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The Flask class is the core of the entire framework. It runs as a WSGI application and centrally manages all functionalities including routing, templates, configuration, hooks, etc. Creation method: app = Flask( __name__ ) * * * ## Constructor Parameters The following are all parameters that can be passed when creating a Flask instance: | Parameter | Type | Default | Description | | --- | --- | --- | --- | | import_name | str | Required | The application module/package name. Flask uses it to locate templates and static files. Pass __name__ for a single file, or the package name for a package | | static_url_path | str | None | The URL path exposed for static files. If not set, uses the folder name of static_folder, e.g., /static | | static_folder | str | "static" | The folder path for static files (CSS, JS, images), relative to root_path or absolute path | | static_host | str | None | The hostname for static file routing, only needed when host_matching=True | | host_matching | bool | False | Whether to match routes by Host header | | subdomain_matching | bool | False | Whether to enable subdomain routing matching | | template_folder | str | "templates" | The folder path for Jinja2 template files | | instance_path | str | None (auto-discovered) | The instance folder path, stores files not committed to version control at runtime. Set to absolute path | | instance_relative_config | bool | False | When True, relative paths in config files are based on instance_path instead of root_path | | root_path | str | None (auto-discovered) | The root path of the application, usually automatically inferred, only needs manual setting for namespace packages | * * * ## Core Attributes | Attribute | Type | Description | | --- | --- | --- | | name | str | Application name. If import_name is __main__, takes the main filename | | config | Config | Configuration dictionary object, stores all configuration items | | debug | bool | Whether debug mode is enabled. Equivalent to config | | testing | bool | Whether test mode is enabled. Equivalent to config | | secret_key | str | Security key, used for Session signing. Equivalent to config | | permanent_session_lifetime | timedelta | Lifetime of permanent Session, default 31 days | | url_map | Map | Werkzeug URL mapping table, stores all routing rules | | view_functions | dict | endpoint → view function mapping dictionary | | blueprints | dict | Registered blueprint name → Blueprint instance mapping | | extensions | dict | Extension storage dictionary, extensions store their state by module name | | logger | Logger | Python standard logging.Logger, application name as logger name | | jinja_env | Environment | Jinja2 environment instance, controls template loading method | | json | JSONProvider | JSON processing provider instance, can be replaced with custom JSON library | | aborter | Aborter | HTTP exception thrower, called by abort() function | | cli | AppGroup | Click command group, can add custom CLI commands | | instance_path | str | Absolute path of instance folder | | root_path | str | Absolute path of application root directory | * * * ## Overridable Class Attributes The following attributes can be overridden in subclasses to customize framework behavior: | Attribute | Default | Description | | --- | --- | --- | | request_class | Request | Class used to create request objects | | response_class | Response | Class used to create response objects | | session_interface | SecureCookieSessionInterface() | Session backend interface, can be replaced with Redis/Database Session | | jinja_environment | Environment | Jinja2 environment class | | app_ctx_globals_class | _AppCtxGlobals | Class for g object | | config_class | Config | Class for configuration objects | | aborter_class | Aborter | Class for exception thrower | | json_provider_class | DefaultJSONProvider | Class for JSON processor | | url_rule_class | Rule | URL rule class (Werkzeug) | | url_map_class | Map | URL mapping class (Werkzeug) | | test_client_class | None (default FlaskClient) | Class for test client | | test_cli_runner_class | None (default FlaskCliRunner) | Class for CLI test runner | | jinja_options | {} | Extra options passed to Jinja2 environment | * * * ## URL Routing Methods | Method | Signature | Description | | --- | --- | --- | | route | (rule, **options) | Decorator, binds a URL rule to a view function | | get | (rule, **options) | Decorator, binds only GET method (equivalent to methods=) | | post | (rule, **options) | Decorator, binds only POST method | | put | (rule, **options) | Decorator, binds only PUT method | | delete | (rule, **options) | Decorator, binds only DELETE method | | patch | (rule, **options) | Decorator, binds only PATCH method | | add_url_rule | (rule, endpoint, view_func, provide_automatic_options, **options) | Programmatically add URL rule. The route decorator internally calls this method | | url_for | (endpoint, *, _anchor, _method, _scheme, _external, **values) | Generate URL based on endpoint and parameters. In blueprints, endpoint needs blueprint name prefix | | register_blueprint | (blueprint, **options) | Register a blueprint, can specify url_prefix, subdomain and other override parameters | | iter_blueprints | () | Iterate through all blueprints in registration order | > The options supported by the route decorator include: methods (HTTP method list), endpoint (custom endpoint name), defaults (default parameters), subdomain (subdomain), etc. * * * ## Request Lifecycle Hooks (Decorators) The following methods can all be used as decorators to insert custom logic at different stages of the request. | Method | Execution Time | Return Non-None Behavior | Typical Use | | --- | --- | --- | --- | | before_request | Before route dispatch | Return value is used as final response, view function is skipped | Login check, permission verification | | after_request | After response is built, before sending | Must receive and return Response object | Add security headers, CORS headers | | teardown_request | When request context pops | Return value is ignored | Release request-level resources | | teardown_appcontext | When application context pops | Return value is ignored | Close database connections | | errorhandler | When specified status code or exception occurs | Return value is used as error page response | Custom 404/500 pages | | url_value_preprocessor | After URL variables are extracted, before view is called | Return value is ignored | Preprocess URL parameters | | url_defaults | When url_for builds URL | Modify the values dictionary passed in | Add default parameters to url_for | | template_filter | — (registration nature) | — | Register Jinja2 template filter | | template_test | — (registration nature) | — | Register Jinja2 template test | | template_global | — (registration nature) | — | Register Jinja2 global function | | shell_context_processor | When flask shell starts | Return dict, merged into shell context | Pre-import common objects into flask shell | * * * ## Request Processing Methods The following methods are internally called during request processing and can be overridden in subclasses: | Method | Description | | --- | --- | | preprocess_request(ctx) | Request preprocessing: executes before_request functions and URL preprocessors | | dispatch_request(ctx) | Core dispatch: matches URL, calls corresponding view function | | make_response(rv) | Converts view function return value to Response object | | finalize_request(ctx, rv) | Finalization: calls make_response and after_request hooks | | process_response(ctx, response) | Post-response processing: executes after_request functions and Session saving | | full_dispatch_request(ctx) | Complete request dispatch entry: preprocess → dispatch → finalize | | handle_http_exception(ctx, e) | Handle HTTP exceptions, find corresponding errorhandler | | handle_user_exception(ctx, e) | Handle user code exceptions, distinguish HTTP exceptions from regular exceptions | | handle_exception(ctx, e) | Final exception handling, returns 500 response | | ensure_sync(func) | Ensure function executes synchronously. Async functions are wrapped to be synchronous