背景：
这两天在看后端代码覆盖率平台代码的时候，发现启动服务只需要执行flask run命令即可。但是找了半天都没有看到工程中Flask app实例对象是在哪里创建的。工程中定义了一个create_app()函数，可是没有看到调用它的地方。带着疑惑，尝试在工程中create_app()函数主动raise一个异常，来看看flask run从入口函数是怎么运行到create_app()，是如何调用create_app()函数的。
1、在命令行中执行flask启动命令flask run，其内部执行的是工程名\venv\Lib\site-packages\flask\cli.py 中的入口函数：
def main() -> None:if int(click.__version__[0]) < 8:warnings.warn("Using the `flask` cli with Click 7 is deprecated and"" will not be supported starting with Flask 2.1."" Please upgrade to Click 8 as soon as possible.",DeprecationWarning,)# TODO omit sys.argv once https://github.com/pallets/click/issues/536 is fixedcli.main(args=sys.argv[1:])if __name__ == "__main__":main()可以看到，主要的逻辑是在cli.main()函数中。
2、cli.main()函数
def main(self, *args, **kwargs):# Set a global flag that indicates that we were invoked from the# command line interface. This is detected by Flask.run to make the# call into a no-op. This is necessary to avoid ugly errors when the# script that is loaded here also attempts to start a server.os.environ["FLASK_RUN_FROM_CLI"] = "true"if get_load_dotenv(self.load_dotenv):load_dotenv()obj = kwargs.get("obj")if obj is None:obj = ScriptInfo(create_app=self.create_app, set_debug_flag=self.set_debug_flag)kwargs["obj"] = objkwargs.setdefault("auto_envvar_prefix", "FLASK")return super().main(*args, **kwargs)其中在load_dotenv()函数中加载工程下的".env"或".flaskenv"文件进行设置环境变量。.flaskenv文件的内容可以如下：
FLASK_APP=coverage_statistics# 默认环境变量设置为development
FLASK_ENV=development
# FLASK_ENV=production
# FLASK_DEBUG = True3、接着调用super.main()
...中间省略很多步
n、调用find_best_app() 该函数根据给定的模块名称，尝试在该模块中找到一个最有可能的应用，没有找到就会报错。
def find_best_app(script_info, module):"""Given a module instance this tries to find the best possibleapplication in the module or raises an exception."""from . import Flask# Search for the most common names first.for attr_name in ("app", "application"):app = getattr(module, attr_name, None)if isinstance(app, Flask):return app# Otherwise find the only object that is a Flask instance.matches = [v for v in module.__dict__.values() if isinstance(v, Flask)]if len(matches) == 1:return matches[0]elif len(matches) > 1:raise NoAppException("Detected multiple Flask applications in module"f" {module.__name__!r}. Use 'FLASK_APP={module.__name__}:name'"f" to specify the correct one.")# Search for app factory functions.for attr_name in ("create_app", "make_app"):app_factory = getattr(module, attr_name, None)if inspect.isfunction(app_factory):try:app = call_factory(script_info, app_factory)if isinstance(app, Flask):return appexcept TypeError as e:if not _called_with_wrong_args(app_factory):raiseraise NoAppException(f"Detected factory {attr_name!r} in module {module.__name__!r},"" but could not call it without arguments. Use"f" \"FLASK_APP='{module.__name__}:{attr_name}(args)'\""" to specify arguments.") from eraise NoAppException("Failed to find Flask application or factory in module"f" {module.__name__!r}. Use 'FLASK_APP={module.__name__}:name'"" to specify one.")1、先尝试在模块中查找是否有app或application名称的文件，没有则进入第二步；
2、然后查找模块中是否有Flask实例的属性对象，有一个则表明找到了，有多个则报错，没有则进入第三步；
3、查找是否有app工程方法“create_app”或"make_app"，如果存在，则调用该工程方法创建app实例并返回，如果没有，则报错。
至此，终于知道Flask app对象是怎么创建，在哪里创建的了。
)
)
