import os
import arcpy
def AssignDefault(tbl, fld):
try:
fname = arcpy.ListFields(tbl, fld)[0].name
mxd = arcpy.mapping.MapDocument("CURRENT")
default = os.path.splitext(arcpy.Describe(mxd.filePath).name)[0]
arcpy.AssignDefaultToField_management(tbl, fname, default)
print default
except:
print("Could not assign default")
AssignDefault("Roads", "Project_Name")