timedelta('1 minute'))
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1
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>>> obj.unwrap(expires)
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1
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>>> obj.unwrap(expires + Timedelta('1 minute'))
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Traceback (most recent call last):
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...
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Expired: 2014-01-01 00:00:00+00:00
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"""
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def unwrap(self, dt):
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"""
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Get the cached value.
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Returns
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-------
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value : object
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The cached value.
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Raises
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------
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Expired
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Raised when `dt` is greater than self.expires.
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"""
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if dt > self.expires:
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raise Expired(self.expires)
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return self.value
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class ExpiringCache(object):
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"""
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A cache of multiple CachedObjects, which returns the wrapped the value
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or raises and deletes the CachedObject if the value has expired.
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Parameters
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----------
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cache : dict-like
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An instance of a dict-like object which needs to support at least:
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`__del__`, `__getitem__`, `__setitem__`
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If `None`, than a dict is used as a default.
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Methods
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-------
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get(self, key, dt)
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Get the value of a cached object for the given `key` at `dt`, if the
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CachedObject has expired then the object is removed from the cache,
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and `KeyError` is raised.
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set(self, key, value, expiration_dt)
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Add a new `value` to the cache at `dt` wrapped in a CachedObject which
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expires at `expiration_dt`.
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Usage
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-----
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>>> from pandas import Timestamp, Timedelta
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>>> expires = Timestamp('2014', tz='UTC')
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>>> value = 1
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>>> cache = ExpiringCache()
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>>> cache.set('foo', value, expires)
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>>> cache.get('foo', expires - Timedelta('1 minute'))
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1
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>>> cache.get('foo', expires + Timedelta('1 minute'))
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Traceback (most recent call last):
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...
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KeyError: 'foo'
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"""
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def __init__(self, cache=None):
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if cache is not None:
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self._cache = cache
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else:
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self._cache = {}
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def get(self, key, dt):
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try:
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return self._cache[key].unwrap(dt)
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except Expired:
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del self._cache[key]
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raise KeyError(key)
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def set(self, key, value, expiration_dt):
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self._cache[key] = CachedObject(value, expiration_dt)
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