[flask advanced] deeply understand the application context and request context of flask from the source code

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Preface

Before that flask The introductory series probably wrote flask Application context and request context , It's more simple , But we need to know later flask Operation mechanism of , You must first have a deep understanding flask Application context and request context in , In this section, Daniel will continue to take you to analyze his mystery from the source code ！

Context source code analysis

stay site-packages Find flask Folder , Find... In it ctx.py file , There are some context related contents ！

Pictured above , Click the structure on the left , It's easy to find our application context AppContext and RequestContext.

and AppContext It's against us. flask An encapsulation of core objects , We can have a look AppContext Constructor of this class ：

    def __init__(self, app: "Flask") -> None:
        self.app = app
        self.url_adapter = app.create_url_adapter(None)
        self.g = app.app_ctx_globals_class()

        # Like request context, app contexts can be pushed multiple times
        # but there a basic "refcount" is enough to track them.
        self._refcnt = 0

You can see such a line of code ： self.app = app, there app We passed app = Flask(name) Instantiated flask The core object ,flask Encapsulate this core object into AppContext.

Pictured above ,AppContext Four methods are defined below , These four methods are actually operations AppContext From stack to stack , I won't go into details here , Write... At the back flask We will also mention .

Next, let's look at the request context RequestContext

You can see RequestContext The constructor of is also applicable to our flask The core object is encapsulated , And it encapsulates request Request object ！

We continue to slide down , Here's the picture ：

We are RequestContext We also see four and AppContext In the same way ！ It is also a stack structure

Thus summarize ：Flask The core objects are stored in AppContext in ,Request Request object in RequestContext in ！

Some people may have some doubts here , We operate this directly Flask Isn't the core object good , Why package , This actually involves our design pattern , Sometimes we design an object , But we found that some parameters are external to this object , At this time, we need to encapsulate these parameters and this object into a new object ,flask Just did such a thing , take Flask The core object and some external parameters are encapsulated into AppContext, Context object ,RequestContext So it is with ！

summary

Read the source code , We need to find out now Flask, AppContext, Request, RequestContext The relationship between .

• Flask Is our core object , The core object carries various functions , For example, it saves the information of our configuration file , Provides registered routes 、 A series of functions such as view functions .
• AppContext take Flask The core object is encapsulated , And additional parameters are attached .
• Request Object holds our request information , For example, related url Parameters of .
• RequestContext take Request Objects are encapsulated , And additional parameters are attached .

We often use Flask Core objects and Request object , Through the understanding of the source code , Now we can think , We can not only pass from … import The import method uses Flask Core objects and Request object , And it can also be passed AppContext,RequestContext To indirectly obtain , So that's the point flask The core of the operation mechanism , Namely LocalProxy( agent ）, Okay , Here is a brief mention , Let's continue in the next section ！

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