How can I get current base URI in flask?

You can use the base_url method on flask's request function.

from flask import Flask, request

app = Flask(__name__)

@app.route('/foo')
def index():
    return request.base_url
    

if __name__ == '__main__':
    app.run()

This returns the following if the app route is /foo:

http://localhost:5000/foo

Use flask.request.url to retrieve your requested url. Have a look at: http://flask.pocoo.org/docs/1.0/api/#flask.Request (or the v0.12 docs)