"Base Cache class."
import time
import warnings
from django.core.exceptions import ImproperlyConfigured
from django.utils.module_loading import import_string
class InvalidCacheBackendError(ImproperlyConfigured):
pass
class CacheKeyWarning(RuntimeWarning):
pass
class InvalidCacheKey(ValueError):
pass
# Stub class to ensure not passing in a `timeout` argument results in
# the default timeout
DEFAULT_TIMEOUT = object()
# Memcached does not accept keys longer than this.
MEMCACHE_MAX_KEY_LENGTH = 250
def default_key_func(key, key_prefix, version):
"""
Default function to generate keys.
Construct the key used by all other methods. By default, prepend
the `key_prefix`. KEY_FUNCTION can be used to specify an alternate
function with custom key making behavior.
"""
return '%s:%s:%s' % (key_prefix, version, key)
def get_key_func(key_func):
"""
Function to decide which key function to use.
Default to ``default_key_func``.
"""
if key_func is not None:
if callable(key_func):
return key_func
else:
return import_string(key_func)
return default_key_func
class BaseCache:
_missing_key = object()
def __init__(self, params):
timeout = params.get('timeout', params.get('TIMEOUT', 300))
if timeout is not None:
try:
timeout = int(timeout)
except (ValueError, TypeError):
timeout = 300
self.default_timeout = timeout
options = params.get('OPTIONS', {})
max_entries = params.get('max_entries', options.get('MAX_ENTRIES', 300))
try:
self._max_entries = int(max_entries)
except (ValueError, TypeError):
self._max_entries = 300
cull_frequency = params.get('cull_frequency', options.get('CULL_FREQUENCY', 3))
try:
self._cull_frequency = int(cull_frequency)
except (ValueError, TypeError):
self._cull_frequency = 3
self.key_prefix = params.get('KEY_PREFIX', '')
self.version = params.get('VERSION', 1)
self.key_func = get_key_func(params.get('KEY_FUNCTION'))
def get_backend_timeout(self, timeout=DEFAULT_TIMEOUT):
"""
Return the timeout value usable by this backend based upon the provided
timeout.
"""
if timeout == DEFAULT_TIMEOUT:
timeout = self.default_timeout
elif timeout == 0:
# ticket 21147 - avoid time.time() related precision issues
timeout = -1
return None if timeout is None else time.time() + timeout
def make_key(self, key, version=None):
"""
Construct the key used by all other methods. By default, use the
key_func to generate a key (which, by default, prepends the
`key_prefix' and 'version'). A different key function can be provided
at the time of cache construction; alternatively, you can subclass the
cache backend to provide custom key making behavior.
"""
if version is None:
version = self.version
return self.key_func(key, self.key_prefix, version)
def add(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
"""
Set a value in the cache if the key does not already exist. If
timeout is given, use that timeout for the key; otherwise use the
default cache timeout.
Return True if the value was stored, False otherwise.
"""
raise NotImplementedError('subclasses of BaseCache must provide an add() method')
def get(self, key, default=None, version=None):
"""
Fetch a given key from the cache. If the key does not exist, return
default, which itself defaults to None.
"""
raise NotImplementedError('subclasses of BaseCache must provide a get() method')
def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
"""
Set a value in the cache. If timeout is given, use that timeout for the
key; otherwise use the default cache timeout.
"""
raise NotImplementedError('subclasses of BaseCache must provide a set() method')
def touch(self, key, timeout=DEFAULT_TIMEOUT, version=None):
"""
Update the key's expiry time using timeout. Return True if successful
or False if the key does not exist.
"""
raise NotImplementedError('subclasses of BaseCache must provide a touch() method')
def delete(self, key, version=None):
"""
Delete a key from the cache and return whether it succeeded, failing
silently.
"""
raise NotImplementedError('subclasses of BaseCache must provide a delete() method')
def get_many(self, keys, version=None):
"""
Fetch a bunch of keys from the cache. For certain backends (memcached,
pgsql) this can be *much* faster when fetching multiple values.
