summed_total = 0
with arcpy.da.SearchCursor(fc, "field to be totaled") as cursor:
    for row in cursor:
        summed_total = summed_total + row[0]

summed_total = 0
field = "field to be summed"
with arcpy.SearchCursor(fc):
    for row in cursor:
        summed_total = summed_total + row.GetValue(field)

import arcpy, numpy

fc = r'C:pathtoyourfc'

# Convert to numpy array.  "test" is the field name
field = arcpy.da.TableToNumPyArray (fc, "test", skip_nulls=True)
sum = field["test"].sum()