Question

In my python code I have this line:

try:
    result = eval(command, self.globals, self.locals)
except SyntaxError:
    exec(command, self.globals, self.locals)

The command variable can be any string. Hence the python debugger pdb may be started in eval/exec and still be active when eval/exec is returning. What I want to do is make sure normal program execution is resumed when returning from eval/exec. To just give you an idea, this is approximately the behavior that I want:

try:
    result = eval(command, self.globals, self.locals)
    try: self.globals['pdb'].run('continue')
    except: pass
except SyntaxError:
    exec(command, self.globals, self.locals)
    try: self.globals['pdb'].run('continue')
    except: pass

However the try line is shown in the debugger before it is executed, but I dont want the debugger to show my code at all. Also it doesn’t really work… The reason i repeat code is to minimize debugging in my code, else I could just do it after the except block.

So how can I do this?

_{As a sidenote:}

If you try to enter the following lines into the IPython or bpython interpreters you’ll see that they have the same problem and you are able to step into their code.

import pdb
pdb.set_trace()
next

However if you do this in the standard cpython interpreter you are returned to the python prompt. The reason for this is obviously because the two former are implemented in python and the last one is not. But my wish is to get the same behavior even when all code is python.

Answer 1

While I’m somewhat concerned that you are eval/exec’ing a string that you don’t control, I’ll assume you’ve thought that bit through.

I think the simplest thing would be to persuade pdb to check the stack frame on each step and resume automatically when you return to the desired level. You can do that with a simple bit of hotfixing. In the code below I’ve simplified it down to a simple eval since all you are really asking is to have pdb resume automatically on return to a specific function. Call Pdb().resume_here() in the function that you don’t want traced. N.B. the resumption is global and there’s only one resumption point but I’m sure you can modify that if you wanted.

If you run the code then you’ll enter the debugger in function foo() and you can then single step but as soon as you return to bar() the code continues automatically.

e.g.

import sys
from pdb import Pdb

def trace_dispatch(self, frame, event, arg):
    if frame is self._resume_frame:
        self.set_continue()
        return
    return self._original_trace_dispatch(frame, event, arg)

def resume_here(self):
    Pdb._resume_frame = sys._getframe().f_back

# hotfix Pdb
Pdb._original_trace_dispatch = Pdb.trace_dispatch
Pdb.trace_dispatch = trace_dispatch
Pdb.resume_here = resume_here
Pdb._resume_frame = None

def foo():
    import pdb
    pdb.set_trace()
    print("tracing...")
    for i in range(3):
        print(i)

def bar():
    Pdb().resume_here()
    exec("foo();print('done')")
    print("returning")

bar()