[ v iet- h ung  ng uyen ]

Peeking π with Python

March 14, 2023 · 3 min read

Happy π day everybody! I’m going to show you how to use serverless functions to build a quick API for find digits in π-thon. It may not the most useful use case for the serverless functions, but it’s a fun one. More importantly, you can see the API in action with an embedded π finder in my blog below. Just input any sequences of digits to find their positions among the first ten million digits in π.

The post is inspired by coding challenge Peeking Inside Pi from The Coding Train

Peeking...
3.141592653589793...
Use keypad to input number

Ingredients for API

Set up

Before we start, make sure you have the following Vercel CLI installed in your machine and login.

npm i -g vercel
vercel login

npm i -g vercel
vercel login

To accelerate the development, we will use the template Flask + Vercel It has all the boilerplate code to deploy a Flask app to Vercel. However, the CLI does not support this template yet, so we need to copy/paste it manually.

Also, create a file named data/pi.txt under the root directory and store just decimal digits of π in it. Now, the folder structure should look like this:

/
├── api/
│   └── index.py
├── data/
│   └── pi.txt
├── .gitignore
├── requirements.txt
└── vercel.json

/
├── api/
│   └── index.py
├── data/
│   └── pi.txt
├── .gitignore
├── requirements.txt
└── vercel.json

The API endpoint

The API endpoint is /find/<digits>, where <digits> is the sequence of digits we want to find in π. For example, if we want to find 123 in π, we can send a GET request to /find/123. The API will return a JSON object with the following information:

Implementation

The implementation is pretty straightforward. We need to read the file data/pi.txt and store all digits in the memory. For the API route, we can use the find method of the string to find the positions of number in π. Finally, we can return the JSON object with the information we need.

api/index.py
from flask import Flask, Response
from os.path import join

app = Flask(__name__)

# Read the digits of pi from the file before the server starts
def preload():
  with open(join('data', 'pi.txt'), 'r') as f:
    return f.read()

data = preload()
length = len(data)
RANGE = 5

@app.route("/find/<digits>", method=['GET'])
def pi_peek(digits: str):
  # Check all characters is digit
  for c in digits:
    if not c.isdigit():
      return Response(f'Input must be a string of numbers', 400)

  # Find index of digits
  index = data.find(digits)
  if (index < 0):
    return {
      'length': length,
      'before': '',
      'between': '',
      'after': f'3.{data[:10]}...'
    }

  # Get before and after
  beforeStart = max(index - RANGE, 0)
  before = data[beforeStart: index]
  if beforeStart > 0:
    before = f'...{before}'
  else:
    before = f'3.{before}'
  afterStart = min(index + len(digits), length)
  afterEnd = min(afterStart + RANGE, length)
  after = f'{data[afterStart: afterEnd]}...'

  return {
    'length': length,
    'index': index,
    'before': before,
    'between': digits,
    'after': after
  }

from flask import Flask, Response
from os.path import join

app = Flask(__name__)

# Read the digits of pi from the file before the server starts
def preload():
  with open(join('data', 'pi.txt'), 'r') as f:
    return f.read()

data = preload()
length = len(data)
RANGE = 5

@app.route("/find/<digits>", method=['GET'])
def pi_peek(digits: str):
  # Check all characters is digit
  for c in digits:
    if not c.isdigit():
      return Response(f'Input must be a string of numbers', 400)

  # Find index of digits
  index = data.find(digits)
  if (index < 0):
    return {
      'length': length,
      'before': '',
      'between': '',
      'after': f'3.{data[:10]}...'
    }

  # Get before and after
  beforeStart = max(index - RANGE, 0)
  before = data[beforeStart: index]
  if beforeStart > 0:
    before = f'...{before}'
  else:
    before = f'3.{before}'
  afterStart = min(index + len(digits), length)
  afterEnd = min(afterStart + RANGE, length)
  after = f'{data[afterStart: afterEnd]}...'

  return {
    'length': length,
    'index': index,
    'before': before,
    'between': digits,
    'after': after
  }

Deployment

To deploy the API, we can use the Vercel CLI. First, we need to log in to Vercel with vercel login. Then, we can deploy the API with vercel --prod. Now, you can call the API with the URL provided by Vercel, with the endpoint <assigned>.vercel.app/find/<digits>, where <assigned> is the assigned name of your project by Vercel.

For example, if the assigned name is pi-peek, we can call the API with https://pi-peek.vercel.app/find/123. And here the result in JSON

{
  "after": "71378...",
  "before": "...45047",
  "between": "123",
  "index": 1923,
  "length": 10000000
}

{
  "after": "71378...",
  "before": "...45047",
  "between": "123",
  "index": 1923,
  "length": 10000000
}

That’s it! We have built a quick API to find digits in π. You can find the full source code in GitHub.