pdb: Your interactive python debugger

Yao Yao on October 4, 2017

Effective Python Item 57: Consider Interactive Debugging with pdb


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

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