Extracting Parameters from Request Path

In most web frameworks we can define a request handler with app.get('/items/{id}') and item will get passed in to the function as a parameter. We see it and use it every day. But, have you ever thought how its done? Lets find out!

# test.py
from fastapi import FastAPI

app = FastAPI()
@app.get("/items/{id}")
def get_item(id):
    return {"id": id}

# install dependencies if you have't already
pip install fastapi uvicorn

# Run it
uvicorn test:app --reload

curl localhost:8000/items/123
# Result
{"id": "123"}

The idea is to take a string like '/items/{id}' and make it into a regular expression where we can use named groups to name things out and extract the parts.

# parse_with_regex.py
import re

def extract_params(path_definition, incoming_path):
    # Convert it to 'items/(?P<id>.+)'
    pattern = path_definition.replace('{', '(?P<').replace('}', '>[^/]+)')

    matches = re.match(pattern, incoming_path)
    return matches.groupdict()


if __name__ == '__main__':
    print(extract_params(
        path_definition='/items/{id}',
        incoming_path='/items/123'
    ))
# output:
# {'id': '123'}

Code readability suffers here. This works at first. But as we add more test cases it starts falling apart very quick. Like when our path definition wants to be a bit more greedy, {id+} for example in AWS API Gateway. Also, what happens when our test contains the extra '/' at the end. The regular expression will get too complicated very fast. It is already not that readable with all string replacements. Let's see if we can do better!

Let's explore the fact that HTTP paths are separated by '/'. So, we can split by it and compare segements

def extract_params(path_definition, incoming_path):

    definition_parts = path_definition.split('/')
    incoming_path_parts = incoming_path.split('/')

    result = {}

    for index, part in enumerate(definition_parts):
        if '{' in part:
            result[part[1:-1]] = incoming_path_parts[index]

    return result


if __name__ == '__main__':
    print(extract_params(
        path_definition='/items/{id}',
        incoming_path='/items/123'
    ))

# output:
# {'id': '123'}

With this approach, the code is easily extendable. Just add more conditions as we inspect the segments.