[flask] obtain request information, redirect and error handling

Preface article ：

1. 【Flask】Web Departure and implementation are based on Flask The smallest application
2. 【Flask】 Static file and template rendering

Get request data

We know , about Web Applications , It is important to respond to the data sent by the client to the server . stay Flask in , This information is determined by the overall situation request The object provides .

request object

The first step in using it is from flask Import in module ：

from flask import request

In the previous article, I introduced , stay httpt Agreement , There are many possibilities for a request , Such as GET,POST etc. . We can go through request.method To get . stay html Sometimes we use form Forms （POST,PUT Method will make data request ） To submit data , Then the server gets form The way of data in the form is through form Property acquisition . A simple case is as follows ：

@app.route('/login', methods=['POST', 'GET'])
def login():
    error = None
    if request.method == 'POST':
        if valid_login(request.form['username'],
                       request.form['password']):
            return log_the_user_in(request.form['username'])
        else:
            error = 'Invalid username/password'
    # the code below is executed if the request method
    # was GET or the credentials were invalid
    return render_template('login.html', error=error)

When key Not in the form In the middle of the day , Will pack KeyError It's abnormal . Then align according to the specific situation try except Operation to capture .

about html End in URL In the middle of the transmission (?key=value), We can use args Get parameters for properties of ：

searcgword=request.args.get('key', '')

Upload files

Another data type is file . Use Flask It's also more convenient . We just need to guarantee form Set... In the form enctype='multipart/form-data', Otherwise, the file cannot be transferred .

The uploaded files are stored in transit or temporary files in the file system . This data is obtained using request Of files attribute . Every uploaded file will be saved in that dictionary . Its behavior is Python Of file object , But with save() Method to allow you to store data in the file system of the server . A simple case is as follows ：

from flask import request

@app.route('/upload', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        f = request.files['the_file']
        f.save('/var/www/uploads/uploaded_file.txt')
    ...

If you want to know how files are named on the client before uploading to your application , You can visit filename attribute . But remember , This value can be forged , So never believe that value . If you want to use the file name of the client to store the file on the server , adopt Werkzeug For you secure_filename() The function passes it , as follows ：

from werkzeug.utils import secure_filename

@app.route('/upload', methods=['GET', 'POST'])
def upload_file():
    if request.method == 'POST':
        file = request.files['the_file']
        file.save(f"/var/www/uploads/{
      secure_filename(file.filename)}")
    ...

Cookies

If you want to visit cookies Data in , We can go through cookies attribute . If you want to set cookies, We need to borrow set_cookie Method . Again cookies The attribute is request A dictionary of objects . If you want to use Session, Please do not use it directly cookie, But use Flask Medium Session, It will be cookie Add some security to it .

Read cookies Cases in ：

from flask import request

@app.route('/')
def index():
    username = request.cookies.get('username')
    # use cookies.get(key) instead of cookies[key] to not get a
    # KeyError if the cookie is missing.

Storage cookies Content case ：

from flask import make_response

@app.route('/')
def index():
    resp = make_response(render_template(...))
    resp.set_cookie('username', 'the username')
    return resp

Please note that ,cookie Is set on the response object . Because you usually only return strings from view functions , therefore Flask They will be converted into response objects for you . If you clearly want to do this , You can use make_response() function , It is then modified . Sometimes you may want to set in a location where the response object does not already exist cookie. This can be achieved by using the deferred request callback pattern . For these things , You need to know more about the official documents .

Redirection and error handling

To redirect the user to another endpoint , Please use redirect() function ; To abort a request in advance with an error code , Please use abort() function ：

from flask import abort, redirect, url_for

@app.route('/')
def index():
    return redirect(url_for('login'))

@app.route('/login')
def login():
    abort(401)
    this_is_never_executed()

This is a rather pointless example , Because users will redirect from the index to pages they cannot access （401 Access denied ）, But it shows how it works .

By default , Each error code will display a black and white error page . If you want to customize the error page , have access to errorhandler() Decorator ：

from flask import render_template

@app.errorhandler(404)
def page_not_found(error):
    return render_template('page_not_found.html'), 404

Be careful render_template() After calling 404. This tells Flask The status code of this page should be 404, This means that no . By default, it is assumed to be 200, It means ： Deo gratias .