arcgis 10.3 - Delete empty and null fields using arcpy

I need to remove fields within file GDB and SDE databases that contain only empty or null values. I was hoping to do this for an entire database at a time rather than per feature class. The code below shows the best of what I have so far. It does not, however, accomplish what I had hoped. I am somewhat new to Python and am using ArcMap 10.3.1.

import arcpy  
from arcpy import env  
env.workspace = r"C:\New Folder\Default.gdb"

def DelFields(path):
    env.workspace = path
    # first need to point to FCs within DSs
    datasets = arcpy.ListDatasets(feature_type='All')
    datasets = [''] + datasets if datasets is not None else []


    for ds in datasets:
        for fc in arcpy.ListFeatureClasses(feature_dataset=ds):
            # list comp syntax:
            # [{code}var{code} for var in [list] if var{logic test}]. 
            fieldList = [f.name for f in arcpy.ListFields(fc) if f.type ==
                         "Short" or "Long" or "Float" or "Double" or "Text" or
                         "Date" or "Blob" or "Raster" or "Guid"] # "All" did not work                
            print fieldList # to see what I have so far

            if fieldList: # this is a bit of a long shot I think

                # The UpdateCursor line gives error:
                # TypeError: 'field_names' must be string or non empty sequence of strings
                with arcpy.da.UpdateCursor(fc, [fieldList]) as cursor:
                    for row in cursor: 
                        for i in range (len(fieldList)):
                            if row[i].rstrip() == "": # trying to find empties
                                arcpy.DeleteField_management(path, fieldList) 
                            elif row[i] == "": # trying to find Nulls
                                arcpy.DeleteField_management(path, fieldList)
                                cursor.updateRow(row)


DelFields(r"C:\New Folder\Default.gdb")

Answer

Try this; see the comments for explanation.

import os
import arcpy
from arcpy import env

def DelFields(path):
    env.workspace = path

    # Get a list of feature classes directly (i.e., not within feature datasets)
    #   the geodatabase
    fcs = arcpy.ListFeatureClasses()
    # Add the relative paths of feature classes that are within feature
    #   datasets
    fcs.extend([os.path.join(fd, fc) for fd in arcpy.ListDatasets() for fc in arcpy.ListFeatureClasses(feature_dataset=fd)])

    # Iterate over the feature classes
    for fc in fcs:
        # Test whether the field is required--rather than testing the field type

        fieldList = [f.name for f in arcpy.ListFields(fc) if not f.required]

        # Iterate over the fields
        for field in fieldList:
            # Set a test variable to False
            keep = False
            # Iterate over each feature; if it has a non-null value, then set
            #   the test variable to True and then break out of the cursor
            with arcpy.da.SearchCursor(fc, field) as cursor:
                for row in cursor:

                    v = row[0]
                    if v is not None or \
                      (isinstance(v, basestring) \
                      and (v.lower.strip() != ''\
                        or v != '')):
                        keep = True
                        break

            # If the test variable was unchanged, delete the field
            if keep is False:

                arcpy.DeleteField_management(fc, field)

DelFields(r"C:\tlax490\data\temp\temp.gdb")

EDIT: I've changed the if statement within the cursor to test for your desired null-like strings.

To catch similar cases that you might otherwise miss, you can determine whether the value is a string, and if so, lowercase it and then remove empty spaces on the outsides. If the result is your bracketed null or an empty string, then it will be considered null.