2018-03-13T20:13:46Z

The Flask Mega-Tutorial Part XV: A Better Application Structure

This is the fifteenth installment of the Flask Mega-Tutorial series, in which I'm going to restructure the application using a style that is appropriate for larger applications.

For your reference, below is a list of the articles in this series.

Microblog is already an application of a decent size, so I thought this is a good opportunity to discuss how a Flask application can grow without becoming messy or too difficult to manage. Flask is a framework that is designed to give you the option to organize your project in any way you want, and as part of that philosophy, it makes it possible to change or adapt the structure of the application as it becomes larger, or as your needs or level of experience change.

In this chapter I'm going to discuss some patterns that apply to large applications, and to demonstrate them I'm going to make some changes to the way my Microblog project is structured, with the goal of making the code more maintainable and better organized. But of course, in true Flask spirit, I encourage you to take these changes just as a recommendation when trying to decide on a way to organize your own projects.

The GitHub links for this chapter are: Browse, Zip, Diff.

Current Limitations

There are two basic problems with the application in its current state. If you look at how the application is structured, you are going to notice that there are a few different subsystems that can be identified, but the code that supports them is all intermixed, without any clear boundaries. Let's review what those subsystems are:

  • The user authentication subsystem, which includes some view functions in app/routes.py, some forms in app/forms.py, some templates in app/templates and the email support in app/email.py.
  • The error subsystem, which defines error handlers in app/errors.py and templates in app/templates.
  • The core application functionality, which includes displaying and writing blog posts, user profiles and following, and live translations of blog posts, which is spread through most of the application modules and templates.

Thinking about these three subsystems that I have identified and how they are structured, you can probably notice a pattern. So far, the organization logic that I've been following is based on having modules dedicated to different application functions. There is a module for view functions, another one for web forms, one for errors, one for emails, a directory for HTML templates, and so on. While this is a structure that makes sense for small projects, once a project starts to grow, it tends to make some of these modules really large and messy.

One way to clearly see the problem is to consider how you would start a second project by reusing as much as you can from this one. For example, the user authentication portion should work well in other applications, but if you wanted to use that code as it is, you would have to go into several modules and copy/paste the pertinent sections into new files in the new project. See how inconvenient that is? Wouldn't it be better if this project had all the authentication related files separated from the rest of the application? The blueprints feature of Flask helps achieve a more practical organization that makes it easier to reuse code.

There is a second problem that is not that evident. The Flask application instance is created as a global variable in app/__init__.py, and then imported by a lot of application modules. While this in itself is not a problem, having the application as a global variable can complicate certain scenarios, in particular those related to testing. Imagine you want to test this application under different configurations. Because the application is defined as a global variable, there is really no way to instantiate two applications that use different configuration variables. Another situation that is not ideal is that all the tests use the same application, so a test could be making changes to the application that affect another test that runs later. Ideally you want all tests to run on a pristine application instance.

You can actually see in the tests.py module that I'm resorting to the trick of modifying the configuration after it was set in the application to direct the tests to use an in-memory database instead of the default SQLite database based on disk. I really have no other way to change the configured database, because by the time the tests start the application has been created and configured. For this particular situation, changing the configuration after it was applied to the application appears to work fine, but in other cases it may not, and in any case, it is a bad practice that can lead to obscure and difficult to find bugs.

A better solution would be to not use a global variable for the application, and instead use an application factory function to create the function at runtime. This would be a function that accepts a configuration object as an argument, and returns a Flask application instance, configured with those settings. If I could modify the application to work with an application factory function, then writing tests that require special configuration would become easy, because each test can create its own application.

In this chapter I'm going to refactor the application to introduce blueprints for the three subsystems I have identified above, and an application factory function. Showing you the detailed list of changes is going to be impractical, because there are little changes in pretty much every file that is part of the application, so I'm going to discuss the steps that I took to do the refactoring, and you can then download the application with these changes made.

Blueprints

In Flask, a blueprint is a logical structure that represents a subset of the application. A blueprint can include elements such as routes, view functions, forms, templates and static files. If you write your blueprint in a separate Python package, then you have a component that encapsulates the elements related to specific feature of the application.

