--iversions returns an error if no package has been imported

[pareyesv]

Example (watermark v1.8.1):

%load_ext watermark
%watermark --iversions

the output (traceback) is the following:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-3-f12554b3f891> in <module>
----> 1 get_ipython().run_line_magic('watermark', '--iversions')

~/.pyenv/versions/miniconda3-latest/envs/myenvname/lib/python3.7/site-packages/IPython/core/interactiveshell.py in run_line_magic(self, magic_name, line, _stack_depth)
   2311                 kwargs['local_ns'] = sys._getframe(stack_depth).f_locals
   2312             with self.builtin_trap:
-> 2313                 result = fn(*args, **kwargs)
   2314             return result
   2315

</home/myusername/.pyenv/versions/miniconda3-latest/envs/myenvname/lib/python3.7/site-packages/decorator.py:decorator-gen-126> in watermark(self, line)

~/.pyenv/versions/miniconda3-latest/envs/myenvname/lib/python3.7/site-packages/IPython/core/magic.py in <lambda>(f, *a, **k)
    185     # but it's overkill for just that one bit of state.
    186     def magic_deco(arg):
--> 187         call = lambda f, *a, **k: f(*a, **k)
    188
    189         if callable(arg):

~/.pyenv/versions/miniconda3-latest/envs/myenvname/lib/python3.7/site-packages/watermark/watermark.py in watermark(self, line)
    133                 self._get_git_branch(bool(args.machine))
    134             if args.iversions:
--> 135                 self._print_all_import_versions(self.shell.user_ns)
    136             if args.watermark:
    137                 if self.out:

~/.pyenv/versions/miniconda3-latest/envs/myenvname/lib/python3.7/site-packages/watermark/watermark.py in _print_all_import_versions(vars)
    245                             continue
    246
--> 247         longest = max([len(i[0]) for i in to_print]) + 1
    248         for entry in to_print:
    249             print(('%s' % entry[0]).ljust(longest) + entry[1])

ValueError: max() arg is an empty sequence

[rasbt]

Thanks for the note. I can confirm that when this is the first entry executed in a code cell, the same error occurs (prob. because no package has been imported, like you mentioned). This should be fixed though such that if no package has been imported, the code should just print an empty string instead of an error. (If someone has a better suggestion, instead of printing an empty string, I'd be happy to hear.)

I am currently traveling and probably won't get to it immediately, but will try to fix that in the next couple of days.