我需要在Python程序中模拟一个do-while循环。不幸的是,下面的简单代码不能工作:

list_of_ints = [ 1, 2, 3 ]
iterator = list_of_ints.__iter__()
element = None

while True:
  if element:
    print element

  try:
    element = iterator.next()
  except StopIteration:
    break

print "done"

不是"1,2,3,done",而是输出如下:

[stdout:]1
[stdout:]2
[stdout:]3
None['Traceback (most recent call last):
', '  File "test_python.py", line 8, in <module>
    s = i.next()
', 'StopIteration
']

为了捕获“停止迭代”异常并中断一段时间,我能做些什么 循环正常吗?

下面的伪代码显示了为什么需要这样一个东西的示例。

状态机:

s = ""
while True :
  if state is STATE_CODE :
    if "//" in s :
      tokens.add( TOKEN_COMMENT, s.split( "//" )[1] )
      state = STATE_COMMENT
    else :
      tokens.add( TOKEN_CODE, s )
  if state is STATE_COMMENT :
    if "//" in s :
      tokens.append( TOKEN_COMMENT, s.split( "//" )[1] )
    else
      state = STATE_CODE
      # Re-evaluate same line
      continue
  try :
    s = i.next()
  except StopIteration :
    break

当前回答

下面的代码可能是一个有用的实现,突出了do-while和while之间的主要区别。

在这种情况下,你总是至少要循环一次。

first_pass = True
while first_pass or condition:
    first_pass = False
    do_stuff()

其他回答

while True:
    try:
        # stuff
        stuff_1()
        if some_cond:
            continue
        if other_cond:
            break
        stuff_2()
    finally:
        # condition
        if not condition:
            break

[x]条件只检查后运行的东西 [x]不是函数调用 [x]条件不是函数调用 [x]的东西可以包含流量控制 []避免检查条件,如果东西叫break(可以用另一个布尔值)

你想知道:

为了捕捉“停止迭代”异常并正确地打破while循环,我能做些什么?

你可以这样做,如下所示,这也利用了Python 3.8中引入的赋值表达式特性(又名“海象运算符”):

list_of_ints = [1, 2, 3]
iterator = iter(list_of_ints)

try:
    while (element := next(iterator)):
        print(element)
except StopIteration:
    print("done")

另一种可能(从Python 2.6到3.x都适用)是为内置的next()函数提供一个默认参数,以避免StopIteration异常:

SENTINEL = object()  # Unique object.
list_of_ints = [1, 2, 3]
iterator = iter(list_of_ints)

while True:
    element = next(iterator, SENTINEL)
    if element is SENTINEL:
        break
    print(element)

print("done")

异常将打破循环,所以您不妨在循环之外处理它。

try:
  while True:
    if s:
      print s
    s = i.next()
except StopIteration:   
  pass

我猜你的代码的问题是,除了没有定义内部中断的行为。通常break只上一层,例如try内部的break直接进入finally(如果存在的话)一个out of try,而不是out of循环。

相关PEP: http://www.python.org/dev/peps/pep-3136 相关问题:打破嵌套循环

用于包含try语句的do - while循环

loop = True
while loop:
    generic_stuff()
    try:
        questionable_stuff()
#       to break from successful completion
#       loop = False  
    except:
        optional_stuff()
#       to break from unsuccessful completion - 
#       the case referenced in the OP's question
        loop = False
   finally:
        more_generic_stuff()

或者,当不需要'finally'从句时

while True:
    generic_stuff()
    try:
        questionable_stuff()
#       to break from successful completion
#       break  
    except:
        optional_stuff()
#       to break from unsuccessful completion - 
#       the case referenced in the OP's question
        break

快速攻击:

def dowhile(func = None, condition = None):
    if not func or not condition:
        return
    else:
        func()
        while condition():
            func()

像这样使用:

>>> x = 10
>>> def f():
...     global x
...     x = x - 1
>>> def c():
        global x
        return x > 0
>>> dowhile(f, c)
>>> print x
0