Flask: Creating objects that remain over multiple requests
Question:
I’ve been able to create objects that are created at every request from this link: http://flask.pocoo.org/docs/appcontext/#locality-of-the-context.
I’m actually creating an API based off of http://blog.miguelgrinberg.com/post/designing-a-restful-api-using-flask-restful.
I want to be able to load an object once and just have it return a processed response rather than it loading at every request. The object is not a DB, just requires unpickling a large file.
I’ve looked through the documentation, but I’m still confused about this whole Flask two states thing.
Answers:
The Flask contexts only apply per request. Use a module global to store data you only want to load once.
You could just load the data on startup, as a global:
some_global_name = load_data_from_pickle()
WSGI servers that support multiple processes either fork the process, or start a new Python interpreter as needed. When forking, globals are copied to the child process.
You can also use before_first_request() hook to load that data into your process; this is only called if the process has to handle an actual request. This would be after the process fork, giving your child process unique data:
@app.before_first_request
def load_global_data():
global some_global_name
some_global_name = load_data_from_pickle()
Update the answer written by Martijn Pieters.
before_first_request() has been deprecated since Flask 2.2 and will be removed in Flask 2.3. It’s suggested to "Run setup code while creating the application instead."
I’ve been able to create objects that are created at every request from this link: http://flask.pocoo.org/docs/appcontext/#locality-of-the-context.
I’m actually creating an API based off of http://blog.miguelgrinberg.com/post/designing-a-restful-api-using-flask-restful.
I want to be able to load an object once and just have it return a processed response rather than it loading at every request. The object is not a DB, just requires unpickling a large file.
I’ve looked through the documentation, but I’m still confused about this whole Flask two states thing.
The Flask contexts only apply per request. Use a module global to store data you only want to load once.
You could just load the data on startup, as a global:
some_global_name = load_data_from_pickle()
WSGI servers that support multiple processes either fork the process, or start a new Python interpreter as needed. When forking, globals are copied to the child process.
You can also use before_first_request() hook to load that data into your process; this is only called if the process has to handle an actual request. This would be after the process fork, giving your child process unique data:
@app.before_first_request
def load_global_data():
global some_global_name
some_global_name = load_data_from_pickle()
Update the answer written by Martijn Pieters.
before_first_request() has been deprecated since Flask 2.2 and will be removed in Flask 2.3. It’s suggested to "Run setup code while creating the application instead."