Create a route with optional URL parameters in Flask

The Problem

How do I write a Flask route with optional URL parameters?

The Solution

To create a Flask route with an optional URL parameter, we can use two route decorators on the same function. One will describe the URL without the optional parameter, and the other will describe the URL with the optional parameter.

@app.route('/products', defaults={'product_id': None})
@app.route('/products/<product_id>')
def show_product(product_id):
    if product_id:
      # code to show individual product
    else:
      # code to show whole catalog

This same method will work for multiple optional URL parameters.

@app.route('/products', defaults={'product_id': None, 'store_id': None})
@app.route('/products/<product_id>', defaults={'store_id': None})
@app.route('/products/<product_id>/<store_id>')
def show_product(product_id, store_id):
    if product_id:
      if store_id:
        # code to show individual product at specific store
      else:
        # code to show individual product everywhere
    else:
      # code to show whole catalog

Note that it would be impossible to extend the above function to accept a store_id without a product_id, as the positional nature of the arguments would create an ambiguity about which ID was being specified.