Question

我有一个非常不可靠的API，我使用Python请求。我一直在考虑使用requests_cache并将expire_after设置为999999999999，就像我见过其他人一样。唯一的问题是，我不知道API何时再次运行，如果数据已更新。如果requests_cache将自动自动更新并删除旧条目。

我试过阅读文档，但我无法在任何地方看到这一点。

Answer 1

在requests_cache时间过去之前，

expire_after 不会更新。在这种情况下，它不会检测到您的API已恢复到工作状态。

我注意到该项目已添加了我过去实施的选项;您现在可以在配置缓存时设置old_data_on_error选项;请参阅CachedSession documentation：

old_data_on_error - 如果更新失败，如果True它将返回过期的缓存响应。

如果后端更新失败，它将重用现有的缓存数据，而不是删除该数据。

在过去，我创建了自己的requests_cache会话设置（加上小补丁），如果后端发出500错误或超时（使用短暂超时），则会重用expire_after之外的缓存值处理有问题的API层，而不是依赖expire_after：

import logging

from datetime import (
    datetime,
    timedelta
)
from requests.exceptions import (
    ConnectionError,
    Timeout,
)
from requests_cache.core import (
    dispatch_hook,
    CachedSession,
)

log = logging.getLogger(__name__)
# Stop logging from complaining if no logging has been configured.
log.addHandler(logging.NullHandler())


class FallbackCachedSession(CachedSession):
    """Cached session that'll reuse expired cache data on timeouts

    This allows survival in case the backend is down, living of stale
    data until it comes back.

    """

    def send(self, request, **kwargs):
        # this *bypasses* CachedSession.send; we want to call the method
        # CachedSession.send() would have delegated to!
        session_send = super(CachedSession, self).send
        if (self._is_cache_disabled or
                request.method not in self._cache_allowable_methods):
            response = session_send(request, **kwargs)
            response.from_cache = False
            return response

        cache_key = self.cache.create_key(request)

        def send_request_and_cache_response(stale=None):
            try:
                response = session_send(request, **kwargs)
            except (Timeout, ConnectionError):
                if stale is None:
                    raise
                log.warning('No response received, reusing stale response for '
                            '%s', request.url)
                return stale

            if stale is not None and response.status_code == 500:
                log.warning('Response gave 500 error, reusing stale response '
                            'for %s', request.url)
                return stale

            if response.status_code in self._cache_allowable_codes:
                self.cache.save_response(cache_key, response)
            response.from_cache = False
            return response

        response, timestamp = self.cache.get_response_and_time(cache_key)
        if response is None:
            return send_request_and_cache_response()

        if self._cache_expire_after is not None:
            is_expired = datetime.utcnow() - timestamp > self._cache_expire_after
            if is_expired:
                self.cache.delete(cache_key)
                # try and get a fresh response, but if that fails reuse the
                # stale one
                return send_request_and_cache_response(stale=response)

        # dispatch hook here, because we've removed it before pickling
        response.from_cache = True
        response = dispatch_hook('response', request.hooks, response, **kwargs)
        return response


def basecache_delete(self, key):
    # We don't really delete; we instead set the timestamp to
    # datetime.min. This way we can re-use stale values if the backend
    # fails
    try:
        if key not in self.responses:
            key = self.keys_map[key]
        self.responses[key] = self.responses[key][0], datetime.min
    except KeyError:
        return

from requests_cache.backends.base import BaseCache
BaseCache.delete = basecache_delete

CachedSession的上述子类绕过original send() method而不是直接转到原始requests.Session.send()方法，即使超时已经过了但后端失败，也会返回现有的缓存值。禁用删除有利于将超时值设置为0，因此如果新请求失败，我们仍然可以重用该旧值。

使用FallbackCachedSession代替常规CachedSession对象。

如果您想使用requests_cache.install_cache()，请确保在FallbackCachedSession关键字参数中将session_factory传递给该函数：

import requests_cache

requests_cache.install_cache(
    'cache_name', backend='some_backend', expire_after=180,
    session_factory=FallbackCachedSession)

在我将上述内容混合在一起之后的某段时间，我的方法比requests_cache实施的方法更全面;即使您之前明确将其标记为已删除，我的版本也会回归到陈旧的响应中。

Answer 2

尝试做类似的事情：

class UnreliableAPIClient:
  def __init__(self):
    self.some_api_method_cached = {} # we will store results here

  def some_api_method(self, param1, param2)
    params_hash = "{0}-{1}".format(param1, param2) # need to identify input
    try:
      result = do_call_some_api_method_with_fail_probability(param1, param2)
      self.some_api_method_cached[params_hash] = result # save result
    except:
      result = self.some_api_method_cached[params_hash] # resort to cached result
      if result is None:
        raise # reraise exception if nothing cached
    return result

当然，您可以使用它来制作简单的装饰 - http://www.artima.com/weblogs/viewpost.jsp?thread=240808

在更新信息时，requests_cache会自动更新缓存吗？

2 个答案: