Diff fails if stack Ignore parameter set to True

[goopilot]

Subject of the issue

When running "sceptre diff" on a stack with ignore set to True, getting an error message "ValueError: max() arg is an empty sequence"

Your environment

$ sceptre --version
Sceptre, version 4.4.2

$ python --version
Python 3.8.10

OS: Windows

Steps to reproduce

Set ignore to Trues for any sceptre project:

template:
  type: file
  path: XXXX.yaml
ignore: True

Expected behaviour

Stack should be ignored with corresponding message

Actual behaviour

$ sceptre --var-file vars.yaml --var runlocal=true diff XXXX/YYYYY
Traceback (most recent call last):
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\runpy.py", line 194, in _run_module_as_main
return _run_code(code, main_globals, None,
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\runpy.py", line 87, in run_code
exec(code, run_globals)
File "C:\Users\asalamatov\AppData\Local\Programs\Python\Python38\Scripts\sceptre.exe_main.py", line 7, in
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\click\core.py", line 1157, in call
return self.main(*args, **kwargs)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\click\core.py", line 1078, in main
rv = self.invoke(ctx)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\click\core.py", line 1688, in invoke
return _process_result(sub_ctx.command.invoke(sub_ctx))
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\click\decorators.py", line 33, in new_func
return f(get_current_context(), *args, **kwargs)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\helpers.py", line 46, in decorated
return func(*args, **kwargs)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\diff.py", line 142 File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\click\decorators.py", line 33, in new_func
return f(get_current_context(), *args, **kwargs)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\helpers.py", line 46, in decorated
return func(*args, **kwargs)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\diff.py", line 142, in diff_command
num_stacks_with_diff = output_diffs(
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\diff.py", line 176, in output_diffs
output_buffer_with_normalized_bar_lengths(line_buffer, output_buffer)
File "c:\users\asalamatov\appdata\local\programs\python\python38\lib\site-packages\sceptre\cli\diff.py", line 190, in output_buffer_with_normalized_bar_lengths
max_length = len(max(buffer, key=len))
ValueError: max() arg is an empty sequence

[zaro0508]

The Sceptre docs says.. The ignore configuration only applies to the launch command. You can still run create, update, or delete commands on a stack marked with ignore: True; these commands will ignore the ignore setting and act upon the stack the same as any other..

@goopilot I think we can agree that Sceptre should not throw an exception in this case. Do you expect ignore to support the diff command as well?

[goopilot]

@zaro0508 Thanks for the quick response.

Yes I expect that any command run on a stack with any valid configuration should not throw an exception.

My use case is a pipeline which is running diff as a first step on a folder with whole bunch of stacks and since some major stacks have ignore the whole pipeline fails.

I did quick workaround for myself (#1463) but now as you mentioned that diff doesn't support ignore, I think more work need to be done to add ignore implementation for diff.

[alex-harvey-z3q]

Any chance you are still planning on sending in a PR @goopilot ?

[goopilot]

Any chance you are still planning on sending in a PR @goopilot ?

The chance is around 0% I'm more of a user of the sceptre than a developer. But I'd appreciate if somebody can take this over.