How to convert eta level to pressure level in WRF model

Question

I am working WRF outputl file and require to extract wind and temp values at 850 hPa, 700 hPa and 500 hPa pressure levels. As the WRF vertical coordinates are based on eta level, I am having trouble in getting parameters at different pressure level.

I am working python. It would be nice if anyone point in right direction. I want the concept as it can be used with any post processing tools.

Edit:-

I think the question is not clear. The wrfout file contain 27 vertical level and the value ranges between 1.0 to 0. In this case, how do I know that 0.98 corresponds to 950 hPa or 850 hPa etc..