Pdb is Python's built-in debugger. To make it easier to start pdb, I added this line to my .vimrc:
noremap <D-d> Oimport pdb; pdb.set_trace()<Esc>:w<Return>
When I type Command-d in normal mode, vim will insert the line
import pdb; pdb.set_trace() and save the current file. When I run the python
program, and that line is excuted, the Python program will pause and start pdb.