Allow duplicate rule outputs if they are empty?

[vinisalazar]

Hi,

Is your feature request related to a problem? Please describe.
I have a rule in my workflow that has four outputs. However, I want two of these outputs to only be generated if a value in config is set to true. The way I achieved this by doing this:

rule my_rule:
    [...]
    output:
        optional_output_one: "file1.txt" if config["optional_output_one"] else (),
        optional_output_two: "file2.txt" if config["optional_output_two"] else (),

However, I get the following error when linting my workflow:

Duplicate output file pattern in rule my_rule. First two duplicate for entries optional_output_one and optional_output_two

Describe the solution you'd like
I modified the following line in function check_output_duplicates of rules.py:

if value in seen:

to

if value and value in seen:

and it fixed the problem. This disables the check if it's an empty value for the output file.

Describe alternatives you've considered
If you have any other suggestions or patterns on how to implement this logic that is better than what I've done, that would be very helpful.

Additional context
I'm not entirely sure if this change would introduce undesired or unexpected behaviour; however, if you agree with it, I'd appreciate the opportunity of contributing to Snakemake.

Thank you for any assistance you can provide.

Best,
Vini