I am new to python, so this code might not necessarily be clean, it's just for learning purposes. I had the idea of writing this function that would display the tree down the specified path. Then, i added the global variable number_of_py to count how many python files were in that tree. That worked as well. Finally, i decided to turn the whole thing into a generator, but the recursion breaks. My understanding of generators is that once next() is called python just executes the body of the function and "yields" a value until we hit the end of the body. Can someone explain why this doesn't work? Thanks. import os from sys import argv script, path = argv number_of_py = 0 lines_of_code = 0 def list_files(directory, key=''): global number_of_py files = os.listdir(directory) for f in files: real_path = os.path.join(directory, f) if os.path.isdir(real_path): list_files(real_path, key=key+' ') else: if real_path.split('.')[-1] == 'py': number_of_py += 1 with open(real_path) as g: yield len(g.read()) print key+real_path for i in list_files(argv[1]): lines_of_code += i print 'total number of lines of code: %d' % lines_of_code print 'total number of py files: %d' % number_of_py