Why does the Python Debugger not Work Sometimes
Why does the Python Debugger not Work Sometimes#
Ever set a breakpoint with breakpoint() or import ipdb; ipdb.set_trace()- or perhaps you are using an IDE breakpoint - and the program does not break and enter the debugger?
Why is that? It is quite frustrating when trying to figure out the context and cause of a bug.
The reason is that the debugger relies on a shell or tty (teletype writer) access through “stdout/stderr/stdin” forwarding.
If pdb is entered in a process that does not have a tty available then it will just exit.
Testing and PDB#
Pytest I think had this problem because it captured stdout. To tell it to not capture stdout so that you can view the debugging consule use the -s flag
You also might encounter a problem with pytest-xdist - a package that distributes testing compute among cpu cores.
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/bdb.py:90: in trace_dispatch
return self.dispatch_call(frame, arg)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
self = <_pytest.debugging.pytestPDB._get_pdb_wrapper_class.<locals>.PytestPdbWrapper object at 0x114b41940>
frame = <frame at 0x7fb9852d0790, file '/Users/stephen/projects/forks/bitcart/api/schemes.py', line 29, code set_created>
arg = None
def dispatch_call(self, frame, arg):
"""Invoke user function and return trace function for call event.
If the debugger stops on this function call, invoke
self.user_call(). Raise BdbQuit if self.quitting is set.
Return self.trace_dispatch to continue tracing in this scope.
"""
# XXX 'arg' is no longer used
if self.botframe is None:
# First call of dispatch since reset()
self.botframe = frame.f_back # (CT) Note that this may also be None!
return self.trace_dispatch
if not (self.stop_here(frame) or self.break_anywhere(frame)):
# No need to trace this function
return # None
# Ignore call events in generator except when stepping.
if self.stopframe and frame.f_code.co_flags & GENERATOR_AND_COROUTINE_FLAGS:
return self.trace_dispatch
self.user_call(frame, arg)
> if self.quitting: raise BdbQuit
E bdb.BdbQuit
/Library/Frameworks/Python.framework/Versions/3.9/lib/python3.9/bdb.py:135: BdbQuit
to fix that you can temporarily tell pytest-xdist to not distribute tests with the slave processes that don’t have a tty.
Give it these flags:
pytest -n0 --dist no
Sources#
https://github.com/pytest-dev/pytest/issues/390