utils: get_caller_name: handle cases when source is unavailable

[rokm]

Fix get_caller_name to gracefully handle cases when a module's source code is unavailable. In such cases, FrameInfo.code_context is None an cannot be iterated over:

Traceback (most recent call last):
  File "program.py", line 6, in <module>
    app = dash.Dash(__name__)
  File "dash/dash.py", line 399, in __init__
  File "dash/_utils.py", line 290, in get_caller_name
TypeError: 'NoneType' object is not iterable

This happens with PyInstaller, where both dash modules and the program's entry-point script are collected as byte-compiled .pyc modules, without corresponding source .py files.

But it can be also triggered by simply byte-compiling the python script into .pyc, removing the source .py, and running the .pyc.

[alexcjohnson]

Thanks @rokm - is there anything else we can do in these cases or is the information about caller name irretrievably lost? We use this to infer the app module name so that users can omit __name__ from app = Dash(__name__) - but if we need to make exceptions "in cases X, Y, Z you still need to provide __name__" this gets much more confusing for users.

cc @T4rk1n

[T4rk1n]

It's not possible for inspect to get the frame source code if inspect can't access the source file which seems to be the case when:

• Running interactive prompt
• Running compiled bytecode like .pyc and pyinstaller.

For the interactive prompt, the name is always __main__ and the default will works. There might be a way to do without the name check and just use the frame index as it should always be the frame above Dash.__init__

[alexcjohnson]

Superseded by #2672 - thanks for your help @rokm!