Flask Web Development Notes

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Flask Web Development Notes

这是我暑假看 Miguel 的那本著名狗书时的笔记，书的版本是最新的 second edition。当时看的是原文版，以后会写一篇文章说阅读英语原文的经验

Ch2 Basic Application Structure

Run an app

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export FLASK_APP=name.py
export FLASK_DEBUG=1
flask run

Dynamic route

Flask supports the types string, int, float, and path for routes. The path type is a special string type.

Response

view function could take as much as three arguments, status code and a dictionary of headers

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@app.route('/')
def index():
	return '<h1>Bad Request</h1>', 400

Compatibility with older version

app.run(debug=True)

app.add_url_rule(): builds the map between URL and view function

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flask run --host 0.0.0.0
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git reset --hard
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sudo apt-get install sqlite3 libsqlite3-dev

Ch3 Templates

business logic and presentation logic, Jinja2 takes the work of presentation. More specifically, the application only sends variables to Jinja2, after that how the HTML page is presented is up to Jinja2.

What this chapter’s mainly about

Static web pages. Topics in this chapter:

  • What are templates; how to inherit
  • How to write URLs in Flask; url_for() helper function; relative & absolute URLs, the latter one less useful
  • How to send static files (in template format)
  • Miscellaneous: Bootstrap integration

Ch4 Web Forms

Form classes （technical)

FlaskForm defines the list of fields in each form, each represented by an object. Each field object have one or more validators attached. A validator is a function. Note the three terms here, FlaskForm, field, validator.

Form submission with GET

Bad. GET doesn’t have a body, so if submit with GET method, the data will be appended to the URL as a query string, and becomes visible in the browser’s address bar.

Therefore, in view function that deals with forms, add a POST method like that:

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@app.route('/', methods=['GET', 'POST'])

Miscellaneous

This method returns True when form is submitted submitted using POST method, and the data accepted by all field validators; used to determine, whether the form needs to be rendered or processed.

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if form.validate_on_submit():
	###

The first and only required argument to url_for() is the endpoint name. By default, the endpoint of a route is the name of the view function attached to it. 

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return redirect(url_for('index'))

Ch5 Databases

Terms

column: attribute of the entity; row: actual data element that assigns values to columns.

table, record –> collection, document, from Relational to NoSQL

primary key: a special column; foreign keys: columns

Sum

  • With ORM middleman, one doesn’t have to SQL semantics
  • Apply different URLs in config could migrate from different (relational) databases
  • Databases migration:
    • Framework: Flask-Migrate, the Alembic wrapper
    • Procedures: flask db init, creates a migrations directory (optional, only on a new project); flask db migrate -m "message", automatically creates migration script, while revision manually; flask db upgrade
    • flask db downgrade and ... upgrade gives db migrations log history like git

Ch7 Large Application Structure

Add custom commands via click

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@app.cli.command()
def thatWillBetheNameofCommand():
	"""This will be shown in the help messages"""
	pass

Blueprint detail

  • In chapter 7 about blueprint, the module where blueprint object is must import its routes’ view functions, because

    Importing these modules causes the routes and error handlers to be associated with the blueprint.

  • From Eg.7.7: the blueprint name before endpoint can be omitted, as url_for('.index'). Redirects within the same blueprint could use the shorter form, while crossing blueprints requires the fully qualified endpoint name that includes the blueprint name.

Ch8 User Authentication

Hashing functions are repeatable.

  • Q: How could the check_password_hash(hash, password) check pwd without knowing the salt of it?

WTForm: write your own validator with validate_ prefix

  • for wtforms validate_ prefix defines custom validators; while in Python standard library unittest the test_ prefix is used.
  • any function referred the database, is defined in model.py, as methods in those ORM models. They access the database using self.attribute.

Flask-Login detail

Flask-Login requires the application to designate a function to be invoked when the extension needs to load a user from the database given its identifier, registered with @login_manager.user_loader decorator.

_get_user() searches for a user ID in the user session –> invoke the function registered with the user_loader decorator, with the ID as argument

Ch14 API

The process of converting an internal representation to a transport format such as JSON is called serialization.

