下面这段python代码中fahr未定义是何含义，pythonfahr,cel = float(

下面这段python代码中fahr未定义是何含义，pythonfahr,cel = float(

cel = float(5 / 9*(fahr - 32))Traceback (most recent call last):  File "<pyshell#31>", line 1, in <module>    cel = float(5 / 9*(fahr - 32))NameError: name 'fahr' is not defined

    fahr=100    #然后    cel = float(5 / 9*(fahr - 32))

def f(fahr):    cel = float(5 / 9*(fahr - 32))    return cel

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