File not found with os.listdir(dir)
os.listdir(dir) returns a list of file names relative to
dir; if this is not your current working directory (i.e.
.) you cannot directly open, copy, or otherwise manipulate the files that it returns. You have to resolve the relative file name with something like
for filename in os.listdir(dir): with open(os.path.join(dir, filename)) as filehandle: ...