How to pass data from React Form -> Flask Backend -> React Component (does it have something to do with CORS)?

Question

Hi I am a complete React beginner and have a fairly basic question. I am looking to perform the following steps:

(1) Provide user a form to input some text (2) Ingest the input into the Flask backend and return a new value after performing some operations (3) Provide the result from (2) to the user in front end view

I'd like this process to be a single page app where the user does not get redirected to another page on step (3).

Here is my App.js code:

import React from 'react';
import './App.css';

class App extends React.Component {

 constructor(props) {
    super(props);
    this.state = {value: '',
                  playerName: ''};

    this.handleChange = this.handleChange.bind(this);
    this.handleSubmit = this.handleSubmit.bind(this);
  }

  handleChange(event) {
    this.setState({value: event.target.value});
  }

  handleSubmit(event) {
    console.log("making request")
    fetch('/result')
      .then(response => {
        console.log(response)
        return response.json()
      })
      .then(json => {
      console.log=(json)
      this.setState({playerName: json[0]})
      })
  }

  render() {
    return (
      <div>
        <form onSubmit={this.handleSubmit} action="http://localhost:5000/result" method="get">
        <label>
          Player ID:
          <input type="text" name="player_id"/>
          <input type="submit" onChange={this.handleChange} value={this.state.value} />
        </label>
      </form>
        <h1> Player Name: {this.state.playerName} </h1>
      </div>
    );
  }
}


export default App

Here is my main.py code:

from flask import Flask, request, jsonify, send_from_directory
from sqlalchemy import create_engine
import pandas as pd

app = Flask(__name__, static_folder='../frontend/build')

@app.route('/result', methods = ['GET'])
def result():
    if request.method == 'GET':
        player_id = request.args.get('player_id', None)
        if player_id:
            data = get_player(player_id)
            name = str(data['name'][0])
            return jsonify(name)
        return "No player information is given"


def get_player(player_id):
    engine = create_engine(
        'postgres://fzmokkqt:********************-***-******-@********.com:*****/******')
    sql = """SELECT * from players WHERE id={player_id}"""
    data = pd.read_sql_query(sql.format(player_id=player_id), con=engine)
    return data

@app.route('/', defaults={'path': ''})
@app.route('/<path:path>')
def serve(path):
    if path != "" and os.path.exists("frontend/build/" + path):
        return send_from_directory('../frontend/build', path)
    else:
        return send_from_directory('../frontend/build', 'index.html')

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0')

When I run this code, I receive the intended result however it is not placed in the HTML as described in the App.js code. Instead, the result is rendered into a new page (on localhost:5000 instead of localhhost:3000 which is where the React code is rendered).

Solution

Sending data to your flask back end is a POST request but you are using a GET request. So your fetch in React js should be

   fetch("/result", {
        method:"POST",
        cache: "no-cache",
        headers:{
            "content_type":"application/json",
        },
        body:JSON.stringify(this.state.value)
        }
    ).then(response => {

    return response.json()
  })
  .then(json => {

  this.setState({playerName: json[0]})
  })

Your flask method should also be a POST method

@app.route('/result', methods = ['POST'])
def result():
    player_id = request.json
    if player_id:
       data = get_player(player_id)
       name = str(data['name'][0])
       return jsonify(name)
   return "No player information is given"

You would need CORS in your flask so

pip install flask-cors then in your code

from flask_cors import CORS

app = Flask(__name__, static_folder='../frontend/build')

CORS(app)

Also you dint post your react js package so make sure you have your proxy added in your package.json like this

  "name": "client",
  "version": "0.1.0",
  "private": true,
  "proxy": "http://0.0.0.0:5000",

I assume the rest of your code works so this is just to answer your question in relation to setting up a connection between Flask and React JS.

You would be able to recieve data in flask from React but you would have a problem sending json data from the database in flask to reactjs . For the database handling I usually use Flask-SQLAlchemy and data I get from the database isnt a json object so I have to serialize the model into JSON and this achieved through a package called flask-marshmellow. So look into use those two packages