Why are my wildcard attributes not being filled in Snakemake? - workflow

Question

I am following the tutorial in the documentation (https://snakemake.readthedocs.io/en/stable/tutorial/advanced.html) and have been stuck on the "Step 4: Rule parameter" exercise. I would like to access a float from my config file using a wildcard in my params directive.
I seem to be getting the same error whenever I run snakemake -np in the command line:
InputFunctionException in line 46 of /mnt/c/Users/Matt/Desktop/snakemake-tutorial/Snakefile:
Error:
AttributeError: 'Wildcards' object has no attribute 'sample'
Wildcards:
Traceback:
File "/mnt/c/Users/Matt/Desktop/snakemake-tutorial/Snakefile", line 14, in get_bcftools_call_priors
This is my code so far
import time
configfile: "config.yaml"
rule all:
input:
"plots/quals.svg"
def get_bwa_map_input_fastqs(wildcards):
print(wildcards.__dict__, 1, time.time()) #I have this print as a check
return config["samples"][wildcards.sample]
def get_bcftools_call_priors(wildcards):
print(wildcards.__dict__, 2, time.time()) #I have this print as a check
return config["prior_mutation_rates"][wildcards.sample]
rule bwa_map:
input:
"data/genome.fa",
get_bwa_map_input_fastqs
#lambda wildcards: config["samples"][wildcards.sample]
output:
"mapped_reads/{sample}.bam"
params:
rg=r"#RG\tID:{sample}\tSM:{sample}"
threads: 2
shell:
"bwa mem -R '{params.rg}' -t {threads} {input} | samtools view -Sb - > {output}"
rule samtools_sort:
input:
"mapped_reads/{sample}.bam"
output:
"sorted_reads/{sample}.bam"
shell:
"samtools sort -T sorted_reads/{wildcards.sample} "
"-O bam {input} > {output}"
rule samtools_index:
input:
"sorted_reads/{sample}.bam"
output:
"sorted_reads/{sample}.bam.bai"
shell:
"samtools index {input}"
rule bcftools_call:
input:
fa="data/genome.fa",
bam=expand("sorted_reads/{sample}.bam", sample=config["samples"]),
bai=expand("sorted_reads/{sample}.bam.bai", sample=config["samples"])
#prior=get_bcftools_call_priors
params:
prior=get_bcftools_call_priors
output:
"calls/all.vcf"
shell:
"samtools mpileup -g -f {input.fa} {input.bam} | "
"bcftools call -P {params.prior} -mv - > {output}"
rule plot_quals:
input:
"calls/all.vcf"
output:
"plots/quals.svg"
script:
"scripts/plot-quals.py"
and here is my config.yaml
samples:
A: data/samples/A.fastq
#B: data/samples/B.fastq
#C: data/samples/C.fastq
prior_mutation_rates:
A: 1.0e-4
#B: 1.0e-6
I don't understand why my input function call in bcftools_call says that the wildcards object is empty of attributes, yet an almost identical function call in bwa_map has the attribute sample that I want. From the documentation it seems like the wildcards would be propogated before anything is run, so why is it missing?
This is the full output of the commandline call snakemake -np:
{'_names': {'sample': (0, None)}, '_allowed_overrides': ['index', 'sort'], 'index': functools.partial(<function Namedlist._used_attribute at 0x7f91b1a58f70>, _name='index'), 'sort': functools.partial(<function Namedlist._used_attribute at 0x7f91b1a58f70>, _name='sort'), 'sample': 'A'} 1 1628877061.8831172
Job stats:
job count min threads max threads
-------------- ------- ------------- -------------
all 1 1 1
bcftools_call 1 1 1
bwa_map 1 1 1
plot_quals 1 1 1
samtools_index 1 1 1
samtools_sort 1 1 1
total 6 1 1
[Fri Aug 13 10:51:01 2021]
rule bwa_map:
input: data/genome.fa, data/samples/A.fastq
output: mapped_reads/A.bam
jobid: 4
wildcards: sample=A
resources: tmpdir=/tmp
bwa mem -R '#RG\tID:A\tSM:A' -t 1 data/genome.fa data/samples/A.fastq | samtools view -Sb - > mapped_reads/A.bam
[Fri Aug 13 10:51:01 2021]
rule samtools_sort:
input: mapped_reads/A.bam
output: sorted_reads/A.bam
jobid: 3
wildcards: sample=A
resources: tmpdir=/tmp
samtools sort -T sorted_reads/A -O bam mapped_reads/A.bam > sorted_reads/A.bam
[Fri Aug 13 10:51:01 2021]
rule samtools_index:
input: sorted_reads/A.bam
output: sorted_reads/A.bam.bai
jobid: 5
wildcards: sample=A
resources: tmpdir=/tmp
samtools index sorted_reads/A.bam
[Fri Aug 13 10:51:01 2021]
rule bcftools_call:
input: data/genome.fa, sorted_reads/A.bam, sorted_reads/A.bam.bai
output: calls/all.vcf
jobid: 2
resources: tmpdir=/tmp
{'_names': {}, '_allowed_overrides': ['index', 'sort'], 'index': functools.partial(<function Namedlist._used_attribute at 0x7f91b1a58f70>, _name='index'), 'sort': functools.partial(<function Namedlist._used_attribute at 0x7f91b1a58f70>, _name='sort')} 2 1628877061.927639
InputFunctionException in line 46 of /mnt/c/Users/Matt/Desktop/snakemake-tutorial/Snakefile:
Error:
AttributeError: 'Wildcards' object has no attribute 'sample'
Wildcards:
Traceback:
File "/mnt/c/Users/Matt/Desktop/snakemake-tutorial/Snakefile", line 14, in get_bcftools_call_priors
If anyone knows what is going wrong I would really appreciate an explaination. Also if there is a better way of getting information out of the config.yaml into the different directives, I would gladly appreciate those tips.
Edit:
I have searched around the internet quite a bit, but have yet to understand this issue.

Answer 1

Wildcards for each rule are based on that rule's output file(s). The rule bcftools_call has one output file (calls/all.vcf), which has no wildcards. Because of this, when get_bcftools_call_priors is called, it throws an exception when it tries to access the unset wildcards.sample attribute.
You should probably set a global prior_mutation_rate in your config file and then access that in the bcftools_call rule:
rule bcftools_call:
...
params:
prior=config["prior_mutation_rate"],