Strange thing about attribute '__setitem__' of PixelAccess object

Hello everybody,

I'm doing an image processing project. When processing the sampling rate 
and quantization rate of the image, I come across a strange phenomenon.

Why doesn't __setitem__ exist before help() function but appear after it?
It is the same even after I execute the express 'data[x,y] = value'. It 
only appears after help() function.
Doesn't the express 'data[x,y] = value' call the method __setitem__ ?

Thank you

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