Quick Method to list all of the feature classes, even in folders, in any geodatabase and in feature datasets

walk_data.py

import arcpy
from os.path import abspath, join
directory = r"C:\Users\cmoravec\Documents\ArcGIS"

def walk_directory(directory):
    output_list = list()
    for base_dir, child_dirs, filenames in arcpy.da.Walk(directory, datatype='FeatureClass'):
        for filename in filenames:
            data = join(abspath(base_dir), filename)
            output_list.append(data)
    return output_list
        
output_data = walk_directory(directory)
print output_data