My use case is when I know ahead of time that some executions of a particular rule will take longer than others, on a multi-core machine. I want the "longer-running" rule to run first. I was thinking I could specify the priority of the rule via a wildcard, but I get:

RuleException in file /private/tmp/snakemake/Snakefile, line 14:
Priority values have to be numeric.

Here's the snakefile:

contigs = {
    "chr1": 1000,
    "chr2": 10000,
    "chr3": 100000,
    "chr4": 100,
}

rule all:
    input:
        [f"{c}.txt" for c in contigs]


rule txt:
    output:
        txt = "{c}.txt"
    priority: lambda wcs: contigs[wcs.c]
    shell:
        "touch {output.txt}"