The contents of a blueprint are initially in a dormant state. To associate these elements, the blueprint needs to be registered with the application. During the registration, all the elements that were added to the blueprint are passed on to the application. So you can think of a blueprint as a temporary storage for application functionality that helps in organizing your code.

Error Handling Blueprint

The first blueprint that I created was one that encapsulates the support for error handlers. The structure of this blueprint is as follows:

app/
    errors/                             <-- blueprint package
        __init__.py                     <-- blueprint creation
        handlers.py                     <-- error handlers
    templates/
        errors/                         <-- error templates
            404.html
            500.html
    __init__.py                         <-- blueprint registration

In essence, what I did is move the app/errors.py module into app/errors/handlers.py and the two error templates into app/templates/errors, so that they are separated from the other templates. I also had to change the render_template() calls in both error handlers to use the new errors template sub-directory. After that I added the blueprint creation to the app/errors/__init__.py module, and the blueprint registration to app/__init__.py, after the application instance is created.

I should note that Flask blueprints can be configured to have a separate directory for templates or static files. I have decided to move the templates into a sub-directory of the application's template directory so that all templates are in a single hierarchy, but if you prefer to have the templates that belong to a blueprint inside the blueprint package, that is supported. For example, if you add a template_folder='templates' argument to the Blueprint() constructor, you can then store the blueprint's templates in app/errors/templates.

The creation of a blueprint is fairly similar to the creation of an application. This is done in the ___init__.py module of the blueprint package:

app/errors/__init__.py: Errors blueprint.

from flask import Blueprint

bp = Blueprint('errors', __name__)

from app.errors import handlers

The Blueprint class takes the name of the blueprint, the name of the base module (typically set to __name__ like in the Flask application instance), and a few optional arguments, which in this case I do not need. After the blueprint object is created, I import the handlers.py module, so that the error handlers in it are registered with the blueprint. This import is at the bottom to avoid circular dependencies.

In the handlers.py module, instead of attaching the error handlers to the application with the @app.errorhandler decorator, I use the blueprint's @bp.app_errorhandler decorator. While both decorators achieve the same end result, the idea is to try to make the blueprint independent of the application so that it is more portable. I also need to modify the path to the two error templates to account for the new errors sub-directory where they were moved.

The final step to complete the refactoring of the error handlers is to register the blueprint with the application:

app/__init__.py: Register the errors blueprint with the application.

app = Flask(__name__)

# ...

from app.errors import bp as errors_bp
app.register_blueprint(errors_bp)

# ...

from app import routes, models  # <-- remove errors from this import!

To register a blueprint, the register_blueprint() method of the Flask application instance is used. When a blueprint is registered, any view functions, templates, static files, error handlers, etc. are connected to the application. I put the import of the blueprint right above the app.register_blueprint() to avoid circular dependencies.

Authentication Blueprint

The process to refactor the authentication functions of the application into a blueprint is fairly similar to that of the error handlers. Here is a diagram of the refactored blueprint:

app/
    auth/                               <-- blueprint package
        __init__.py                     <-- blueprint creation
        email.py                        <-- authentication emails
        forms.py                        <-- authentication forms
        routes.py                       <-- authentication routes
    templates/
        auth/                           <-- blueprint templates
            login.html
            register.html
            reset_password_request.html
            reset_password.html
    __init__.py                         <-- blueprint registration

To create this blueprint I had to move all the authentication related functionality to new modules I created in the blueprint. This includes a few view functions, web forms, and support functions such as the one that sends password reset tokens by email. I also moved the templates into a sub-directory to separate them from the rest of the application, like I did with the error pages.

When defining routes in a blueprint, the @bp.route decorate is used instead of @app.route. There is also a required change in the syntax used in the url_for() to build URLs. For regular view functions attached directly to the application, the first argument to url_for() is the view function name. When a route is defined in a blueprint, this argument must include the blueprint name and the view function name, separated by a period. So for example, I had to replace all occurrences of url_for('login') with url_for('auth.login'), and same for the remaining view functions.

