加载器负责从诸如文件系统的资源加载模板。环境会把编译的模块像 Python 的sys.modules 一样保持在内存中。与 sys.models 不同，无论如何这个 缓存默认有大小限制，且模板会自动重新加载。 所有的加载器都是 BaseLoader 的子类。如果你想要创建自己的加载器，继 承 BaseLoader 并重载 get_source 。

class jinja2.BaseLoader

Baseclass for all loaders. Subclass this and override get_source to implement a custom loading mechanism. The environment provides a get_template method that calls the loader’s load method to get the Template object.

A very basic example for a loader that looks up templates on the file system could look like this:

from jinja2 import BaseLoader, TemplateNotFound
from os.path import join, exists, getmtime

class MyLoader(BaseLoader):

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

    def get_source(self, environment, template):
        path = join(self.path, template)
        if not exists(path):
            raise TemplateNotFound(template)
        mtime = getmtime(path)
        with file(path) as f:
            source = f.read().decode('utf-8')
        return source, path, lambda: mtime == getmtime(path)

get_source(environment, template)

Get the template source, filename and reload helper for a template. It’s passed the environment and template name and has to return a tuple in the form(source, filename, uptodate) or raise a TemplateNotFound error if it can’t locate the template.

The source part of the returned tuple must be the source of the template as unicode string or a ASCII bytestring. The filename should be the name of the file on the filesystem if it was loaded from there, otherwise None. The filename is used by python for the tracebacks if no loader extension is used.

The last item in the tuple is the uptodate function. If auto reloading is enabled it’s always called to check if the template changed. No arguments are passed so the function must store the old state somewhere (for example in a closure). If it returns False the template will be reloaded.

load(environment, name, globals=None)

Loads a template. This method looks up the template in the cache or loads one by calling get_source(). Subclasses should not override this method as loaders working on collections of other loaders (such as PrefixLoader or ChoiceLoader) will not call this method but get_source directly.

这里有一个 Jinja2 提供的内置加载器的列表:

class jinja2.FileSystemLoader(searchpath, encoding='utf-8')

Loads templates from the file system. This loader can find templates in folders on the file system and is the preferred way to load them.

The loader takes the path to the templates as string, or if multiple locations are wanted a list of them which is then looked up in the given order:

>>> loader = FileSystemLoader('/path/to/templates')
>>> loader = FileSystemLoader(['/path/to/templates', '/other/path'])

Per default the template encoding is 'utf-8' which can be changed by setting theencoding parameter to something else.

class jinja2.PackageLoader(package_name, package_path='templates', encoding='utf-8')

Load templates from python eggs or packages. It is constructed with the name of the python package and the path to the templates in that package:

loader = PackageLoader('mypackage', 'views')

If the package path is not given, 'templates' is assumed.

Per default the template encoding is 'utf-8' which can be changed by setting theencoding parameter to something else. Due to the nature of eggs it’s only possible to reload templates if the package was loaded from the file system and not a zip file.

class jinja2.DictLoader(mapping)

Loads a template from a python dict. It’s passed a dict of unicode strings bound to template names. This loader is useful for unittesting:

>>> loader = DictLoader({'index.html': 'source here'})

Because auto reloading is rarely useful this is disabled per default.

class jinja2.FunctionLoader(load_func)

A loader that is passed a function which does the loading. The function becomes the name of the template passed and has to return either an unicode string with the template source, a tuple in the form (source, filename, uptodatefunc) or None if the template does not exist.

>>> def load_template(name):
...     if name == 'index.html':
...         return '...'
...
>>> loader = FunctionLoader(load_template)

The uptodatefunc is a function that is called if autoreload is enabled and has to returnTrue if the template is still up to date. For more details have a look atBaseLoader.get_source() which has the same return value.

class jinja2.PrefixLoader(mapping, delimiter='/')

A loader that is passed a dict of loaders where each loader is bound to a prefix. The prefix is delimited from the template by a slash per default, which can be changed by setting the delimiter argument to something else:

loader = PrefixLoader({
    'app1':     PackageLoader('mypackage.app1'),
    'app2':     PackageLoader('mypackage.app2')
})

By loading 'app1/index.html' the file from the app1 package is loaded, by loading'app2/index.html' the file from the second.

class jinja2.ChoiceLoader(loaders)

This loader works like the PrefixLoader just that no prefix is specified. If a template could not be found by one loader the next one is tried.

>>> loader = ChoiceLoader([
...     FileSystemLoader('/path/to/user/templates'),
...     FileSystemLoader('/path/to/system/templates')
... ])

This is useful if you want to allow users to override builtin templates from a different location.

class jinja2.ModuleLoader(path)

This loader loads templates from precompiled templates.

Example usage:

>>> loader = ChoiceLoader([
...     ModuleLoader('/path/to/compiled/templates'),
...     FileSystemLoader('/path/to/templates')
... ])

Templates can be precompiled with Environment.compile_templates().