In a previous post I showed how to start the most basic Flask web application. In this post I show a dynamic URL which renders content that depends on using the URL as user input. The mathematical function known as factorial is defined as
\[n! \triangleq \prod_{i=1}^n i\] when \(i \geq 1\) and when \(n=0\) then \(0! \triangleq 1\). We will set a dynamic URL which takes an integer for which the factorial will be calculated and displayed. We can specify this by passing '/factorial/<int:number>'. The prefix /factorial/ is just to remind us that we’re calculating the factorial of a number. The angle brackets <TYPE:...> indicate that something is unspecified input in between the brackets with type TYPE. In this case we want <int: ...> because the factorial function (not to be confused with its generalizations, like the gamma function) is defined only for non-negative integers. It is natural to use number since we expect a number, but other names are possible for this.
from math import factorial
from flask import Flask
app = Flask(__name__)
@app.route('/factorial/<int:number>')
def calculate_factorial(number):
return str(factorial(number))
if __name__ == '__main__':
app.run(debug=True)That’s it. We’re just taking the number, calculating its factorial, casting the result to a string, and returning it. Flask takes care of the rest.