To register the auth blueprint with the application, I used a slightly different format:

app/__init__.py: Register the authentication blueprint with the application.

# ...
from app.auth import bp as auth_bp
app.register_blueprint(auth_bp, url_prefix='/auth')
# ...

The register_blueprint() call in this case has an extra argument, url_prefix. This is entirely optional, but Flask gives you the option to attach a blueprint under a URL prefix, so any routes defined in the blueprint get this prefix in their URLs. In many cases this is useful as a sort of "namespacing" that keeps all the routes in the blueprint separated from other routes in the application or other blueprints. For authentication, I thought it was nice to have all the routes starting with /auth, so I added the prefix. So now the login URL is going to be http://localhost:5000/auth/login. Because I'm using url_for() to generate the URLs, all URLs will automatically incorporate the prefix.

Main Application Blueprint

The third blueprint contains the core application logic. Refactoring this blueprint requires the same process that I used with the previous two blueprints. I gave this blueprint the name main, so all url_for() calls that referenced view functions had to get a main. prefix. Given that this is the core functionality of the application, I decided to leave the templates in the same locations. This is not a problem because I have moved the templates from the other two blueprints into sub-directories.

The Application Factory Pattern

As I mentioned in the introduction to this chapter, having the application as a global variable introduces some complications, mainly in the form of limitations for some testing scenarios. Before I introduced blueprints, the application had to be a global variable, because all the view functions and error handlers needed to be decorated with decorators that come from app, such as @app.route. But now that all routes and error handlers were moved to blueprints, there are a lot less reasons to keep the application global.

So what I'm going to do, is add a function called create_app() that constructs a Flask application instance, and eliminate the global variable. The transformation was not trivial, I had to sort out a few complications, but let's first look at the application factory function:

app/__init__.py: Application factory function.

# ...
db = SQLAlchemy()
migrate = Migrate()
login = LoginManager()
login.login_view = 'auth.login'
login.login_message = _l('Please log in to access this page.')
mail = Mail()
bootstrap = Bootstrap()
moment = Moment()
babel = Babel()

def create_app(config_class=Config):
    app = Flask(__name__)
    app.config.from_object(config_class)

    db.init_app(app)
    migrate.init_app(app, db)
    login.init_app(app)
    mail.init_app(app)
    bootstrap.init_app(app)
    moment.init_app(app)
    babel.init_app(app)

    # ... no changes to blueprint registration

    if not app.debug and not app.testing:
        # ... no changes to logging setup

    return app

You have seen that most Flask extensions are initialized by creating an instance of the extension and passing the application as an argument. When the application does not exist as a global variable, there is an alternative mode in which extensions are initialized in two phases. The extension instance is first created in the global scope as before, but no arguments are passed to it. This creates an instance of the extension that is not attached to the application. At the time the application instance is created in the factory function, the init_app() method must be invoked on the extension instances to bind it to the now known application.

Other tasks performed during initialization remain the same, but are moved to the factory function instead of being in the global scope. This includes the registration of blueprints and logging configuration. Note that I have added a not app.testing clause to the conditional that decides if email and file logging should be enabled or not, so that all this logging is skipped during unit tests. The app.testing flag is going to be True when running unit tests, due to the TESTING variable being set to True in the configuration.

So who calls the application factory function? The obvious place to use this function is the top-level microblog.py script, which is the only module in which the application now exists in the global scope. The other place is in tests.py, and I will discuss unit testing in more detail in the next section.

As I mentioned above, most references to app went away with the introduction of blueprints, but there were some still in the code that I had to address. For example, the app/models.py, app/translate.py, and app/main/routes.py modules all had references to app.config. Fortunately, the Flask developers tried to make it easy for view functions to access the application instance without having to import it like I have been doing until now. The current_app variable that Flask provides is a special "context" variable that Flask initializes with the application before it dispatches a request. You have already seen another context variable before, the g variable in which I'm storing the current locale. These two, along with Flask-Login's current_user and a few others you haven't seen yet, are somewhat "magical" variables, in that they work like global variables, but are only accessible during the handling of a request, and only in the thread that is handling it.

