pydevd_resolver.py: "unsupported operand type(s) for +: 'dict' and 'list'" in TupleResolver.get_contents_debug_adapter_protocol throws

[aorcl]

Environment data

• PTVSD version: python-2019.3.6139
• OS and version: Windows10
• Python version (& distribution if applicable, e.g. Anaconda): 3.7.2.64 standard python
• Using VS Code or Visual Studio: VSCode

Actual behavior

When trying to inspect a collection during debug, the variable inspector hangs and throws a message

Expected behavior

Inspection working

Steps to reproduce:

1. Start debugging a program that uses collections module.
2. Trying to inspect a collections variable throws "unsupported operand type(s) for +: 'dict' and 'list'":

Traceback (most recent call last):  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 695, in trace_dispatch
    self.do_wait_suspend(thread, frame, event, arg)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_frame.py", line 78, in do_wait_suspend
    self._args[0].do_wait_suspend(*args, **kwargs)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\pydevd.py", line 1285, in do_wait_suspend
    keep_suspended = self._do_wait_suspend(thread, frame, event, arg, suspend_type, from_this_thread, frames_tracker)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\pydevd.py", line 1307, in _do_wait_suspend
    self.process_internal_commands()
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\pydevd.py", line 1024, in process_internal_commands
    int_cmd.do_it(self)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_comm.py", line 485, in do_it
    self.method(dbg, *self.args, **self.kwargs)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_comm.py", line 655, in internal_get_variable_json
    for child_var in variable.get_children_variables(fmt=fmt):
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_suspended_frames.py", line 97, in get_children_variables
    lst = resolver.get_contents_debug_adapter_protocol(self.value, fmt=fmt)
  File "c:\Users\adolgano\.vscode\extensions\ms-python.python-2019.3.6139\pythonFiles\lib\python\ptvsd\_vendored\pydevd\_pydevd_bundle\pydevd_resolver.py", line 374, in get_contents_debug_adapter_protocol
    ret = from_default_resolver + ret
TypeError: unsupported operand type(s) for +: 'dict' and 'list'

The code tries indeed to add a list to a dict with a '+'

if from_default_resolver:
    ret = from_default_resolver + ret
return ret

[aorcl]

Thank you!