Jinja 2.1 和更高的版本支持外部字节码缓存。字节码缓存使得在首次使用时把生成的字节码 存储到文件系统或其它位置来避免处理模板。

这在当你有一个在首个应用初始化的 web 应用， Jinja 一次性编译大量模板拖慢应用时尤其 有用。

要使用字节码缓存，把它实例化并传给 Environment 。

class jinja2.BytecodeCache

To implement your own bytecode cache you have to subclass this class and overrideload_bytecode() and dump_bytecode(). Both of these methods are passed a Bucket.

A very basic bytecode cache that saves the bytecode on the file system:

from os import path

class MyCache(BytecodeCache):

    def __init__(self, directory):
        self.directory = directory

    def load_bytecode(self, bucket):
        filename = path.join(self.directory, bucket.key)
        if path.exists(filename):
            with open(filename, 'rb') as f:
                bucket.load_bytecode(f)

    def dump_bytecode(self, bucket):
        filename = path.join(self.directory, bucket.key)
        with open(filename, 'wb') as f:
            bucket.write_bytecode(f)

A more advanced version of a filesystem based bytecode cache is part of Jinja2.

clear()

Clears the cache. This method is not used by Jinja2 but should be implemented to allow applications to clear the bytecode cache used by a particular environment.

dump_bytecode(bucket)

Subclasses have to override this method to write the bytecode from a bucket back to the cache. If it unable to do so it must not fail silently but raise an exception.

load_bytecode(bucket)

Subclasses have to override this method to load bytecode into a bucket. If they are not able to find code in the cache for the bucket, it must not do anything.

class jinja2.bccache.Bucket(environment, key, checksum)

Buckets are used to store the bytecode for one template. It’s created and initialized by the bytecode cache and passed to the loading functions.

The buckets get an internal checksum from the cache assigned and use this to automatically reject outdated cache material. Individual bytecode cache subclasses don’t have to care about cache invalidation.

environment

创建 bucket 的 Environment

key

该 bucket 的唯一键

code

如果已加载，则为字节码，否则为 None 。

bytecode_from_string(string)

Load bytecode from a string.

bytecode_to_string()

Return the bytecode as string.

load_bytecode(f)

Loads bytecode from a file or file like object.

reset()

Resets the bucket (unloads the bytecode).

write_bytecode(f)

Dump the bytecode into the file or file like object passed.

内建的字节码缓存:

class jinja2.FileSystemBytecodeCache(directory=None,pattern='_jinja2%s.cache')

A bytecode cache that stores bytecode on the filesystem. It accepts two arguments: The directory where the cache items are stored and a pattern string that is used to build the filename.

If no directory is specified the system temporary items folder is used.

The pattern can be used to have multiple separate caches operate on the same directory. The default pattern is '_jinja2%s.cache'. %s is replaced with the cache key.

>>> bcc = FileSystemBytecodeCache('/tmp/jinja_cache', '%s.cache')

This bytecode cache supports clearing of the cache using the clear method.

class jinja2.MemcachedBytecodeCache(client, prefix='jinja2/bytecode/',timeout=None, ignore_memcache_errors=True)

This class implements a bytecode cache that uses a memcache cache for storing the information. It does not enforce a specific memcache library (tummy’s memcache or cmemcache) but will accept any class that provides the minimal interface required.

Libraries compatible with this class:

• werkzeug.contrib.cache
• python-memcached
• cmemcache

(Unfortunately the django cache interface is not compatible because it does not support storing binary data, only unicode. You can however pass the underlying cache client to the bytecode cache which is available asdjango.core.cache.cache._client.)

The minimal interface for the client passed to the constructor is this:

class MinimalClientInterface

set(key, value[, timeout])

Stores the bytecode in the cache. value is a string and timeout the timeout of the key. If timeout is not provided a default timeout or no timeout should be assumed, if it’s provided it’s an integer with the number of seconds the cache item should exist.

get(key)

Returns the value for the cache key. If the item does not exist in the cache the return value must be None.

The other arguments to the constructor are the prefix for all keys that is added before the actual cache key and the timeout for the bytecode in the cache system. We recommend a high (or no) timeout.

This bytecode cache does not support clearing of used items in the cache. The clear method is a no-operation function.

New in version 2.7: Added support for ignoring memcache errors through theignore_memcache_errors parameter.