Replacing app with Flask's current_app variable eliminates the need of importing the application instance as a global variable. I was able to change all references to app.config with current_app.config without any difficulty through simple search and replace.

The app/email.py module presented a slightly bigger challenge, so I had to use a small trick:

app/email.py: Pass application instance to another thread.

from flask import current_app

def send_async_email(app, msg):
    with app.app_context():
        mail.send(msg)

def send_email(subject, sender, recipients, text_body, html_body):
    msg = Message(subject, sender=sender, recipients=recipients)
    msg.body = text_body
    msg.html = html_body
    Thread(target=send_async_email,
           args=(current_app._get_current_object(), msg)).start()

In the send_email() function, the application instance is passed as an argument to a background thread that will then deliver the email without blocking the main application. Using current_app directly in the send_async_email() function that runs as a background thread would not have worked, because current_app is a context-aware variable that is tied to the thread that is handling the client request. In a different thread, current_app would not have a value assigned. Passing current_app directly as an argument to the thread object would not have worked either, because current_app is really a proxy object that is dynamically mapped to the application instance. So passing the proxy object would be the same as using current_app directly in the thread. What I needed to do is access the real application instance that is stored inside the proxy object, and pass that as the app argument. The current_app._get_current_object() expression extracts the actual application instance from inside the proxy object, so that is what I passed to the thread as an argument.

Another module that was tricky was app/cli.py, which implements a few shortcut commands for managing language translations. The current_app variable does not work in this case because these commands are registered at start up, not during the handling of a request, which is the only time when current_app can be used. To remove the reference to app in this module, I resorted to another trick, which is to move these custom commands inside a register() function that takes the app instance as an argument:

app/cli.py: Register custom application commands.

import os
import click

def register(app):
    @app.cli.group()
    def translate():
        """Translation and localization commands."""
        pass

    @translate.command()
    @click.argument('lang')
    def init(lang):
        """Initialize a new language."""
        # ...

    @translate.command()
    def update():
        """Update all languages."""
        # ...

    @translate.command()
    def compile():
        """Compile all languages."""
        # ...

Then I called this register() function from microblog.py. Here is the complete microblog.py after all the refactoring:

microblog.py: Main application module refactored.

from app import create_app, db, cli
from app.models import User, Post

app = create_app()
cli.register(app)

@app.shell_context_processor
def make_shell_context():
    return {'db': db, 'User': User, 'Post' :Post}

Unit Testing Improvements

As I hinted in the beginning of this chapter, a lot of the work that I did so far had the goal of improving the unit testing workflow. When you are running unit tests you want to make sure the application is configured in a way that it does not interfere with your development resources, such as your database.

The current version of tests.py resorts to the trick of modifying the configuration after it was applied to the application instance, which is a dangerous practice as not all types of changes will work when done that late. What I want is to have a chance to specify my testing configuration before it gets added to the application.

The create_app() function now accepts a configuration class as an argument. By default, the Config class defined in config.py is used, but I can now create an application instance that uses different configuration simply by passing a new class to the factory function. Here is an example configuration class that would be suitable to use for my unit tests:

tests.py: Testing configuration.

from config import Config

class TestConfig(Config):
    TESTING = True
    SQLALCHEMY_DATABASE_URI = 'sqlite://'

What I'm doing here is creating a subclass of the application's Config class, and overriding the SQLAlchemy configuration to use an in-memory SQLite database. I also added a TESTING attribute set to True, which I currently do not need, but could be useful if the application needs to determine if it is running under unit tests or not.

If you recall, my unit tests relied on the setUp() and tearDown() methods, invoked automatically by the unit testing framework to create and destroy an environment that is appropriate for each test to run. I can now use these two methods to create and destroy a brand new application for each test:

tests.py: Create an application for each test.

class UserModelCase(unittest.TestCase):
    def setUp(self):
        self.app = create_app(TestConfig)
        self.app_context = self.app.app_context()
        self.app_context.push()
        db.create_all()

    def tearDown(self):
        db.session.remove()
        db.drop_all()
        self.app_context.pop()

The new application will be stored in self.app, but creating an application isn't enough to make everything work. Consider the db.create_all() statement that creates the database tables. The db instance needs to know what the application instance is, because it needs to get the database URI from app.config, but when you are working with an application factory you are not really limited to a single application, there could be more than one created. So how does db know to use the self.app instance that I just created?

