Why are all list dictionaries empty except the first one?

Question

Python

It is necessary to write to the file user_id; category, 1, but only the 1st user is correctly recorded

import csv


def writer(user_id):
    user_cart = []
    user_order = []
    user_cart_quantity = {}  # {user_cart: user_order}
    for row in reader:
        if row['user_id'] == str(user_id):
            # список со всеми категориями с повтором
            user_cart.append(row['cart'])
            # список уникальных дат заказов
            if row['order_completed_at'] not in user_order:
                user_order.append(row['order_completed_at'])

    # получение словаря key - категория, value - количество заказов
    for i in user_cart:
        if i not in user_cart_quantity:
            if user_cart.count(i) > len(user_order) / 2: # заказов > 50%
                user_cart_quantity.update({f'{i}': user_cart.count(i)})

                with open('data/tesr.txt', 'a') as fw:
                    fw.write(f'{user_id};{i}, 1\n')

    print(user_id)
    print(user_order)
    print(len(user_order))
    print(user_cart_quantity)


with open('data/train.csv') as f:
    reader = csv.DictReader(f)
    user_id = 2
    while user_id < 20000:
        writer(user_id)
        user_id += 1

Answer 1

in each cycle, reset the file pointer to the beginning f.seek(0)
ps What you have written is a complete nightmare. can be optimized by making only 1 pass through the file

Answer 2

You are using the global reader object. After the first user, the reader has reached the end of the file and has nothing more to read.
Change the code so that each user has their own dictionary, and sort the data into them. For example, make a top-level dictionary where the key is user_id.