Calculating fields in a script

I am working on a script that automates several steps in a workflow process we use. What I want to do is calculate certain fields like I can when I use Field Calculator while working on a feature in ArcMap. For instance I want to make FieldA = FieldXYZ. In Field Calculator this looks like FieldA = !FieldXYZ!
What I can't figure out is how to translate that same process to the code in the script I am working on.
Do I use arcpy.CalculateField_management()? If so how exactly do I set one field equal to another field in the expression for this tool?

Thanks in advance!