examples/flask/1/echo_post.py
from flask import Flask, request
app = Flask(__name__)
 
@app.route("/")
def hello():
    return render_template('index.html')

@app.route("/echo", methods=['POST'])
def echo(): 
    return "You said: " + request.form['text']
 
 
if __name__ == "__main__":
    app.run()
examples/flask/1/templates/index.html
<form action="/echo" method="POST">
<input name="text">
<input type="submit" value="Echo">
</form>

Internal Server Error