How to use expand in snakemake when some particular combinations of wildcards are not desired?

Question

Let's suppose that I have the following files, on which I want to apply some processing automatically using snakemake:

test_input_C_1.txt
test_input_B_2.txt
test_input_A_2.txt
test_input_A_1.txt

The following snakefile uses expand to determine all the potential final results file:

rule all:
    input: expand("test_output_{text}_{num}.txt", text=["A", "B", "C"], num=[1, 2])

rule make_output:
    input: "test_input_{text}_{num}.txt"
    output: "test_output_{text}_{num}.txt"
    shell:
        """
        md5sum {input} > {output}
        """

Executing the above snakefile results in the following error:

MissingInputException in line 4 of /tmp/Snakefile:
Missing input files for rule make_output:
test_input_B_1.txt

The reason for that error is that expand uses itertools.product under the hood to generate the wildcards combinations, some of which happen to correspond to missing files.

How to filter out the undesired wildcards combinations?

Answer 1

The expand function accepts a second optional non-keyword argument to use a different function from the default one to combine wildcard values.

One can create a filtered version of itertools.product by wrapping it in a higher-order generator that checks that the yielded combination of wildcards is not among a pre-established blacklist:

from itertools import product

def filter_combinator(combinator, blacklist):
    def filtered_combinator(*args, **kwargs):
        for wc_comb in combinator(*args, **kwargs):
            # Use frozenset instead of tuple
            # in order to accomodate
            # unpredictable wildcard order
            if frozenset(wc_comb) not in blacklist:
                yield wc_comb
    return filtered_combinator

# "B_1" and "C_2" are undesired
forbidden = {
    frozenset({("text", "B"), ("num", 1)}),
    frozenset({("text", "C"), ("num", 2)})}

filtered_product = filter_combinator(product, forbidden)

rule all:
    input:
        # Override default combination generator
        expand("test_output_{text}_{num}.txt", filtered_product, text=["A", "B", "C"], num=[1, 2])

rule make_output:
    input: "test_input_{text}_{num}.txt"
    output: "test_output_{text}_{num}.txt"
    shell:
        """
        md5sum {input} > {output}
        """

---

The missing wildcards combinations can be read from the configfile.

Here is an example in json format:

{
    "missing" :
    [
        {
            "text" : "B",
            "num" : 1
        },
        {
            "text" : "C",
            "num" : 2
        }
    ]
}

The forbidden set would be read as follows in the snakefile:

forbidden = {frozenset(wc_comb.items()) for wc_comb in config["missing"]}