Return a dict mapping each key in keys to its value. If the given
key is missing, it will be missing from the response dict.
"""
d = {}
for k in keys:
val = self.get(k, self._missing_key, version=version)
if val is not self._missing_key:
d[k] = val
return d
def get_or_set(self, key, default, timeout=DEFAULT_TIMEOUT, version=None):
"""
Fetch a given key from the cache. If the key does not exist,
add the key and set it to the default value. The default value can
also be any callable. If timeout is given, use that timeout for the
key; otherwise use the default cache timeout.
Return the value of the key stored or retrieved.
"""
val = self.get(key, self._missing_key, version=version)
if val is self._missing_key:
if callable(default):
default = default()
self.add(key, default, timeout=timeout, version=version)
# Fetch the value again to avoid a race condition if another caller
# added a value between the first get() and the add() above.
return self.get(key, default, version=version)
return val
def has_key(self, key, version=None):
"""
Return True if the key is in the cache and has not expired.
"""
return self.get(key, self._missing_key, version=version) is not self._missing_key
def incr(self, key, delta=1, version=None):
"""
Add delta to value in the cache. If the key does not exist, raise a
ValueError exception.
"""
value = self.get(key, self._missing_key, version=version)
if value is self._missing_key:
raise ValueError("Key '%s' not found" % key)
new_value = value + delta
self.set(key, new_value, version=version)
return new_value
def decr(self, key, delta=1, version=None):
"""
Subtract delta from value in the cache. If the key does not exist, raise
a ValueError exception.
"""
return self.incr(key, -delta, version=version)
def __contains__(self, key):
"""
Return True if the key is in the cache and has not expired.
"""
# This is a separate method, rather than just a copy of has_key(),
# so that it always has the same functionality as has_key(), even
# if a subclass overrides it.
return self.has_key(key)
def set_many(self, data, timeout=DEFAULT_TIMEOUT, version=None):
"""
Set a bunch of values in the cache at once from a dict of key/value
pairs. For certain backends (memcached), this is much more efficient
than calling set() multiple times.
If timeout is given, use that timeout for the key; otherwise use the
default cache timeout.
On backends that support it, return a list of keys that failed
insertion, or an empty list if all keys were inserted successfully.
"""
for key, value in data.items():
self.set(key, value, timeout=timeout, version=version)
return []
def delete_many(self, keys, version=None):
"""
Delete a bunch of values in the cache at once. For certain backends
(memcached), this is much more efficient than calling delete() multiple
times.
"""
for key in keys:
self.delete(key, version=version)
def clear(self):
"""Remove *all* values from the cache at once."""
raise NotImplementedError('subclasses of BaseCache must provide a clear() method')
def validate_key(self, key):
"""
Warn about keys that would not be portable to the memcached
backend. This encourages (but does not force) writing backend-portable
cache code.
"""
for warning in memcache_key_warnings(key):
warnings.warn(warning, CacheKeyWarning)
def incr_version(self, key, delta=1, version=None):
"""
Add delta to the cache version for the supplied key. Return the new
version.
"""
if version is None:
version = self.version
value = self.get(key, self._missing_key, version=version)
if value is self._missing_key:
raise ValueError("Key '%s' not found" % key)
self.set(key, value, version=version + delta)
self.delete(key, version=version)
return version + delta
def decr_version(self, key, delta=1, version=None):
"""
Subtract delta from the cache version for the supplied key. Return the
new version.
"""
return self.incr_version(key, -delta, version)
def close(self, **kwargs):
"""Close the cache connection"""
pass
def memcache_key_warnings(key):
if len(key) > MEMCACHE_MAX_KEY_LENGTH:
yield (
'Cache key will cause errors if used with memcached: %r '
'(longer than %s)' % (key, MEMCACHE_MAX_KEY_LENGTH)
)
for char in key:
if ord(char) < 33 or ord(char) == 127:
yield (
'Cache key contains characters that will cause errors if '
'used with memcached: %r' % key
)
break