def METHOD_NAME(self, default_config, custom_conf_dir, monkeypatch):
    """Verify that environment variables are able to be referenced in config
    files.
    """
    default_dir = default_config.parent
    aux_conf = default_dir / "auxiliary.conf"
    write_config(
        aux_conf,
        """
        [virustotaldl]
        enabled = no
        #dlintelkey = SomeKeyWithDLAccess
        dlpath = /tmp/
        """,
    )
    custom_conf = custom_conf_dir / "auxiliary.conf"
    write_config(
        custom_conf,
        """
        [virustotaldl]
        enabled = yes
        dlintelkey = %(ENV:DLINTELKEY)s
        """,
    )
    custom_secret = "MyReallySecretKeyWithAPercent(%)InIt"
    monkeypatch.setenv("DLINTELKEY", custom_secret)
    config = Config("auxiliary")
    section = config.get("virustotaldl")
    # Inherited from default config
    assert section.dlpath == "/tmp/"
    # Overridden from custom config
    assert section.enabled is True
    # Overridden from custom config and uses environment variable
    assert section.dlintelkey == custom_secret