A trick of setting breakpoint in pdb

When using pdb to debug a python program:

python -m pdb foo.py

I want to set a breakpoint, but meet following error:

(Pdb) b bar.py:46
*** 'bar.py' not found from sys.path

A small trick is setting breakpoint in main first and run the program:

(Pdb) b main
Breakpoint 1 at ......
(Pdb) r
......

After breakpoint set for main is hit, set breakpoint again at bar.py:46. This time it should work:

(Pdb) b bar.py:46
Breakpoint 2 at ......