Convert expression in Calculate Field from VB to PYTHON3

Question

I'm having trouble converting these expression to Python 3, could someone help me?

arcpy.CalculateField_management(Featureclass1, "test", "Len( [ClassRoomCd] )", "VB", "")

arcpy.CalculateField_management(Featureclass2, "ClassRoomCd", "/"0/"+ [ClassRoomCd]", "VB", "")

I tried the following expressions, but I got "the calculation tool is invalid" error.

arcpy.CalculateField_management(Featureclass1, "test", "Len( !ClassRoomCd! )", "PYTHON", "")

arcpy.CalculateField_management(Featureclass2, "ClassRoomCd", "/"0/"+ !ClassRoomCd!", "PYTHON", "")

I'm not sure what the last argument (the empty one) means.

Thank you in advance.

Rema · Answer

Dear Raul
I'm sorry for thanking you so late.
That's what I wanted to know!
I read the link you atattched for me, and got what the "code block" is.
Thank you.

Amir-Sarrafzadeh-Arasi · Answer

Dear Rema, 
My Pleasure

Best Wishes

Rema · Answer

Dear 
Amir Sarrafzadeh Arasi

Thank you for your reply.
I tryed the script that you atatched, and that was educational! 
The script was so simple that even I can understand. 
I succeeded to fill a new field with  the values  I defined in the script.
This experience motivated me to study python.
Thank you so much!