Todo

  • decorators in Python
    They are functions that takes a func as an arg then returns another arg

  • OOP concepts: constructor, method&attribute
    (so far I assume that) Methods are funcs, attributes are data.

  • How blocks are really defined in bootstrap/base.html

  • what <> means in Python code (<{'key1': v, 'k2': v2}>)

  • What is label in HTML
    makes the form text clickable; same style with it.

  • The remaining class variables are the attributes of the model, defined as instances of the db.Column class.

    what is instance, class variable, attribute

  • with keyword in Python

  • __name__, __file__ and other dunder variable (basedir = os.path.abspath(os.path.dirname(__file__)) )

    • __name__ has two values, "__main__" (when being executed) or the module’s name (when imported). With respect to Object, __name__ is the special attribute of a class, function, method, descriptor, or generator instance.
    • __file__, the absolute path of the module

  • functionality of the __init__.py file in a directory

  • OOP: @property decorator in a class

  • DB: users = db.relationship('User', backref='role', lazy='dynamic'). “users” is a column of the table role. What does lazy kwarg mean

  • DB: Role.query.filter_by(name='Administrator').first(), type of the return value

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    User.query.filter_by(username='galt').first()  # output: <User 'john'>
    a = User.query.filter_by(username='galt').first()
    type(a)  # output: <class 'app.models.User'>

    On success, return value is the row in the table. a.id would return the id column of user galt.

  • Linux command export
  • form.hidden_tag() element for CSRF protection
  • How much freedom does ORM/ODM provide in choosing DB, only different rational databases?
    Yes.
  • {{ wtf.quick_form(form) }}, how will this template be rendered, search keyword in the book
    • {{ name }} construct references a variable, a placeholder
  • How to initialize SelectField, 7.31
  • Post.query.order_by will this query affect rows? 8.1

After Flask:

  • WTForm package validator source code, I assume it’s just RegEx

  • For example, the execution of the send_async_email() function can be sent to a Celery task queue. (p.107)

  • Decorator: How different orders affects the registered function

  • Thread: worker thread in Flask doc

  • Iterator and that: 

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    follows = [{'user': item.follower, 'timestamp': item.timestamp}
    for item in pagination.items]

Check yourself:

  • How to add a new row in a table, how to change a column of a row
  • Page 182, how the foreign key of a table is added

  • synopsis: brief summary

  • delimiter (programming)

  • miscellaneous

  • callable, page 65

  • cardinality alchemy, transparent (Ch5)

  • build a perfect replica of the virtual environment…

Appendix

Object in Python

type of methods:

  • instance method, the 1st argument of it must be self; any method taking self as the 1st arg is a instance method
  • class method, @classmethod, cls as the 1st arg. Calling: Obj().count(), where Obj is the name of a class
  • static method， @staticmethod. E.g. Obj().someStaticMethod()

super() in Python2.x takes its own class as the first variable, like super(Child, self).__init__(age, height). Also the parent shall be defined as class Parent(object), having object as base class.

REST API

  • more business logic on client side
  • client could discover resources using different URL compositions
  • versioning for backward compatibility

Compare to Mage Tutorial

  1. _get
  2. plain text and HTML email
  3. from GitHub’s commit history you could view specifically what changed in every commit

Test & Caution

  1. Think of boundary values while designing programs.

    • current_user.confirmed
    • @login_required
    • self.email.lower() ensure that the string is lowercase
    • verify_password(email, password): before loading user from DB to check pwd, email must be checked first not to be ''

  2. Crucial cases where proxies provided by Flask likecurrent_app can’t fake:

    • current_app can’t fake its type as the actual object type
    • when sending Signals or passing data to a background thread

    best practice:

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    app = current_app._get_current_object()
    my_signal.send(app)
  3.  from sqlalchemy.exc import IntegrityError
 try:
     db.session.commit()
 except IntegrityError:
     db.session.rollback()

  4. form.body.data = post.body; how to render page according to attributes of the user; Context processors make variables available to all templates during rendering

  5. from app.exceptions import

The power of Flask Shell

Page 214, static method add_self_follows() that manipulates existed rows