[Python] Flask - Problem with passing variables between routes OR with sessions

I have been struggling with this for nearly a week.

I am learning Flask, building an application for collectors. Collectors can view the database, but can add/edit only after login. The app requires for an item to be added first, before a photo can be added under the same page. After a photo is added, the app returns back to the viewing page. That requires an id to be passed between the app routes.

At this stage I am either doing something absolutely stupid while passing values, or I have issues with my sessions.

Simplified version of app.py can be seen below (certain routes are omitted, I can provide them if necessary):

import os
import sqlite3

from flask import Flask, flash, jsonify, redirect, render_template, request, session, url_for
from flask_session import Session
from functools import wraps
from werkzeug.security import check_password_hash, generate_password_hash
from werkzeug.utils import secure_filename

# Configure application

UPLOAD_FOLDER = 'static\\uploads'
ALLOWED_EXTENSIONS = {'png', 'jpg', 'jpeg'}
app = Flask(__name__)
app.config\['UPLOAD_FOLDER'\] = UPLOAD_FOLDER
app.config\['SECRET_KEY'\] = '123123cdsfqernwkfnvgfelnrvgn3v5gq35kj'

# Configure session to use filesystem (instead of signed cookies)

app.config\["SESSION_PERMANENT"\] = False
app.config\["SESSION_TYPE"\] = "filesystem"
Session(app)

# Database

db = sqlite3.connect('database.db', check_same_thread=False)
cursor = db.cursor()

def login_required(f):
"""
Decorate routes to require login.

https://flask.palletsprojects.com/en/latest/patterns/viewdecorators/
"""

@wraps(f)
def decorated_function(*args, **kwargs):
if session.get("user_id") is None:
return redirect("/login")
return f(*args, **kwargs)

return decorated_function

@app.route("/view/\<p_id\>", methods=\["GET", "POST"\])
def view(p_id):
"""View an individual P"""

rows = cursor.execute("SELECT * FROM pez WHERE p_id = ?", (p_id, )).fetchall()[0]
series = rows[1]
subseries = rows[2]
name = rows[3]
issue = rows[4]


if request.method == "GET":
return render_template("view.html", series = series,
subseries = subseries,
name = name,
issue = issue,
p_id = p_id)

@app.route("/add_main_img", methods=\["GET", "POST"\])
@login_required
def add_main_img():
"""Add new item to database"""

p_id = request.args.get("p_id")

if request.method == "GET":
return render_template("add_main_img.html")

else:
# check if there is a file part
if 'file' not in request.files:
flash('No file part')
return render_template("add_main_img.html")
file = request.files['file']

# check filename is not empty (i.e. user didn't select a file) and allowed and upload it
if file.filename == '':
flash("No image selected for upload")
return redirect(request.url)
if file and allowed_file(file.filename):
filename = secure_filename(file.filename)
file.save(os.path.join(app.config['UPLOAD_FOLDER'], filename))
flash("Image succesfully uploaded")
return redirect(url_for('view', p_id=p_id))
else:
flash("Allowed image types are .png, .jpg, .jpeg")
return redirect(request.url)



view.html:

{% extends "layout.html" %}

{% block title %}
View
{% endblock %}

{% block main %}

<table>
<tr>
<th>Series</th>
<td>{{ series }}</td>
</tr>
<tr>
<th>Subseries</th>
<td>{{ subseries }}</td>
</tr>
<tr>
<th>Dispenser/Character</th>
<td>{{ name }}</td>
</tr>
<tr>
<th>Issue</th>
<td>{{ issue }}</td>
</tr>
</table>

<button type="submit" id="main_img" name="main_img"><a href="{{ url_for('add_main_img', p_id=p_id) }}">Add main/profile img</a></button>


{% endblock %}


add_image:

{% extends "layout.html" %}

{% block title %}
Add main image
{% endblock %}

{% block main %}

<h2>Upload new</h2>
<form action="/add_main_img" method="post" enctype = "multipart/form-data">
<div>
<input class="form-control mx-auto w-auto" type="file" name="file" placeholder="Add Profile Image">
</div>
<input type="submit" >
</form>

{% endblock %}




The route view -> add_image -> upload seems to work, but it's failing to pass the id afterwards.

I then get the following traceback (not the complete one, let me know if more details are needed):

werkzeug.routing.exceptions.BuildError

werkzeug.routing.exceptions.BuildError: Could not build url for endpoint 'view'. Did you forget to specify values ['p_id']?

File "C:\Users\denchevd\Desktop\personal\Code\FinalProject\app.py", line 37, in decorated_function

return f(*args, **kwargs)
^^^^^^^^^^^^^^^^^^

File "C:\Users\denchevd\Desktop\personal\Code\FinalProject\app.py", line 415, in add_main_img

return redirect(url_for('view', p_id=p_id))
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


And this is where things get funky. I managed to partially redeem the situation by converting the def variable to a string. However, I can't find any reason why that worked:

@app.route("/add_main_img", methods=["GET", "POST"])
@login_required
def add_main_img():

"""Add new item to database"""
p_id = str(request.args.get("p_id"))


This made the app to get stuck at a different place at @view:

IndexError
IndexError: list index out of range
File "C:\Users\denchevd\Desktop\personal\Code\FinalProject\app.py", line 235, in view
rows = cursor.execute("SELECT * FROM pez WHERE p_id = ?", (p_id, )).fetchall()[0] ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^


It seems that p_id is being passed as 'None' and is not convertible to int.

am I doing something stupid while passing the values or am I having a problem with the session? If the latter, please suggest a fix to it, as I am unable to work it out on my own.

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