The answer is in the application context. Remember the current_app variable, which somehow acts as a proxy for the application when there is no global application to import? This variable looks for an active application context in the current thread, and if it finds one, it gets the application from it. If there is no context, then there is no way to know what application is active, so current_app raises an exception. Below you can see how this works in a Python console. This needs to be a console started by running python, because the flask shell command automatically activates an application context for convenience.

>>> from flask import current_app
>>> current_app.config['SQLALCHEMY_DATABASE_URI']
Traceback (most recent call last):
    ...
RuntimeError: Working outside of application context.

>>> from app import create_app
>>> app = create_app()
>>> app.app_context().push()
>>> current_app.config['SQLALCHEMY_DATABASE_URI']
'sqlite:////home/miguel/microblog/app.db'

So that's the secret! Before invoking your view functions, Flask pushes an application context, which brings current_app and g to life. When the request is complete, the context is removed, along with these variables. For the db.create_all() call to work in the unit testing setUp() method, I pushed an application context for the application instance I just created, and in that way, db.create_all() can use current_app.config to know where is the database. Then in the tearDown() method I pop the context to reset everything to a clean state.

You should also know that the application context is one of two contexts that Flask uses. There is also a request context, which is more specific, as it applies to a request. When a request context is activated right before a request is handled, Flask's request and session variables become available, as well as Flask-Login's current_user.

Environment Variables

As you have seen as I built this application, there are a number of configuration options that depend on having variables set up in your environment before you start the server. This includes your secret key, email server information, database URL, and Microsoft Translator API key. You'll probably agree with me that this is inconvenient, because each time you open a new terminal session those variables need to be set again.

A common pattern for applications that depend on lots of environment variables is to store these in a .env file in the root application directory. The application imports the variables in this file when it starts, and that way, there is no need to have all these variables manually set by you.

There is a Python package that supports .env files called python-dotenv, and it is already installed because I used it with the .flaskenv file earlier in the tutorial. While the .env and .flaskenv files are similar, Flask expects Flask's own configuration variables to be in .flaskenv, while application configuration variables (including some that can be of a sensitive nature) to be in .env. The .flaskenv file can be added to your source control, as it does not contain any secrets or passwords. The .env file is not supposed to be added to source control to ensure that your secrets are protected.

The flask command automatically imports into the environment any variables defined in the .flaskenv and .env files. This is sufficient for the .flaskenv file, because its contents are only needed when running the application through the flask command. The .env file, however, is going to be used also in the production deployment of this application, which is not going to use the flask command. For that reason, it is a good idea to explicitly import the contents of the .env file.

Since the config.py module is where I read all the environment variables, I'm going to import the .env file before the Config class is created, so that the variables are already set when the class is constructed:

config.py: Import a .env file with environment variables.

import os
from dotenv import load_dotenv

basedir = os.path.abspath(os.path.dirname(__file__))
load_dotenv(os.path.join(basedir, '.env'))

class Config(object):
    # ...

So now you can create a .env file with all the environment variables that your application needs. It is important that you do not add your .env file to source control. You do not want to have a file that contains passwords and other sensitive information included in your source code repository.

The .env file can be used for all the configuration-time variables, but it cannot be used for Flask's FLASK_APP and FLASK_DEBUG environment variables, because these are needed very early in the application bootstrap process, before the application instance and its configuration object exist.

