pythondebuggingbreakpointsevaluationpdb

How to automatically print a variable in pdb?

I am debugging a code in Python using Pdb:

a = [12, 3, 4, 10, 23, 1]


def compute(x):
    return 2 * x


for i in a:
    b = compute(i)

To trace the value of a variable inside the loop, I set a breakpoint at the first line of the loop body and use the continue command to move to the next breakpoint, then use print command to inspect the variable:

$ python -m pdb program.py
> /tmp/program.py(1)<module>()
-> a = [12, 3, 4, 10, 23, 1]
(Pdb) l
  1  -> a = [12, 3, 4, 10, 23, 1]
  2
  3
  4     def compute(x):
  5         return 2 * x
  6
  7
  8     for i in a:
  9         b = compute(i)
[EOF]
(Pdb) b 9
Breakpoint 1 at /tmp/program.py:9
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
(Pdb) p i
12
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
(Pdb) p i
3
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
(Pdb) p i
4
(Pdb)

Is there any way to automatically print the variable's value every time execution stops at the breakpoint?

Solution

  • Yes, It can be done using display command. Use it as follows:

    (Pdb) display variable

    This command will automatically show the current and previous values of the variable each time the debugger stops:

    $ python -m pdb program.py
> /tmp/program.py(1)<module>()
-> a = [12, 3, 4, 10, 23, 1]
(Pdb) b 9
Breakpoint 1 at /tmp/program.py:9
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
(Pdb) display i
display i: 12
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
display i: 3  [old: 12]
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
display i: 4  [old: 3]
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
display i: 10  [old: 4]
(Pdb) c
> /tmp/program.py(9)<module>()
-> b = compute(i)
display i: 23  [old: 10]