input: from typing import List
  
  def extract_urls(url: str) -> List[str]:
      """ Extract all href attributes from 'a' tags in the given webpage.
      The function uses BeautifulSoup to parse the webpage and retrieve the URLs.
      >>> extract_urls("http://example.com")
      ['/page1', 'http://example.com/page2', '/page3']
      """
      
      import requests
      from bs4 import BeautifulSoup
  
      # Get the web page
      webpage = requests.get(url)
  
      # Parse the web page
      soup = BeautifulSoup(webpage.content, 'html.parser')
  
      # Find all 'a' tags
      links = soup.find_all('a')
  
      # Extract the URLs in the 'href' attribute
      extracted_urls = [link.get('href') for link in links]
      return extracted_urls
output:     return [url for url in extracted_urls if url is not None]