Python，函数输出不符合预期

Question

I wrote a function to create a directory in python. The function adds an extension for example _1 to the directoryname if the original name was already taken. The function works and creates the folder as expected; But there is something wrong with the returnvalue. When I print it out I get None. Here is my code. Probably not the cleanest :s. This has something to do with calling itself inside the function, but I'm not sure how to fix it.

import os


##function to generate a dir with extension if it already exists
def createDir(directory,ext):
  thePath=directory+ext
  if not os.path.exists(thePath):
    os.makedirs(thePath)
    output=thePath + '/'
    print 'I return ' + output #I got "I return /media/usb0/incomplete_noNameYet_2/ (Because incomplete_noNameYet and incomplete_noNameYet_1 already existed)" This is fine!
    return output
  else:
    if ext =='':
      ext='_1'
    else:
      ext= '_' + str(int(ext[1:len(ext)])+1)
    createDir(directory,ext)

def main():
  print createDir('/media/usb0/incomplete_noNameYet','') #I got none.   

if __name__ == '__main__':
  main() 

Answer 1

You are ignoring the return value of the recursive call ; add a return there too:

return createDir(directory,ext)

otherwise the return value of createDir() is discarded, and the parent function call returns with no explicit return call, defaulting to None .

Answer 2

The problem is that on the else branch, you are calling createDir and not returning the value. The initial call to createDir carries out the second two recursive calls (including their side effects) but then discards the return value and returns None.