Lecture 41

Beautiful Soup

MCS 275 Spring 2022
Emily Dumas

Lecture 41: Beautiful Soup

Course bulletins:

• Please complete your course evaluations.
• Project 4 is due 6pm 29 April (one week from today)
• Remember to install beautifulsoup4 with

  
              python3 -m pip install beautifulsoup4
          

  so you'll be ready for Worksheet 15!

Homework 14

Available now. Due Tuesday at Noon. It's the last homework.

Due to no labs this week, the homework is scaled back. No requirement to write new code.

All you need to do is submit a screenshot showing you can run a Flask+SQLite application on your computer.

See the assignment itself for details. Contact me if you need help.

BS4 basics

        soup = bs4.BeautifulSoup(fp_or_str, "html.parser") # parse!
        str(soup) # the HTML
        soup.prettify() # prettier HTML
        soup.title # first (and only) title tag
        soup.p  # first p tag
        soup.find("p") # first p tag (alternative)
        soup.p.em  # first em tag within the first p tag
        soup.find_all("a") # list of all a tags
    

Working with tags

        str(tag) # HTML for this tag and everything inside it
        tag.name # name of the tag, e.g. "a" or "ul"
        tag.attrs # dict of tag's attributes
        tag["href"] # get a single attribute
        tag.text # All the text nodes inside tag, concatenated
        tag.string # If tag has only text inside it, returns that text
                   # But if it has other tags as well, returns None
        tag.parent # enclosing tag
        tag.contents # list of the children of this tag
        tag.children # iterable of children of this tag
        tag.banana # first descendant banana tag (sub actual tag name!)
        tag.find(...) # first descendant meeting criteria
        tag.find_all(...) # descendants meeting criteria
        tag.find_next_sibling(...) # next sibling tag meeting criteria
        

Searching

Arguments supported by all the find* methods:

        tag.find_all(True) # all descendants
        tag.find_all("tagname") # descendants by tag name
        tag.find_all(href="https://example.com/") # by attribute
        tag.find_all(class_="post") # by class
        tag.find_all(re.compile("^fig")) # tag name regex match
        tag.find_all("a",limit=15) # first 15 a tags
        tag.find_all("a",recursive=False) # all a *children*
    

Also work with find(), find_next_sibling(), ...

Simulating CSS

soup.select(SELECTOR) returns a list of tags that match a CSS selector, e.g.

        soup.select(".wide") # all tags of class "wide"
        
        # ul tags within divs of class messagebox
        soup.select("div.messagebox ul")
        

There are many CSS selectors and functions we haven't discussed, so this gives a powerful alternative search syntax.

        # all third elements of unordered lists
        soup.select("ul > li:nth-of-type(3)")
    

The CSS selector examples here were based on those in the Beautiful Soup documentation.

Sketch of a scraper

        from urllib.request import urlopen
        from bs4 import BeautifulSoup
        import csv

        # grab and parse the HTML
        with urlopen("https://space.wasps/sol-system/") as fobj:
            soup = BeautifulSoup(fobj,"html.parser")

        # find the div we care about
        plansdiv = soup.find("div",id="secret_plans")
    
        # save all links in that div to a CSV file
        with open("plan_links.csv") as outfile:
            writer = csv.writer(outfile)
            writer.writerow(["dest","linktext"])
            for anchor in plansdiv.find_all("a"):
                writer.writerow([anchor["href"], anchor.text])
    

Example scraper

The math department posts a list of upcoming graduate courses at:

https://mscs.uic.edu/graduate/current-students/advising-and-registration/graduate-courses/

Let's write a scraper to convert the Fall 2022 data to a CSV file.

HTML tables

HTML table tag represents a table made up of rectangular cells arranged in aligned rows and columns.

HTML table tags

• table - entire table
• tr - row (inside a table)
• td - data cell (inside a row)
• th - header cell (inside a row)

Scraper tips

• Develop using a local snapshot of the HTML
• Avoid complicated transformaiton; try to faithfully extract the data into a structured format
• Be mindful of maintenance cost (e.g. time); keeping a scraper working as a site that changes over time is difficult. Does size/value of data justify it? [1, 2]
• Try to minimize dependence on markup details that seem most likely to change