The following example shows a .env file that defines a secret key, configures email to go out on a locally running mail server on port 25 and no authentication, sets up the Microsoft Translator API key, and leaves the database configuration to use the defaults:

SECRET_KEY=a-really-long-and-unique-key-that-nobody-knows
MAIL_SERVER=localhost
MAIL_PORT=25
MS_TRANSLATOR_KEY=<your-translator-key-here>

Requirements File

At this point I have installed a fair number of packages in the Python virtual environment. If you ever need to regenerate your environment on another machine, you are going to have trouble remembering what packages you had to install, so the generally accepted practice is to write a requirements.txt file in the root folder of your project listing all the dependencies, along with their versions. Producing this list is actually easy:

(venv) $ pip freeze > requirements.txt

The pip freeze command will dump all the packages that are installed on your virtual environment in the correct format for the requirements.txt file. Now, if you need to create the same virtual environment on another machine, instead of installing packages one by one, you can run:

(venv) $ pip install -r requirements.txt

235 comments

  • #126 Miguel Grinberg said 2019-09-06T18:53:30Z

    @Thomas: When you initialize the extension by passing the app instance there is no need to use a context, the extension already knows what the application is. When you initialize it without the app and then call init_app, the extension does not keep a link to the app instance, and instead relies on an app context to be available. Most Flask extensions work like this.

  • #127 cizario said 2019-09-07T12:05:05Z

    Hi Miguel; as per official Flask tutorial and many resources out there (github repos), i see that it is very common (and recommended as well) to put the Flask app factory function in the package's init.py file. i'm little bit confused, Why ? Why we shouldn't move the code to another file, let say factory.py and leave the init.py empty or at least just insert imports, all, version, author, docstrings ... if so how do i rearrange my code ?

    thx a lot

  • #128 Miguel Grinberg said 2019-09-08T07:25:26Z

    @cizario: you can put it on a factory.py module if you like, that should not be a problem. The only thing you need to keep in mind is that the module(s) that define the routes of he application need to be imported, so that they are registered with Flask. I do this import at the bottom of init.py, but you can do it in factory.py and it should work the same.

  • #129 Sebastian said 2019-10-20T10:33:51Z

    I know you have a compelling reason for writing everything in a specific way. I'm trying to track down why (in your book) you needed to split the import block to two parts (and make pep8 checkers unhappy). Is it "just" efficiency or is there also another reason? Could you clarify?

    import os from dotenv import load_dotenv dotenv_path = os.path.join(os.path.dirname(__file__), '.env') if os.path.exists(dotenv_path): load_dotenv(dotenv_path) import sys import click
  • #130 Miguel Grinberg said 2019-10-20T19:01:51Z

    @Sebastian: In many cases the environment variables need to be loaded when code is imported. If having two import blocks bother you, then you can move the dotenv stuff into a separate module, let's call it environment.py. Then you can just do:

    import environment import sys import click

  • #131 Can said 2019-12-04T19:47:12Z

    Overall I really like this structure but is there a way to cleanly support multiple test files with this approach? Splitting tests.py into a tests/ directory makes app/ a sibling of the tests directory which creates import issues.

  • #132 Miguel Grinberg said 2019-12-05T10:59:36Z

    @Can: what import issues did you find? That is exactly the way, app and tests as sibling packages. As long as you always run the app and the tests with the parent directory as your current directory both packages should be able to import just fine.

  • #133 Tim Jones said 2019-12-16T09:53:37Z

    Hi Miguel, I'm a bit late to the party but your tutorial is an absolute life-saver! I had a question about structure, where you have the templates under the global folder rather than in a templates folder with the other blueprint files. I would have thought that having everything in one place would be a benefit in terms of portability. You say that it's a matter of choice, so I'd be interested why you prefer the structure you proposed for the templates.

  • #134 Miguel Grinberg said 2019-12-16T10:13:56Z

    @Tim: It's really a minor thing in my view. I prefer to have all my templates together. I do separate them by blueprint, but all under the global templates directory. Having a templates directory in each blueprint is perfectly fine, I can't really give you any down sides to it, other than maybe when you need to inherit from a base template it becomes a bit odd to have to do these inheritance declarations across completely different directory branches.

    I do feel a bit more strongly about this practice with static files. If you have static files for each template in their own static folder, then on a production deployment you end up having multiple static file folders, and that makes the configuration harder to maintain than having a single one.

  • #135 Alex Yang said 2020-01-09T05:13:45Z

    Hi Miguel, I don't know if you still check this tutorial, but I've been following it recently and have really enjoyed it: thank you for your work! I've recently come across an odd bug and the only person I thought could help was you. I followed the new format that you laid out with auth and main directories within app. I put the route for login in auth, but when I need its url for base.html, it won't work when I do auth.login. It only works if I do main.login, even though the route exists in the auth directory. Not sure what's going on, any insight would be appreciated!

