Flask context and "working outside application context"

First, the reason

1. Application of the object beyond the scope of the application context
2. Stack _app_ctx_stack is empty

Second, the solution

act = app.app_context()
act.push()
# Current_app.xxxxx application operation
act.pop()

Note: pycharm breakpoint debugging, current_app may appear LocalProxy: unbound. Mouse over current_app will show the correct content.

Third, in-depth understanding

1. Flask context
   1. The AppContext : encapsulates Flask instance object and other methods and parameters.
   2. The RequestContext : encapsulates Request objects and other methods and parameters.
   3. They include Push () , POP () , The __enter __ () , The __exit __ () method.
   4. Programming process using Flask and Request objects from the context of acquisition.
2. Out stack
   1. A request is received, the package RequestContext , check LocalStack if (application context stack) the stack is empty, if empty, the AppContext push ( Push () ) to the stack (_app_ctx_stack), the then RequestContext push stack (_request_ctx_stack) in. When the request, two elements of the stack would be popped ( POP () ).
   2. current_app and request are ( LocalProxy ) directed application context stack top element and the request context. It returns Flask instance object app request objects Request .
   3. Manually stack, the stack
      1. act = app.app_context () Get AppContext
      2. act.push () push _app_ctx_stack stack
      3. act.pop () out of the stack, removing elements
      4. The stack is generally applied manually to the off-line access applications and unit tests.

Fourth, the context manager

1. basic concepts
   1. Protocol context: implements The __enter __ () and The __exit __ () two methods, i.e., to achieve a protocol context;
   2. Context Manager: object that implements the context of the agreement.
   3. The context expression: with the statement, and with must return a context manager.
2. __enter__()
   1. with demo () as d: d values is The __enter __ () method returns the value
3. __exit__()
   1. __exit__ method returns a Boolean value. If with the contents of the code block abnormal, __exit__ returns False , and will throw an exception. Conversely returns True .
   2. __exit__ method requires pass . 4 parameters, in addition to self other three are exc_type , ext_value , TB . When an exception occurs, exc_type value of the type of anomaly; ext_value is described abnormality; TB is abnormal stack information.
4. application
   1. Links and release of resources.
   2. Therefore, the manual out of the stack can be written as :

with app.app_context() as atc:
    #atc.xxxx
    #xxxx