Python 如何快速识别 Snakemake 中的规则是否需要输入函数

我正在关注他们文档页面上的snakemake教程，并真正理解了输入函数https://snakemake.readthedocs.io/en/stable/tutorial/advanced.html#step-3-input-functions的概念

基本上，他们对config.yaml的定义如下:

samples:
  A: data/samples/A.fastq
  B: data/samples/B.fastq

和Snakefile，无任何输入功能:

configfile: "config.yaml"

rule all:
    input:
        "plots/quals.svg"

rule bwa_map:
    input:
        "data/genome.fa",
        "data/samples/{sample}.fastq"
    output:
        "mapped_reads/{sample}.bam"
    threads: 12
    shell:
        "bwa mem -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'])
    output:
        "calls/all.vcf"
    shell:
        "bcftools mpileup -f {input.fa} {input.bam} | "
        "bcftools call -mv - > {output}"

rule plot_quals:
    input:
        "calls/all.vcf"
    output:
        "plots/quals.svg"
    script:
        "scripts/plot-quals.py"

在本教程中，他们提到此扩展发生在初始化步骤中:

bam = expand("sorted_reads/{sample}.bam",sample=config['samples']),
bai = expand("sorted_reads/{sample}.bam.bai",sample=config['samples'])

在此阶段，无法确定规则bwa_map的FASTQ路径.然而，如果我们按原样运行，代码会工作，为什么会这样？

然后，他们建议使用输入函数将bwa_map延迟到下一阶段(DAG阶段)，如下所示:

def get_bwa_map_input_fastqs(wildcards):
    return config["samples"][wildcards.sample]

rule bwa_map:
    input:
        "data/genome.fa",
        get_bwa_map_input_fastqs
    output:
        "mapped_reads/{sample}.bam"
    threads: 8
    shell:
        "bwa mem -t {threads} {input} | samtools view -Sb - > {output}"

我真的很困惑，输入函数什么时候有意义，什么时候没有？