Is there a function in python which returns the Amplitude/Sound Pressure Level of a sound file(like .wav) file? I found a function called scipy.io.wavefile.read() which returns the sampling rate and an array. But in the documentation, it is not specified that whether the array returned by this function is of Amplitude or Sound Pressure Level. Which one is it?
No, you need to know the full Transfer function from the acoustic response at the sensors face to your A/D converter.  Transducer responses vary with frequency. You need a calibrated reference at a minimum.  

A reference sensor will have a response curve from the manufacturer.  An average would be useful as well.