that's expected, since os.listdir only returns the names of the files/dirs, so objects are not found, unless you're running it in the current directory.
You have to join to scanned directory to compute the full path for it to work:
for filename in os.listdir(file_to_search):
if os.path.isdir(os.path.join(file_to_search,filename)):
dirlist.append(filename)
note the list comprehension version:
dirlist = [filename for filename in os.listdir(file_to_search) if os.path.isdir(os.path.join(file_to_search,filename))]
