Calculate text field based on codeblock query of numeric field

Question

I am trying to populate a field with a L/R bank designation based on a route measure (as part of a larger script), but I am struggling. I have a shapefile created (tir_pts_lyr), I have a blank column (L_R_Bank), I have the measure value column (M_off_str). So if the measure value is negative, I want "L_R_Bank" to be populated with an "L", if the measure is positive, an "R", and if zero, a "C".
I'm trying to use the sample codeblocks in the ArcHelp, but they are lacking in detail for my knowledge level. I have no clue what to put in the expression field for getClass and how the next codeblock line relates to that. Any hints for a newbie.

#calc left/right bank using M-distance measure
# Set local variables
inTable = "tir_pts_lyr"
fieldName = "L_R_Bank"
expression = "getClass(float(!SHAPE.M_Off_str!))"
codeblock = """def getClass(M_Off_str):
    if M_Off_str < 0:
        return "L"
    if M_Off_str > 0:
        return "R"
    else:
        return "C" """
#Execute CalculateField
arcpy.CalculateField_management(inTable, fieldName, expression, "PYTHON", codeblock)

Answer 1

Through blind luck, I seemed to have solved it by removing the SHAPE. out of the expression.

No idea if this is the best way to do this, but it works, so I'm going with it!