PS. I recommend this command line to play with tkcrawl.py:

python3 tkcrawl.py xkcd.com -q

The fetching process takes about 6.8 seconds to fetch 1765 URLs on my machine. The original crawl.py takes 6.2 seconds.

I guess the next step would be to hook up the logger to a Tk text widget. It would also be nice to have a set of Tk widgets keep track of the state of the connections in the ConnectionPool, showing for each connection what its state it. It's a little tricky because connections get opened and closed at random; I would recommend using a table assigning connections slots in the widget. The number of lines in the widget is limited by max_pool.

(I would code up more of this myself except my Tkinter is rusty!)