Snakemake refuses to unpack input function when rule A is a dependency of rule B, but accepts it when rule A is the final rule - python-3.x

Question

I have a snakemake workflow for a metagenomics project. At a point in the workflow, I map DNA sequencing reads (either single or paired-end) to metagenome assemblies made by the same workflow. I made an input function conform the Snakemake manual to map both single end and paired end reads with one rule. like so
import os.path
def get_binning_reads(wildcards):
pathpe=("data/sequencing_binning_signals/" + wildcards.binningsignal + ".trimmed_paired.R1.fastq.gz")
pathse=("data/sequencing_binning_signals/" + wildcards.binningsignal + ".trimmed.fastq.gz")
if os.path.isfile(pathpe) == True :
return {'reads' : expand("data/sequencing_binning_signals/{binningsignal}.trimmed_paired.R{PE}.fastq.gz", PE=[1,2],binningsignal=wildcards.binningsignal) }
elif os.path.isfile(pathse) == True :
return {'reads' : expand("data/sequencing_binning_signals/{binningsignal}.trimmed.fastq.gz", binningsignal=wildcards.binningsignal) }
rule backmap_bwa_mem:
input:
unpack(get_binning_reads),
index=expand("data/assembly_{{assemblytype}}/{{hostcode}}/scaffolds_bwa_index/scaffolds.{ext}",ext=['bwt','pac','ann','sa','amb'])
params:
lambda w: expand("data/assembly_{assemblytype}/{hostcode}/scaffolds_bwa_index/scaffolds",assemblytype=w.assemblytype,hostcode=w.hostcode)
output:
"data/assembly_{assemblytype}_binningsignals/{hostcode}/{binningsignal}.bam"
threads: 100
log:
stdout="logs/bwa_backmap_samtools_{assemblytype}_{hostcode}.stdout",
samstderr="logs/bwa_backmap_samtools_{assemblytype}_{hostcode}.stdout",
stderr="logs/bwa_backmap_{assemblytype}_{hostcode}.stderr"
shell:
"bwa mem -t {threads} {params} {input.reads} 2> {log.stderr} | samtools view -# 12 -b -o {output} 2> {log.samstderr} > {log.stdout}"
When I make an arbitrary 'all-rule' like this, the workflow runs successfully.
rule allbackmapped:
input:
expand("data/assembly_{assemblytype}_binningsignals/{hostcode}/{binningsignal}.bam", binningsignal=BINNINGSIGNALS,assemblytype=ASSEMBLYTYPES,hostcode=HOSTCODES)
However, when the files created by this rule are required for subsequent rules like so:
rule backmap_samtools_sort:
input:
"data/assembly_{assemblytype}_binningsignals/{hostcode}/{binningsignal}.bam"
output:
"data/assembly_{assemblytype}_binningsignals/{hostcode}/{binningsignal}.sorted.bam"
threads: 6
resources:
mem_mb=5000
shell:
"samtools sort -# {threads} -m {mem_mb}M -o {output} {input}"
rule allsorted:
input:
expand("data/assembly_{assemblytype}_binningsignals/{hostcode}/{binningsignal}.sorted.bam",binningsignal=BINNINGSIGNALS,assemblytype=ASSEMBLYTYPES,hostcode=HOSTCODES)
The workflow closes with this error
WorkflowError in line 416 of
/stor/azolla_metagenome/Azolla_genus_metagenome/Snakefile: Can only
use unpack() on list and dict
To me, this error suggests the input function for the former rule is faulty. This however, seems not to be the case for it ran successfully when no subsequent processing was queued.
The entire project is hosted on github. The entire Snakefile and a github issue.