如何在Snakemake规则中使用dict值和key

samples = {
    "sample1": "/path/to/sample1.fastq",
    "sample2": "/path/to/sample2.fastq",
    "sample3": "/path/to/sample3.fastq"
}

rule example_rule:
    input:
        samples["sample1"]
    output:
        "output.txt"
    shell:
        "command --input {input} --output {output}"

rule all:
    input:
        expand("output_{sample}.txt", sample=samples.keys())

rule example_rule:
    input:
        samples["{sample}"]
    output:
        "output_{sample}.txt"
    shell:
        "command --input {input} --output {output}"