  • #136 Alex Yang said 2020-01-09T05:44:05Z

    A second question, sorry, but how do I use the flask.env to hold the secret key as well as BASIC_AUTH_USERNAME and BASIC_AUTH_PASSWORD (I'm using BasicAuth)? I currently have in my config.py: SECRET_KEY = os.environ.get('SECRET_KEY') or 'test' BASIC_AUTH_USERNAME = 'test7' BASIC_AUTH_PASSWORD = 'test7'

    I tried copying and pasting that into the .env and it didn't run properly. How should I go about this? Thanks!

  • #137 Miguel Grinberg said 2020-01-09T07:40:51Z

    @Alex: the directory where you put a route does not really matter, what matters is what blueprint you attach it to. This route goes in the auth blueprint, so you need to use the @auth.route decorator for it.

  • #138 Miguel Grinberg said 2020-01-09T07:43:23Z

    @Alex: Regarding your 2nd question, after you add the variables to your .env file, you have to change them in config.py to read from the environment, just like SECRET_KEY, DATABASE_URL or others. For example:

    BASIC_AUTH_USERNAME = os.environ.get('BASIC_AUTH_USERNAME')

  • #139 Xinyu said 2020-03-05T09:18:22Z

    Hi Miguel, Very exciting chapter, again!

    I have a quesition about blueprint: One of the objects of blueprint is to structure your Flask application by grouping its functionality into reusable components. I group the functionalities so, that templates are inside a blueprint package. If I want to reuse a blueprint, e.g. "auth", I can, in ideal case, just copy the folder "auth" into a new project and register it, and it is ready to use.

    But there are some dependancies. "auth" uses database and model "User". One can see the dependancies in routes.py:

    from app import db from app.models import User

    That means, the new project must also have a global "db" and the module "app.models" with a compatible class "User". Alternatively, I can do some modification in my "auth" package.

    Is there anyway to make these dependancies clear and explicit? Or can I use something more general (interface?) for e.g. class "User" to reduce the dependancy?

    (The tempates use base.html, which is also a dependancy).

    Thank you!

  • #140 Miguel Grinberg said 2020-03-05T10:58:40Z

    @Xinyu: some people put the models that apply to a blueprint in that blueprint, so you then have a lot of models.py files spread across multiple blueprints, all of which need to be imported to form the complete database schema. I do not like this approach myself, but maybe that's what you are looking for?

  • #141 Ben Kim said 2020-03-09T07:01:48Z

    Thank you for the great tutorial! I am learning a lot just by following your tutorial, and that's not just limited to Flask but also web development and design pattern. Seriously, this is great. I just want to leave a few notes though, I hope you don't mind.

    I've been following your tutorial and typing the code myself while tweaking few minor details along the way. Even if there were errors, I've been able to fix them by reading the tutorial again and also comparing the codes. However, this tutorial specifically was quite difficult to follow - mostly because I was not fully aware how much the Blueprints would affect the whole app architecture, but also because there were a lot of crossover between Blueprints and Application Factory. There were just a lot of changes happening at once. It made me wonder if it would've been easier if Blueprints and Application Factory was introduced earlier in the tutorial. I'd appreciate your input on that matter. Also, on the Error Handling Blueprint part of the tutorial, I think it'd be nice if you mention that errors.py from Chapter 7 Error Handling is changed to handlers.py. The name change kind of caught me off guard.

  • #142 Miguel Grinberg said 2020-03-10T10:34:20Z

    @Ben: growing a Python module into a package is a common refactor operation. The errors.py module from the early chapters was grown into an errors package, there was no name change there. Internally in the errors package there is a handlers.py file, but that is internal to the package, from the outside it's still called errors. Consider if I had named handlers.py as errors.py it would be very confusing, as you would have a package and a module inside the package both with the same name.

