We’ve mentioned that Snakemake rules take either strings or a list of strings as input, and that we can use any Python expression in Snakemake workflows. Here we’ll show how these features help us condense the code of rules.

Consider the rule align_to_genome below.

rule align_to_genome:
    """
    Align a fastq file to a genome index using Bowtie 2.
    """
    output:
        "intermediate/{sample_id}.bam"
    input:
        "data/raw_internal/{sample_id}.fastq.gz",
        "intermediate/NCTC8325.1.bt2",
        "intermediate/NCTC8325.2.bt2",
        "intermediate/NCTC8325.3.bt2",
        "intermediate/NCTC8325.4.bt2",
        "intermediate/NCTC8325.rev.1.bt2",
        "intermediate/NCTC8325.rev.2.bt2"
    shell:
        """
        bowtie2 -x intermediate/NCTC8325 -U {input[0]} > {output}
        """

Here we have seven inputs; the fastq file with the reads and six files with similar file names from the Bowtie 2 genome indexing. Instead of writing all the filenames we can tidy this up by using a Python expression to generate a list of these files instead. If you’re familiar with Python you could do this with list comprehensions like this:

input:
    "data/raw_internal/{sample_id}.fastq.gz",
    [f"intermediate/NCTC8325.{substr}.bt2" for
        substr in ["1", "2", "3", "4", "rev.1", "rev.2"]]

This will take the elements of the list of substrings one by one, and insert that element in the place of {substr}. Since this type of aggregating rules are quite common, Snakemake also has a more compact way of achieving the same thing.

input:
    "data/raw_internal/{sample_id}.fastq.gz",
    expand("intermediate/NCTC8325.{substr}.bt2",
        substr = ["1", "2", "3", "4", "rev.1", "rev.2"])

Important!
When using expand() like this, substr is not a wildcard because it is resolved to the values explicitly given inside the expand expression.

Now change in the rules index_genome and align_to_genome to use the expand() expression.

In the workflow we decide which samples to run by including the SRR ids in the names of the inputs to the rules multiqc and generate_count_table:

rule generate_count_table:
    output:
        "results/tables/counts.tsv"
    input:
        bams = ["intermediate/SRR935090.sorted.bam", 
                "intermediate/SRR935091.sorted.bam", 
                "intermediate/SRR935092.sorted.bam"],
...
rule multiqc:
    output:
        html = "results/multiqc.html",
        stats = "intermediate/multiqc_general_stats.txt"
    input:
        "intermediate/SRR935090_fastqc.zip",
        "intermediate/SRR935091_fastqc.zip",
        "intermediate/SRR935092_fastqc.zip"

The output files from these two rules, results/multiqc.html and
results/tables/counts.tsv, are in turn specified as input to the all rule at the top of the file. Because the first rule is targeted by default when we run Snakemake on the command line (like we mentioned in snakemake-4-the-mrsa-workflow) this is what triggers the rules to run on each of the three samples.

However, this is a potential source of errors since it’s easy to change in one place and forget to change in the other. Because we can use Python code “everywhere” let’s instead define a list of sample ids and put at the very top of the Snakefile, just before the rule all:

SAMPLES = ["SRR935090", "SRR935091", "SRR935092"]

Now use expand() in multiqc and generate_count_table to use SAMPLES for the sample ids. For the multiqc rule it could look like this:

input:
    expand("intermediate/{sample_id}_fastqc.zip", sample_id = SAMPLES)

See if you can update the generate_count_table rule in the same manner!

Quick recap
In this section we’ve learned:

How to use the expand() expression to create a list with file names, inserting all provided wildcard values.