Python“ while”循环（无限迭代）

Iteration means executing the same block of code over and over, potentially many times. A programming structure that implements iteration is called a loop.

迭代意味着一次又一次地执行同一段代码。 实现迭代的编程结构称为循环 。

In programming, there are two types of iteration, indefinite and definite:

在编程中，有两种类型的迭代：不确定的和确定的：

• With indefinite iteration, the number of times the loop is executed isn’t specified explicitly in advance. Rather, the designated block is executed repeatedly as long as some condition is met.

• With definite iteration, the number of times the designated block will be executed is specified explicitly at the time the loop starts.

• 对于不确定的迭代 ，循环的执行次数没有事先明确指定。 相反，只要满足某些条件，就会重复执行指定的块。

• 通过确定的迭代 ，指定的块将在循环开始时明确执行的次数。

In this tutorial, you’ll:

在本教程中，您将：

• Learn about the while loop, the Python control structure used for indefinite iteration
• See how to break out of a loop or loop iteration prematurely
• Explore infinite loops

• 了解while循环，这是用于不确定迭代的Python控制结构
• 了解如何提前退出循环或循环迭代
• 探索无限循环

When you’re finished, you should have a good grasp of how to use indefinite iteration in Python.

完成后，您应该掌握如何在Python中使用不确定的迭代。

while循环 (The while Loop)

Let’s see how Python’s while statement is used to construct loops. We’ll start simple and embellish as we go.

让我们看看如何使用Python的while语句构造循环。 我们将以简单而优美的方式开始。

The format of a rudimentary while loop is shown below:

基本的while循环的格式如下所示：

 while while << exprexpr >> :
    :
    << statementstatement (( ss )) >
>

<statement(s)> represents the block to be repeatedly executed, often referred to as the body of the loop. This is denoted with indentation, just as in an if statement.

<statement(s)>表示要重复执行的块，通常称为循环的主体。 就像if语句一样，用缩进表示。

The controlling expression, <expr>, typically involves one or more variables that are initialized prior to starting the loop and then modified somewhere in the loop body.

控制表达式<expr>通常涉及一个或多个变量，这些变量在启动循环之前已初始化，然后在循环主体中的某处进行了修改。

When a while loop is encountered, <expr> is first evaluated in Boolean context. If it is true, the loop body is executed. Then <expr> is checked again, and if still true, the body is executed again. This continues until <expr> becomes false, at which point program execution proceeds to the first statement beyond the loop body.

遇到while循环时，首先在Boolean上下文中评估<expr> 。 如果为true，则执行循环体。 然后再次检查<expr> ，如果仍然为true，则再次执行主体。 这一直持续到<expr>变为false，这时程序执行将继续执行循环体之外的第一条语句。

Consider this loop:

考虑以下循环：

 >>>  n = 5
>>>  while n > 0 :
...     n -= 1
...     print ( n )
...
4
3
2
1
0

Here’s what’s happening in this example:

这是此示例中发生的事情：

• n is initially 5. The expression in the while statement header on line 2 is n > 0, which is true, so the loop body executes. Inside the loop body on line 3, n is decremented by 1 to 4, and then printed.

• When the body of the loop has finished, program execution returns to the top of the loop at line 2, and the expression is evaluated again. It is still true, so the body executes again, and 3 is printed.

• This continues until n becomes 0. At that point, when the expression is tested, it is false, and the loop terminates. Execution would resume at the first statement following the loop body, but there isn’t one in this case.

• n最初是5 。 第2行的while语句标头中的表达式为n > 0 ，这是正确的，因此将执行循环体。 在第3行的循环体内， n递减1到4 ，然后打印。

• 循环体完成后，程序执行将返回到循环的第二行，并再次对表达式进行求值。 这仍然是正确的，因此主体再次执行，并打印了3 。

• 这一直持续到n变为0为止。 此时，当测试表达式时，它为false，并且循环终止。 执行将在循环体后的第一个语句处恢复，但在这种情况下没有一个。

Note that the controlling expression of the while loop is tested first, before anything else happens. If it’s false to start with, the loop body will never be executed at all:

请注意，在发生其他任何事情之前，将首先测试while循环的控制表达式。 如果开头是错误的，则循环体将永远不会执行：

 >>>  n = 0
>>>  while n > 0 :
...     n -= 1
...     print ( n )
...

In the example above, when the loop is encountered, n is 0. The controlling expression n > 0 is already false, so the loop body never executes.

在上面的示例中，遇到循环时， n为0 。 控制表达式n > 0已经为假，因此循环体从不执行。

Here’s another while loop involving a list, rather than a numeric comparison:

这是涉及列表而不是数字比较的另一个while循环：

 >>>  a = [ 'foo' , 'bar' , 'baz' ]
>>>  while a :
...     print ( a . pop ( - 1 ))
...
baz
bar
foo

When a list is evaluated in Boolean context, it is truthy if it has elements in it and falsy if it is empty. In this example, a is true as long as it has elements in it. Once all the items have been removed with the .pop() method and the list is empty, a is false, and the loop terminates.

