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NDB Context class | App Engine standard environment for Python 2

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An application can use a Context to control its caching policy. Context also offers convenient asynchronous APIs for Memcache and URL Fetch integrated into NDB's asynchronous facilities; in particular, the Memcache API supports auto-batching.

• Cache Management Instance Methods
  • clear_cache()
  • flush()
  • get_cache_policy()
  • get_datastore_policy()
  • get_memcache_policy()
  • get_memcache_timeout_policy()
  • set_cache_policy()
  • set_datastore_policy()
  • set_memcache_policy()
  • set_memcache_timeout_policy()
• Memcache and Urlfetch Instance Methods
  • memcache_add()
  • memcache_cas()
  • memcache_decr()
  • memcache_delete()
  • memcache_get()
  • memcache_gets()
  • memcache_incr()
  • memcache_replace()
  • memcache_set()
  • urlfetch()
• Transaction Management Methods
  • call_on_commit()
  • in_transaction()
• Static Methods
  • default_cache_policy()
  • default_datastore_policy()
  • default_memcache_policy()
  • default_memcache_timeout_policy()

clear_cache()

Clears the in-context cache.

flush()

Flush all auto-batcher queues (sending their work to the servers). The flush happens asynchronously.

Returns a Future. To wait for the flushing to finish, call this object's wait() method.

get_cache_policy()

Returns the cache policy function. This policy function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None (meaning use default_cache_policy).

get_datastore_policy()

Returns the current context Datastore policy function. This policy function takes a Key instance argument and returns a bool indicating if it should use the Datastore. May be None (meaning use default_datastore_policy).

get_memcache_policy()

Returns the current memcache policy function. This policy function takes a Key instance argument and returns a bool indicating if it should be cached. May be None (meaning use default_memcache_policy).

get_memcache_timeout_policy()

Returns the current memcache timeout policy function. This policy function takes a Key instance argument and returns the desired memcache timeout in seconds (or returns zero to use the default timeout). May be None, which uses default_memcache_timeout_policy.

set_cache_policy(func)

Sets the cache policy function.

Arguments

func

This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None.

set_datastore_policy(func)

Sets the Datastore policy function.

Arguments

func

This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be stored in the persistent Datastore. May be None.

set_memcache_policy(func)

Sets the memcache policy function.

Arguments

func

This function takes a Key instance argument and returns a bool indicating whether the entity with this key should be cached in the in-context cache. May be None.

set_memcache_timeout_policy(func)

Sets the memcache timeout policy function.

Arguments

func

This function takes a Key instance argument and returns the desired memcache timeout in seconds (or returns zero to use the default timeout). May be None.

The full list of methods is found in the Memcache API reference. The list below highlights some of the more commonly used methods:

memcache_add(key, value, time, namespace)

Async auto-batching memcache add().

memcache_cas(key, value, time, namespace)

Async auto-batching memcache Client cas() (compare-and-swap).

memcache_decr(key, delta, initial_value, namespace)

Async auto-batching memcache decr().

memcache_delete(key, seconds, namespace)

Async auto-batching memcache delete().

memcache_get(key, namespace, use_cache)

Async auto-batching memcache get().

Returns a Future whose return value is the value retrieved from memcache or None.

memcache_gets(key, namespace, use_cache)

Async auto-batching memcache gets().

memcache_incr(key, delta, initial_value, namespace)

Async auto-batching memcache incr().

memcache_replace(key, value, time, namespace)

Async auto-batching memcache replace().

memcache_set(key, value, time, namespace, use_cache)

Async auto-batching memcache set().

urlfetch(url, payload, method, headers, allow_truncated, follow_redirects, validate_certificate, deadline, callback)

Async auto-batching urlfetch fetch().

call_on_commit(callback)

Queues a callback to call upon successful commit of a transaction.

If not in a transaction, the callback is called immediately.

In a transaction, multiple callbacks may be registered and will be called once the transaction commits in the order in which they were registered. If the transaction fails, the callbacks will not be called.

If a callback raises an exception, it bubbles up normally. This means: If the callback is called immediately, any exception it raises will bubble up immediately. If the call is postponed until commit, remaining callbacks will be skipped and the exception will bubble up through the transaction() call. (However, the transaction is already committed at that point.)

Arguments

callback

Callback function. This callback function takes no parameters and returns nothing.

in_transaction()

Returns True if this Context represents a transaction, False otherwise.

default_cache_policy(key)

Checks for a _use_cache class variable on the entity's model class; if there is one, return it. Otherwise, return None.

default_datastore_policy(key)

Checks for a _use_datastore class variable on the entity's model class; if there is one, return it. Otherwise, return None.

default_memcache_policy(key)

Checks for a _use_memcache class variable on the entity's model class; if there is one, return it. Otherwise, return None.

default_memcache_timeout_policy(key)

Checks for a _memcache_timeout class variable on the entity's model class; if there is one, return it. Otherwise, return None.

Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 4.0 License, and code samples are licensed under the Apache 2.0 License. For details, see the Google Developers Site Policies. Java is a registered trademark of Oracle and/or its affiliates.

Last updated 2025-08-07 UTC.

[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-07 UTC."],[[["This page outlines the use of legacy bundled services and APIs, which are compatible only with first-generation runtimes in the App Engine standard environment."],["A `Context` can be used by an application to control its caching policy, including setting cache, datastore, and memcache policies, as well as memcache timeout policies."],["`Context` provides asynchronous APIs for Memcache, supporting auto-batching, and URL Fetch, enabling efficient data retrieval and management."],["The `Context` also provides methods for transaction management, including queuing callbacks upon successful transaction commit and checking if the current context is within a transaction."],["The `Context` class offers various static methods for setting and checking policies such as default_cache_policy, default_datastore_policy, default_memcache_policy, and default_memcache_timeout_policy."]]],[]]

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