Unique orders and customers per month

# Table: Orders
# 
# +---------------+---------+
# | Column Name   | Type    |
# +---------------+---------+
# | order_id      | int     |
# | order_date    | date    |
# | customer_id   | int     |
# | invoice       | int     |
# +---------------+---------+
# order_id is the column with unique values for this table.
# This table contains information about the orders made by customer_id.
# 
#  
# 
# Write a solution to find the number of unique orders and the number of unique customers with invoices > $20 for each different month.
# 
# Return the result table sorted in any order.
# 
# The result format is in the following example.
#
# Example 1:
# Input:
# Orders table:
# +----------+------------+-------------+------------+
# | order_id | order_date | customer_id | invoice    |
# +----------+------------+-------------+------------+
# | 1        | 2020-09-15 | 1           | 30         |
# | 2        | 2020-09-17 | 2           | 90         |
# | 3        | 2020-10-06 | 3           | 20         |
# | 4        | 2020-10-20 | 3           | 21         |
# | 5        | 2020-11-10 | 1           | 10         |
# | 6        | 2020-11-21 | 2           | 15         |
# | 7        | 2020-12-01 | 4           | 55         |
# | 8        | 2020-12-03 | 4           | 77         |
# | 9        | 2021-01-07 | 3           | 31         |
# | 10       | 2021-01-15 | 2           | 20         |
# +----------+------------+-------------+------------+
# Output:
# +---------+-------------+----------------+
# | month   | order_count | customer_count |
# +---------+-------------+----------------+
# | 2020-09 | 2           | 2              |
# | 2020-10 | 1           | 1              |
# | 2020-12 | 2           | 1              |
# | 2021-01 | 1           | 1              |
# +---------+-------------+----------------+
# Explanation:
# In September 2020 we have two orders from 2 different customers with invoices > $20.
# In October 2020 we have two orders from 1 customer, and only one of the two orders has invoice > $20.
# In November 2020 we have two orders from 2 different customers but invoices < $20, so we don't include that month.
# In December 2020 we have two orders from 1 customer both with invoices > $20.
# In January 2021 we have two orders from 2 different customers, but only one of them with invoice > $20.

import pandas as pd

def unique_orders_and_customers(orders: pd.DataFrame) -> pd.DataFrame:
  filtered = orders[orders['invoice'] > 20].copy()
  if filtered.empty:
    return pd.DataFrame(columns=['month', 'order_count', 'customer_count'])
  filtered['month'] = pd.to_datetime(filtered['order_date']).dt.to_period('M').astype(str)
  result = filtered.groupby('month').agg(
    order_count=('order_id', 'nunique'),
    customer_count=('customer_id', 'nunique')
  ).reset_index()
  return result