python 3.8 - Flask - request.args.getlist - Can only be read once?

Question

In my Flask app, I'm redirecting to an other page with an argument as follows:

return redirect(url_for("edit.editFunction", plants=thisPlant))

Then, in an other file, I'm reading that argument with those line:

    # Get arguments from url_for in administration
    thisPlant = request.args.getlist("plants")
    print(thisPlant)

    # Get ID on edit plant
    plant_id = thisPlant[0]
    print(plant_id)

So far so good as this is return in my console:

['1', 'aa', '11', '11', 'https://i.ibb.co/QNJnLR8/empty.png', 'aa', '1']
1

But when I want to use a post method on the same file, I get this:

    plant_id = thisPlant[0]
IndexError: list index out of range

How can that be? I should read 1 Can someone spot my mistake?

Here is the full file where I use this variable:

import sqlite3
import traceback
import sys
import re
import os

from flask import Blueprint, render_template, redirect, session, request, flash, get_flashed_messages
from application import getUserName, getUserPicture, login_required, confirmed_required, getUserRole, role_required, uploadPicture
from werkzeug.utils import secure_filename

# Set Blueprints
edit = Blueprint('edit', __name__,)

@edit.route("/edit", methods=["GET", "POST"])
@login_required
@confirmed_required
@role_required
def editFunction():    

    # Force flash() to get the messages on the same page as the redirect.
    get_flashed_messages()

    if request.method == "POST":

        # Get variables
        plant_id = request.form.get("plant_id")
        name = request.form.get("name")
        stock = request.form.get("stock")
        price = request.form.get("price")
        description = request.form.get("description")

        # Ensure the plant name was submitted
        if not name:
            flash("must provide plant name")
            return redirect("/edit")

        # Ensure the plant name fits server-side
        if not re.search("^[a-zA-Z0-9]{1,50}$", name):
            flash("Invalid plant name")
            return redirect("/edit")

        # Ensure the plant stock was submitted
        if not stock:
            flash("must provide plant stock")
            return redirect("/edit")

        # Ensure the plant stock fits server-side
        if not re.search("^[0-9]+$", stock):
            flash("Invalid plant stock")
            return redirect("/edit")

        # Ensure the plant price was submitted
        if not stock:
            flash("must provide plant price")
            return redirect("/edit")

        # Ensure the plant price fits server-side
        if not re.search("^[0-9]+$", stock):
            flash("Invalid plant price")
            return redirect("/edit")

        # Ensure the plant description was submitted
        if not description:
            flash("must provide plant description")
            return redirect("/edit")

        # Ensure the plant description fits server-side
        if not re.search("^(?!;).+", description):
            flash("Invalid plant description")
            return redirect("/edit")

        # Check show bool status
        show = "show" in request.form


        # Update plant name, stock, price, description and show status into the table
        try:

            sqliteConnection = sqlite3.connect("database.db")
            cursor = sqliteConnection.cursor()

            cursor.execute("UPDATE plants SET name=:name, stock=:stock, price=:price, description=:description, show=:show WHERE id=:plant_id;", {"name": name, "stock": stock, "price": price, "description": description, "show": show, "plant_id": plant_id})
            sqliteConnection.commit()

            cursor.close()

        except sqlite3.Error as error:
        
            print("Failed to read data from sqlite table", error)
            print("Exception class is: ", error.__class__)
            print("Exception is", error.args)

            print('Printing detailed SQLite exception traceback: ')
            exc_type, exc_value, exc_tb = sys.exc_info()
            print(traceback.format_exception(exc_type, exc_value, exc_tb))

        finally:

            if (sqliteConnection):
                sqliteConnection.close()

        # Save, upload and delete picture file
        file = request.files["picture"]

        if file and file.filename:

            filename = secure_filename(file.filename)
            file.save(os.path.join("./static", filename))
            upload = uploadPicture("./static/" + filename)
            os.remove("./static/" + filename)

