This principle doesn't necessarily mean functions should never return booleans, though.

Booleans are used in a variety of (popular) Python libraries when checking whether a password is correct (e.g. Django's `check_password` returns False if the password is wrong).

This is most likely the reason. It allows your code to follow the happy path for logging in, and treat verification as the exceptional case. Definitely a design choice by the author who self-identifies as a Pythonista.

  def handle_login():
    try:
      ph.verify(hash, "s3kr3tp4ssw0rd")
      log_login()
      redirect_to_page()
    except VerificationError:
      log_bad_login()
      redirect_to_login()

While we're talking Python best practices, I highly recommend using an `else` clause to keep the code being 'excepted' to one line:

  def handle_login():
    try:
      ph.verify(hash, "s3kr3tp4ssw0rd")
    except VerificationError:
      log_bad_login()
      redirect_to_login()
    else:
      log_login()
      redirect_to_page()

It's not a big deal for this code, but in general this is good practice because

1. It makes it very obvious to the next developer which line is the one that is expected to raise that exception

2. One of the other lines could unintentionally raise that exception and mistakenly trigger the except clause. (This is more of an issue with Python's built in exceptions than with something very specific like this `VerificationError` example.)