How can I tell a Python script to halt for debugger attach to process?

Question

Is there a way to tell Python to halt execution at a certain point in a script and wait for a debugger to attach to the process?

Is there something similar to dot-Net's Debugger.Break() in Python?

Answer 1

Different IDEs can use different methods for attaching to the process, however PyCharm, Eclipse, Visual Studio, and VS Code all use pydevd (VS/VSC via their Python plugin via ptvsd) to provide their in-process debugger implementation. As a result, we can target those with one piece of code.

The idea is to wait until pydevd is imported and then stop at a breakpoint.

import sys
import threading

from importlib.abc import MetaPathFinder


class NotificationFinder(MetaPathFinder):
    def find_spec(self, fullname, _path, _target=None):
        if 'pydevd' in fullname:
            with t:
                t.notify()

t = threading.Condition()
sys.meta_path.insert(0, NotificationFinder())

with t:
    t.wait()

breakpoint()

Since pydevd creates __builtins__.breakpoint, this should work regardless of the Python version. I tested in PyCharm (Community Edition 2019.1.3). I started the script, attached with the "Attach to process" option in my IDE, and was able to attach successfully. The script then stopped at breakpoint() as expected.