How to debug a function under the (pdb) prompt?

[jfong at ms4.hinet.net]

For example, there is a file test0.py:
---------
1  def foo():
2      for i in range(3):
3          print(i)
4
5  x = 0
---------
I start the debug session as below. My question is why the break point at line 3 didn't work. How to debug the function foo() at this time?

D:\Works\Python>py -m pdb test0.py
> d:\works\python\test0.py(1)<module>()
-> def foo():
(Pdb) tbreak 5
Breakpoint 1 at d:\works\python\test0.py:5
(Pdb) cont
Deleted breakpoint 1 at d:\works\python\test0.py:5
> d:\works\python\test0.py(5)<module>()
-> x = 0
(Pdb) tbreak 3
Breakpoint 2 at d:\works\python\test0.py:3
(Pdb) foo()
0
1
2
(Pdb)