flask中的url_for

经常在flask代码中看到endpoint，翻译成中文是终点的意思？

不清楚，也不晓得什么意思，反正感觉是有点崩溃的。

直到看了url_for的源码才晓得。

def url_for(endpoint, **values):
    """Generates a URL to the given endpoint with the method provided.

    Variable arguments that are unknown to the target endpoint are appended
    to the generated URL as query arguments.  If the value of a query argument
    is ``None``, the whole pair is skipped.  In case blueprints are active
    you can shortcut references to the same blueprint by prefixing the
    local endpoint with a dot (``.``).

    This will reference the index function local to the current blueprint::

        url_for('.index')

    For more information, head over to the :ref:`Quickstart <url-building>`.

    To integrate applications, :class:`Flask` has a hook to intercept URL build
    errors through :attr:`Flask.url_build_error_handlers`.  The `url_for`
    function results in a :exc:`~werkzeug.routing.BuildError` when the current
    app does not have a URL for the given endpoint and values.  When it does, the
    :data:`~flask.current_app` calls its :attr:`~Flask.url_build_error_handlers` if
    it is not ``None``, which can return a string to use as the result of
    `url_for` (instead of `url_for`'s default to raise the
    :exc:`~werkzeug.routing.BuildError` exception) or re-raise the exception.
    An example::

        def external_url_handler(error, endpoint, values):
            "Looks up an external URL when `url_for` cannot build a URL."
            # This is an example of hooking the build_error_handler.
            # Here, lookup_url is some utility function you've built
            # which looks up the endpoint in some external URL registry.
            url = lookup_url(endpoint, **values)
            if url is None:
                # External lookup did not have a URL.
                # Re-raise the BuildError, in context of original traceback.
                exc_type, exc_value, tb = sys.exc_info()
                if exc_value is error:
                    raise exc_type, exc_value, tb
                else:
                    raise error
            # url_for will use this result, instead of raising BuildError.
            return url

        app.url_build_error_handlers.append(external_url_handler)

    Here, `error` is the instance of :exc:`~werkzeug.routing.BuildError`, and
    `endpoint` and `values` are the arguments passed into `url_for`.  Note
    that this is for building URLs outside the current application, and not for
    handling 404 NotFound errors.

    .. versionadded:: 0.10
       The `_scheme` parameter was added.

    .. versionadded:: 0.9
       The `_anchor` and `_method` parameters were added.

    .. versionadded:: 0.9
       Calls :meth:`Flask.handle_build_error` on
       :exc:`~werkzeug.routing.BuildError`.

    :param endpoint: the endpoint of the URL (name of the function)
    :param values: the variable arguments of the URL rule
    :param _external: if set to ``True``, an absolute URL is generated. Server
      address can be changed via ``SERVER_NAME`` configuration variable which
      defaults to `localhost`.
    :param _scheme: a string specifying the desired URL scheme. The `_external`
      parameter must be set to ``True`` or a :exc:`ValueError` is raised. The default
      behavior uses the same scheme as the current request, or
      ``PREFERRED_URL_SCHEME`` from the :ref:`app configuration <config>` if no
      request context is available. As of Werkzeug 0.10, this also can be set
      to an empty string to build protocol-relative URLs.
    :param _anchor: if provided this is added as anchor to the URL.
    :param _method: if provided this explicitly specifies an HTTP method.
    """
    appctx = _app_ctx_stack.top
    reqctx = _request_ctx_stack.top

    if appctx is None:
        raise RuntimeError(
            'Attempted to generate a URL without the application context being'
            ' pushed. This has to be executed when application context is'
            ' available.'
        )

    # If request specific information is available we have some extra
    # features that support "relative" URLs.
    if reqctx is not None:
        url_adapter = reqctx.url_adapter
        blueprint_name = request.blueprint

        if endpoint[:1] == '.':
            if blueprint_name is not None:
                endpoint = blueprint_name + endpoint
            else:
                endpoint = endpoint[1:]

        external = values.pop('_external', False)

    # Otherwise go with the url adapter from the appctx and make
    # the URLs external by default.
    else:
        url_adapter = appctx.url_adapter

        if url_adapter is None:
            raise RuntimeError(
                'Application was not able to create a URL adapter for request'
                ' independent URL generation. You might be able to fix this by'
                ' setting the SERVER_NAME config variable.'
            )

        external = values.pop('_external', True)

    anchor = values.pop('_anchor', None)
    method = values.pop('_method', None)
    scheme = values.pop('_scheme', None)
    appctx.app.inject_url_defaults(endpoint, values)

    # This is not the best way to deal with this but currently the
    # underlying Werkzeug router does not support overriding the scheme on
    # a per build call basis.
    old_scheme = None
    if scheme is not None:
        if not external:
            raise ValueError('When specifying _scheme, _external must be True')
        old_scheme = url_adapter.url_scheme
        url_adapter.url_scheme = scheme

    try:
        try:
            rv = url_adapter.build(endpoint, values, method=method,
                                   force_external=external)
        finally:
            if old_scheme is not None:
                url_adapter.url_scheme = old_scheme
    except BuildError as error:
        # We need to inject the values again so that the app callback can
        # deal with that sort of stuff.
        values['_external'] = external
        values['_anchor'] = anchor
        values['_method'] = method
        values['_scheme'] = scheme
        return appctx.app.handle_url_build_error(error, endpoint, values)

    if anchor is not None:
        rv += '#' + url_quote(anchor)
    return rv

虽说整体上处于看不明白的状态，但是至少懂了点。

根据其中对url_for的参数endpoint的解释

:param endpoint: the endpoint of the URL (name of the function)

我感觉意思就是函数的名字对应的url映射。

既然可以根据url找到相应的处理函数，那么根据处理函数，是不是也是可以同样找到对应的url呢？

答案当然就是肯定的。url_for就是用于满足这个功能的。

定义函数的时候使用**values表示的是，不定项的具名参数。

在函数内部，values就是一个字典了。

需要传入的参数呢包括endpoint需要的参数。

如果还有些不在endpoint只中呢，就是当成参数放在url的后面。

例如传入一个a=1，而a不属于endpoint中需要的参数

那么这个a=1就会在这个url_for产生的url后面添加如下部分

'?a=1'