python - How do you watch a variable in pdb

Question

I'm debugging a python script, and I want to watch a variable for a change (much like you can watch a memory adress in gdb). Is there a way to do this?

Answer 1

data breakpoints with pdb

...much like you can watch a memory address in gdb...

• GDB uses data breakpoints, this is made easy with hardware support (hardware watchpoints), this typically involves marking the memory pages read-only which then trips an exception handler on memory access. When hardware watchpoints are not available it uses software watchpoints, these are only useful in single threads and are much slower.
• PDB does not support data breakpoints, so the short answer is NO, you cannot do it with PDB out of the box.

printing variables when hitting breakpoints in pdb

For watching a variable when you are hitting a breakpoint, you can use the commands command. E.g. printing some_variable when hitting breakpoint #1 (canonical example from pdb doc).

(Pdb) commands 1
(com) print(some_variable)
(com) end
(Pdb)

Additionally, you can use the condition command to ensure the breakpoint is only hit whenever the variable takes a certain value.

eg:

(Pdb) condition 1 some_variable==some_value

other solutions

You can use tracing / profiling functions to examine things step by step using sys.settrace and checking out the opcodes being executed.

https://docs.python.org/3/library/sys.html#sys.settrace

Here is some code to get you started:

import sys
import dis


def tracefn(frame, event, arg):
    if event == 'call':
        print("## CALL", frame)
        frame.f_trace_opcodes = True
    elif event == 'opcode':
        opcode = frame.f_code.co_code[frame.f_lasti]
        opname = dis.opname[opcode]
        print("## OPCODE", opname)
    return tracefn


watchme = 123

def foo():
    global watchme
    watchme = 122

sys.settrace(tracefn)

foo()

You will probably need to spy on all the STORE_* opcodes. https://docs.python.org/3/library/dis.html