pdb: Your interactive python debugger

Effective Python Item 57: Consider Interactive Debugging with pdb

先看例子：

>>> def func():
...     a = 1
...     b = 2
...     import pdb; pdb.set_trace()  # ①
...     c = 3
...     return 4
... 
>>> func()
> <stdin>(5)func()
(Pdb) bt  # ②
  <stdin>(1)<module>()
> <stdin>(5)func()
(Pdb) p(a)  # ③
1
(Pdb) p(b)  # ③
2
(Pdb) whatis(a)
<class 'int'>

• ① Call pdb.set_trace() to enter the debugger at the calling stack frame. It is useful to hard-code a breakpoint at a given point in a program.
  • debugger console 下的 b(reak) 命令是用来动态创建 breakpoint 的
  • 这里写成一行是方便你注释掉用的（一般你不会在 console 里定义函数，肯定是从你自己的 module 里 import 进来的，所以这里是方便你在你自己的 module 里注释掉）
• ② 进入 debugger console
• ③ 检查 local variables

进入 debugger console 之后，有这些 commands 可以用（完整的说明请参考 Python Documentation: 27.3.1. Debugger Commands）:

• bt: Print the traceback of the current execution call stack.
• u or up: Move your scope up the function call stack to the caller of the current function. This allows you to inspect the local variables in higher levels of the call stack.
• d or down: Move your scope back down the function call stack one level.
• p(expression): Evaluate expression in the current context and print its value.
  • print() can also be used, but is not a debugger command—this executes the Python print() function.
• pp(expression): Like the p command, except using pprint to print
• whatis(expression): Print the type of expression
• s or step: Execute the current line, stop at the first possible occasion (either in a function that is called or on the next line in the current function).
• n or next: Execute the current line, stop at the next line.
  • If the current line includes calling a function, the debugger will not stop until the called function has returned.
• c or continue: Continue execution until the next breakpoint.
• r or return: Continue execution until the current function returns.
• q or quit: Quit from the debugger. The program being executed is aborted.