            # Update database with new image url 
            try:

                sqliteConnection = sqlite3.connect("database.db")
                cursor = sqliteConnection.cursor()
                
                cursor.execute("UPDATE plants SET picture=:picture WHERE name=:name;", {"picture": upload, "name": name})
                sqliteConnection.commit()

                cursor.close()

            except sqlite3.Error as error:
            
                print("Failed to read data from sqlite table", error)
                print("Exception class is: ", error.__class__)
                print("Exception is", error.args)

                print('Printing detailed SQLite exception traceback: ')
                exc_type, exc_value, exc_tb = sys.exc_info()
                print(traceback.format_exception(exc_type, exc_value, exc_tb))

            finally:

                if (sqliteConnection):
                    sqliteConnection.close()

        flash("Plant edited")
        return redirect("/administration")


    else:
        # Get arguments from url_for in administration
        thisPlant = request.args.getlist("plants")
    
        return render_template("edit.html", name=getUserName(), picture=getUserPicture(), role=getUserRole(), plants=thisPlant)

This is the full console output:

127.0.0.1 - - [05/Jan/2021 22:20:44] "GET /administration HTTP/1.1" 200 -
127.0.0.1 - - [05/Jan/2021 22:20:44] "GET /static/styles.css HTTP/1.1" 200 -
127.0.0.1 - - [05/Jan/2021 22:20:45] "POST /administration HTTP/1.1" 302 -
['1', 'aa', '11', '11', 'https://i.ibb.co/QNJnLR8/empty.png', 'aa', '1']
1
127.0.0.1 - - [05/Jan/2021 22:20:45] "GET /edit?plants=1&plants=aa&plants=11&plants=11&plants=https%3A%2F%2Fi.ibb.co%2FQNJnLR8%2Fempty.png&plants=aa&plants=1 HTTP/1.1" 200 -
127.0.0.1 - - [05/Jan/2021 22:20:45] "GET /static/styles.css HTTP/1.1" 200 -
[]
[2021-01-05 22:20:53,655] ERROR in app: Exception on /edit [POST]
Traceback (most recent call last):
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 94, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 109, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 124, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/routes/edit.py", line 28, in editFunction
    plant_id = thisPlant[0]
IndexError: list index out of range
127.0.0.1 - - [05/Jan/2021 22:20:53] "POST /edit HTTP/1.1" 500 -
Error on request:
Traceback (most recent call last):
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 2447, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1952, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1821, in handle_user_exception
    reraise(exc_type, exc_value, tb)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/_compat.py", line 39, in reraise
    raise value
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1950, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1936, in dispatch_request
    return self.view_functions[rule.endpoint](**req.view_args)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 94, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 109, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/application.py", line 124, in decorated_function
    return f(*args, **kwargs)
  File "/home/benoit/Documents/CrazyPlantCrew/routes/edit.py", line 28, in editFunction
    plant_id = thisPlant[0]
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/werkzeug/serving.py", line 304, in run_wsgi
    execute(self.server.app)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/werkzeug/serving.py", line 292, in execute
    application_iter = app(environ, start_response)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 2464, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 2450, in wsgi_app
    response = self.handle_exception(e)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-packages/flask/app.py", line 1881, in handle_exception
    return self.finalize_request(server_error, from_error_handler=True)
  File "/home/benoit/Documents/CrazyPlantCrew/venv/lib/python3.8/site-package

Answer 1

If I've read this correctly...

request.args.getlist only grabs data posted as URL parameters, hence the KeyError when trying to access plants[0] during a POST request. Notice in the server log, print(thisPlant) during the POST request gives an empty list: []

Seems like you just need to restructure that route, so the problematic line is only executed within a GET request.

Something like:

def editFunction():    

    # Force flash() to get the messages on the same page as the redirect.
    get_flashed_messages()

    if request.method = 'GET':
        # Get arguments from url_for in administration
        thisPlant = request.args.getlist("plants")
        print(thisPlant)

        # Get ID on edit plant
        plant_id = thisPlant[0]
        print(plant_id)

        #?Do return here, rather than in 'else' statement at bottom of file
        return render_template("edit.html", name=getUserName(), picture=getUserPicture(), role=getUserRole(), plants=thisPlant)

    elif request.method == "POST":

        # code to handle POST Request

        # ...

        flash("Plant edited")
        return redirect("/administration")