14.7. Input For A Flask Web Application

In this section, we will design a web application that obtains input from the user. In the example in this section, the user must encode the input directly into the URL. In the next section, we’ll provide a more user-friendly approach for obtaining input.

The URL used to interact with a web application can contain input data in addition to the path. This input data is typically encoded into the URL in the form of a query string. Here’s an example of a URL containing a query string:

http://www.bing.com/search?q=python+flask&go=Submit

The query string is the portion that comes after the ? symbol:

q=python+flask&go=Submit

It contains a set of query variables and values, each query variable/value pair separated from the others by the & symbol. This example has a query variable named q whose value is python+flask, and a variable named go whose value is Submit.

Flask applications can access query variables using a dictionary named request.args (dictionaries are discussed in detail in Dictionaries). When a browser sends a request to a Flask application that contains a query string, the data in the query string is placed in the request.args dictionary, where it can be retrieved by the application. For example, in the Bing search URL above, if Bing were a Flask application, it could access the values in the query string like this:

q = request.args['q']
go = request.args['go']

This would retrieve the values ‘python flask’ and ‘Submit’ from the query string and store them, respectively, in q and go.

Here is an enhanced version of the original flaskhello.py program that gets the user’s name from the query string and uses it to greet the user:

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 from flask import Flask, request
 from datetime import datetime

 app = Flask(__name__)

 @app.route('/')
 def hello():
     name = request.args['name']
     return HELLO_HTML.format(
             name, str(datetime.now()))

 HELLO_HTML = """
     <html><body>
         <h1>Hello, {0}!</h1>
         The time is {1}.
     </body></html>"""

 if __name__ == "__main__":
     # Launch the Flask dev server
     app.run(host="localhost", debug=True)

To test this example, you would need to enter the following URL into the browser:

http://localhost:5000/?name=Frank

If the name parameter is omitted, the application will crash when it attempts to retrieve the query parameter from the dictionary, because indexing a dictionary with a key that is not present in the dictionary is illegal. To make the application more robust, we could change line 8 to check to see if the name parameter was submitted:

if 'name' in request.args:
    name = request.args['name']
else:
    name = 'World'

The test 'name' in request.args is True if ‘name’ was present in the query parameters, and False if not.

A shorter way to handle a missing query parameter is to use the dictionary get() method, which allows us to supply a default value to use in case the user omits the query parameter. The if statement above could be rewritten with a single line of code:

name = request.args.get('name', 'World')

This line does the same check as the if statement. If ‘name’ is present in the query parameters, its value is stored in name. Otherwise, the value ‘World’ is stored in name if no name parameter was supplied.

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