How to programmatically execute/step through Python code line by line

Question

I am trying to find a way that I can have a program step through Python code line by line and do something with the results of each line. In effect a debugger that could be controlled programmatically rather than manually. pdb would be exactly what I am looking for if it returned its output after each step as a string and I could then call pdb again to pickup where I left off. However, instead it outputs to stdout and I have to manually input "step" via the keyboard.

Things I have tried:

I am able to redirect pdb's stdout. I could redirect it to a second Python program which would then process it. However, I cannot figure out how to have the second Python program tell pdb to step.
Related to the previous one, if I could get pdb to step all the way through to the end (perhaps I could figure out something to spoof a keyboard repeatedly entering "step"?) and redirect the output to a file, I could then write another program that acted like it was stepping through the program when it was actually just reading the file line by line.
I could use exec to manually run lines of Python code. However, since I would be looking at one line at a time, I would need to manually detect and handle things like conditionals, loops, and function calls which quickly gets very complicated.
I read some posts that say that pdb is implemented using sys.settrace. If nothing else works I should be able to recreate the behavior I need using this.

Is there any established/straight forward way to implement the behavior that I am looking for?

This appears to be a duplicate of Tracing the execution of a Python program line/statement by line programmatically — Michael Dyck
– Michael Dyck, Commented Dec 17, 2016 at 17:48
Below you say you got it working. Did you spoof stdin to step through, or did you find a way to call something like pdb.do_step()? Can you give some details of your solution? — Michael
– Michael, Commented May 18, 2021 at 2:15

jasonharper · Accepted Answer · 2016-12-09 21:38:18Z

2

sys.settrace() is the fundamental building block for stepping through Python code. pdb is implemented entirely in Python, so you can just look at the module to see how it does things. It also has various public functions/methods for stepping under program control, read the library reference for your version of Python for details.

answered Dec 9, 2016 at 21:38

jasonharper

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2 Comments

David Moneysmith Over a year ago

Thank you for your reply. I was looking at link but the commands it describes use the debugger prompt rather than a program. I got the pdb source at link and I've been trying to access the methods directly. Calling run opens the debugger prompt: myPdb = pdb.Pdb() myPdb.run(test) However, I have not found a method that steps through without a prompt. myPdb.do_step("test = "a = 10\nb=20\nprint a + b") which doesn't work. Do you have any advise on where I should go from here?

David Moneysmith Over a year ago

Hey, sorry for taking so long to accept your answer; things got crazy with the holidays. I was able to get it to work with pdb!

Michael Dyck · Accepted Answer · 2016-12-17 17:39:05Z

1

I read some posts that say that pdb is implemented using sys.settrace. If nothing else works I should be able to recreate the behavior I need using this.

Don't view this as a last resort. I think it's the best approach for what you want to accomplish.

answered Dec 17, 2016 at 17:39

Michael Dyck

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