<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def wav_length(fn: str) -> float: """ Returns the length of the WAV file in seconds."""


    args = [config.SOX_PATH, fn, "-n", "stat"]
    p = subprocess.Popen(
        args, stdin=PIPE, stdout=PIPE, stderr=PIPE)
    length_line = str(p.communicate()[1]).split("\\n")[1].split()
    print(length_line)
    assert length_line[0] == "Length"
    return float(length_line[-1])