  • #143 Laura said 2020-03-17T20:58:35Z

    I am trying to follow your steps, and use the application factory structure, but I am facing two problems: 1. In debug mode the application creates twice, as if the create_app() runs twice. This doesn't happen when debug is off. 2. main problem is with my databases, I have the follwoing global variables in app/init.py: db = SQLAlchemy() migrate = Migrate() login = LoginManager() sched = APScheduler()

    and this is how my create_app() function looks like: def create_app(config_class=Config): print('Welcome to Audrey Web Application.') app = Flask(name) app.config.from_object(config_class)

    db.init_app(app) db.create_all() migrate.init_app(app, db) login.init_app(app) ldap.init_app(app) sched.init_app(app) sched.start() atexit.register(lambda: sched.shutdown()) # Locals within the app context # with app.app_context(): # db.init_app(app) # db.create_all() # migrate.init_app(app, db) # login.init_app(app) # sched.init_app(app) # sched.start() # atexit.register(lambda: sched.shutdown()) from app.errors import bp as errors_bp app.register_blueprint(errors_bp) from app.main import bp as main_bp app.register_blueprint(main_bp) return app

    I have an apscheduler that I would like it to run at specific time of the day and run something in the background and store the results in databases. I am using a sqlite data base to store/track jobs.

    However I get the following Run time Error: 'No application found. Either work inside a view function or push' RuntimeError: No application found. Either work inside a view function or push an application context. See http://flask-sqlalchemy.pocoo.org/contexts/.

    I also tried the app_context (commented) above but couldn't solve it. Any advice is really appreciated.

  • #144 Miguel Grinberg said 2020-03-18T15:22:24Z

    @Laura: the app_context needs to be used in your scheduled function. You will need to have access to your application instance there, so that you can call app.app_context().

  • #145 YoungKi said 2020-04-08T10:13:33Z

    
    
    

    from app import create_app app = create_app() app.app_context().push() current_app.config['SQLALCHEMY_DATABASE_URI'] 'sqlite:////home/miguel/microblog/app.db'

    I think the code should be modified as below.

    app.config['SQLALCHEMY_DATABASE_URI'] 'sqlite:////home/miguel/microblog/app.db'

  • #146 Miguel Grinberg said 2020-04-09T17:23:41Z

    @YoungKi: the idea is that you learn how to work with the app context for cases where you cannot easily import your application instance.

  • #147 db migration said 2020-04-27T14:14:56Z

    Hi Miguel, First of all, this tutorial is so great. My problem is: when I installed the files of this chapter and do a db init/migration, I got the import error. There was no init.py in the root directory before and was added in this chapter and then I got the error. (venv) admin@localhost:~/Documents/python-lab/Flask/Microblog$ flask db init Usage: flask db init [OPTIONS]

    Error: While importing "Microblog.microblog", an ImportError was raised:

    Traceback (most recent call last): File "/home/admin/Documents/python-lab/Flask/Microblog/venv/lib/python3.6/site-packages/flask/cli.py", line 240, in locate_app import(module_name) File "/home/admin/Documents/python-lab/Flask/Microblog/init.py", line 12, in from config import Config ModuleNotFoundError: No module named 'config'

  • #148 Miguel Grinberg said 2020-04-27T15:00:09Z

    @db: this chapter does not add a __init__.py to the root folder. Where did you see that? If there are any questions regarding the file structure of the project, you can download the project from the link at the top of the article and compare everything against your version.

  • #149 Pedro said 2020-04-28T20:54:42Z

    Hi Miguel,

    Great job. Thank you very much for sharing this tutorial

  • #150 Mansi said 2020-05-07T19:28:36Z

    Hi Miguel! Thank you for this great tutorial. But I am getting a problem in importing Blueprint object. I have done the exact same thing as you. I am unable to figure out the problem. Can you please help me with this? Error Log: Traceback (most recent call last): File "/home/mansi/.virtualenvs/microblog/lib/python3.5/site-packages/flask/cli.py", line 240, in locate_app import(module_name) File "/home/mansi/git/microblog/microblog.py", line 4, in app = create_app() File "/home/mansi/git/microblog/app/init.py", line 41, in create_app from app.errors import bp as errors_bp ImportError: cannot import name 'bp'

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