• Python's standard debugger is called PDB.
• PDB is a standard command-line debugger.
• The pdb.set_trace() method can be used to stop program execution
  and enter the debugger.
• Your REPL's prompt will change to (Pdb) when you're in the debugger.
• You can access PDB's built-in help system by typing help.
• You can use python -m pdb followed by a script name to run a program under PDB from the start.
• PDB's where command shows the current call stack.
• PDB's next command lets execution continue to the next line of code.
• PDB's continue command lets program execution continue indefinitely, or until you stop it with Ctrl+C.
• PDB's list command shows you the source code at your current location.
• PDB's return command resumes execution until the end of the current function.
• PDB's print command lets you see the values of objects in the debugger.
• Use the  quit command to exit PDB.

Along the way we found that:

• The divmod() function calculates the quotient and remainder for a division
  operation at one time.
• The reversed() function can reverse a sequence.
• You can pass -m to your Python command to have it run a module as a script.
• Debugging makes it clear that Python is evaluating everything at run time.

  Notice that we can use print with or without parentheses. Don't be alarmsed – we haven't regressed to Python 2. In this context print is a PDB command rather than a Python 3 function.