field calculator - Using arcpy.CalculateField_management

I have a table with a column named "Kategorie", filled with values. I added a new column named "RULE". I want to fill this column with Strings depending on "KATEGORIE". I have a PY-Script and it doesn't work.

What is wrong?

Expression:

"getKat(!KATEGORIE!)"

Codeblock:

"""def getKat(KATEGORIE):
    if KATEGORIE = "0":
        myval = "Autobahn"
    if KATEGORIE = "1":
        myval = "Bundesstrasse"
    else:
        myval= 3"""

arcpy.CalculateField_management(ERG, "RULE", expression, "", codeblock)

---

Both answers so far don't work. I think the problem is the line "expression" or "codeblock". I try to print the result of "myval".The result is a error message "name myval is not defined"

expression:

"getKat(!KATEGORIE!)"

codeblock:

"""def getKat(KATEGORIE):
    if KATEGORIE == "0":

        myval == "Autobahn"
    elif KATEGORIE == "1":
        myval == "Bundesstrasse"
    else:
        myval == "3"
    return myval"""
print "myval", myval
arcpy.CalculateField_management(ERG, "RULE", expression, "PYTHON", codeblock)