Python - 返回None

Question

我已经开始使用谷歌Python类，但我得到了一些奇怪的结果，整整一天的调试并没有帮助我解决它。

似乎正在发生的事情是函数返回None而不是我分配它们的值，但是为什么会发生这种情况是在逃避我。我写了一些调试行，并尝试逐步完成它，但我看到导致该行为的原因。

以下是一些调试输出的示例：

C:\Users\toshiba\Dropbox\DEV\python\google-python-exercises\basic>python string2.py
front_back
  X  got: None expected: 'abxcdy'
 OK  got: 'abcxydez' expected: 'abcxydez'
 OK  got: 'KitDontenut' expected: 'KitDontenut'

代码来自googles类，然后是我编写的函数。

# F. front_back
# Consider dividing a string into two halves.
# If the length is even, the front and back halves are the same length.
# If the length is odd, we'll say that the extra char goes in the front half.
# e.g. 'abcde', the front half is 'abc', the back half 'de'.
# Given 2 strings, a and b, return a string of the form
#  a-front + b-front + a-back + b-back
def front_back(a, b):
  # +++your code here+++

  # Debug hardcode setting
  # set to 1 to debug (default 0 off)
  letsDebug = 0

  alpha, bravo = a, b
  if letsDebug == 1:
        endString = a \
        + ' ' \
        + b
        return endString

  lenA = len(alpha)
  lenB = len(bravo)

  if lenA % 2 == 1:
    statAlpha = 'odd'
  else:
    statAlpha = 'even'

  if lenB % 2 == 1:
    statBravo = 'odd'
  else:
    statBravo = 'even'
  if letsDebug == 2:
        endString = a \
        + ' ' \
        + b \
        + ' ' \
        + statAlpha \
        +  ' ' \
        + statBravo 
        return endString

  workB = lenB / 2
  workA = lenA / 2
  if letsDebug == 3:
        endString = a \
        + ' ' \
        + b \
        + ' ' \
        + statAlpha \
        +  ' ' \
        + statBravo \
        + ' ' \
        + str(workA) \
        + ' ' \
        + str(workB) 
        return endString

  if statAlpha == 'even':
    aFront, aBack = alpha[:workA], alpha[-workA:]
  else:
    aFront, aBack = alpha[:(workA+1)], alpha[-workA:]

  if statBravo == 'even':
    bFront, bBack = bravo[:workB], bravo[-workB:]
  else:
    bFront, bBack = bravo[:(workB+1)], bravo[-workB:]

    if letsDebug == 4:
        endString = a \
        + ' ' \
        + str(workA) \
        + ' ' \
        + b \
        + ' ' \
        + str(workB) \
        + ' ' \
        + statAlpha \
        +  ' ' \
        + statBravo \
        + ' ' \
        + aFront \
        + ' ' \
        + bFront \
        + ' ' \
        + aBack \
        + ' ' \
        + bBack \
        + ' ' \
        + aFront + bFront + aBack + bBack
    else:
        endString = aFront + bFront + aBack + bBack

    return endString


# Simple provided test() function used in main() to print
# what each function returns vs. what it's supposed to return.
def test(got, expected):
  if got == expected:
    prefix = ' OK '
  else:
    prefix = '  X '
  print '%s got: %s expected: %s' % (prefix, repr(got), repr(expected))


# main() calls the above functions with interesting inputs,
# using the above test() to check if the result is correct or not.
def main():
  print 'verbing'
  test(verbing('hail'), 'hailing')
  test(verbing('swiming'), 'swimingly')
  test(verbing('do'), 'do')

  print
  print 'not_bad'
  test(not_bad('This movie is not so bad'), 'This movie is good')
  test(not_bad('This dinner is not that bad!'), 'This dinner is good!')
  test(not_bad('This tea is not hot'), 'This tea is not hot')
  test(not_bad("It's bad yet not"), "It's bad yet not")

  print
  print 'front_back'
  test(front_back('abcd', 'xy'), 'abxcdy')
  test(front_back('abcde', 'xyz'), 'abcxydez')
  test(front_back('Kitten', 'Donut'), 'KitDontenut')

if __name__ == '__main__':
  main()

非常感谢任何能破译我在这里的地方的人。

Answer 1

如果在front_back（）中没有return语句覆盖，则有一个路径。这一个：

if statBravo == 'even':

Answer 2

看起来front_back末尾的整个块缩进了太多级别。从if letsDebug == 4:到return endString - 这是在其上方启动的else块的所有部分（语句else的{​​{1}}）。我猜这应该是在函数范围内。