在布尔上下文中评估列表时，如果列表中包含元素，则为true；如果为空，则为false。 在此示例中，只要其中包含元素， a为真。 一旦使用.pop()方法删除了所有项目，并且列表为空，则a为false，并且循环终止。

循环迭代中断 (Interruption of Loop Iteration)

In each example you have seen so far, the entire body of the while loop is executed on each iteration. Python provides two keywords that terminate a loop iteration prematurely:

到目前为止，在每个示例中， while循环的整个主体都是在每次迭代中执行的。 Python提供了两个关键字来提前终止循环迭代：

• break immediately terminates a loop entirely. Program execution proceeds to the first statement following the loop body.

• continue immediately terminates the current loop iteration. Execution jumps to the top of the loop, and the controlling expression is re-evaluated to determine whether the loop will execute again or terminate.

• break立即完全终止循环。 程序执行继续到循环体后的第一条语句。

• continue立即终止当前循环迭代。 执行跳转到循环的顶部，并重新计算控制表达式以确定循环是再次执行还是终止。

The distinction between break and continue is demonstrated in the following diagram:

之间的区别break和continue证明如下图：

Here’s a script file called break.py that demonstrates the break statement:

这是一个名为break.py的脚本文件，用于演示break语句：

Running break.py from a command-line interpreter produces the following output:

从命令行解释器运行break.py会产生以下输出：

 C:UsersjohnDocuments>python break.py
C:UsersjohnDocuments> python break.py
4
4
3
3
Loop ended.
Loop ended.

When n becomes 2, the break statement is executed. The loop is terminated completely, and program execution jumps to the print() statement on line 7.

当n变为2 ，将执行break语句。 循环完全终止，程序执行跳至第7行的print()语句。

The next script, continue.py, is identical except for a continue statement in place of the break:

下一个脚本continue.py与除break的continue语句相同：

The output of continue.py looks like this:

continue.py的输出如下所示：

 C:UsersjohnDocuments>python continue.py
C:UsersjohnDocuments> python continue.py
4
4
3
3
1
1
0
0
Loop ended.
Loop ended.

This time, when n is 2, the continue statement causes termination of that iteration. Thus, 2 isn’t printed. Execution returns to the top of the loop, the condition is re-evaluated, and it is still true. The loop resumes, terminating when n becomes 0, as previously.

这次，当n为2 ， continue语句导致该迭代终止。 因此，不会打印2 。 执行返回到循环的顶部，条件被重新评估，并且仍然为真。 如前所述，循环将继续，直到n变为0时终止。

else条款 (The else Clause)

Python allows an optional else clause at the end of a while loop. This is a unique feature of Python, not found in most other programming languages. The syntax is shown below:

Python在while循环的结尾允许一个可选的else子句。 这是Python的独特功能，大多数其他编程语言都没有。 语法如下所示：

The <additional_statement(s)> specified in the else clause will be executed when the while loop terminates.

当while循环终止时，将执行else子句中指定的<additional_statement(s)> 。

About now, you may be thinking, “How is that useful?” You could accomplish the same thing by putting those statements immediately after the while loop, without the else:

关于现在，您可能会想，“这有什么用？” 您可以通过将这些语句立即放在while循环之后而不使用else来完成相同的事情：

 while while << exprexpr >> :
    :
    << statementstatement (( ss )) >
>
<< additional_statementadditional_statement (( ss )) >
>

What’s the difference?

有什么不同？

In the latter case, without the else clause, <additional_statement(s)> will be executed after the while loop terminates, no matter what.

在后一种情况下，没有else子句，无论循环如何， <additional_statement(s)>将在while循环终止后执行。

When <additional_statement(s)> are placed in an else clause, they will be executed only if the loop terminates “by exhaustion”—that is, if the loop iterates until the controlling condition becomes false. If the loop is exited by a break statement, the else clause won’t be executed.

将<additional_statement(s)>放在else子句中时，仅当循环“通过穷举”终止时（即，循环迭代直到控制条件变为假），才执行它们。 如果循环由break语句退出，则else子句将不会执行。

Consider the following example:

考虑以下示例：

 >>>  n = 5
>>>  while n > 0 :
...     n -= 1
...     print ( n )
...  else :
 ...     print ( 'Loop done.' )
 ...
4
3
2
1
0
Loop done.

In this case, the loop repeated until the condition was exhausted: n became 0, so n > 0 became false. Because the loop lived out its natural life, so to speak, the else clause was executed. Now observe the difference here:

在这种情况下，重复循环直到条件用尽： n变为0 ，因此n > 0变为假。 可以说，由于循环是自然的，所以执行else子句。 现在在这里观察区别：

 >>>  n = 5
>>>  while n > 0 :
...     n -= 1
...     print ( n )
...     if n == 2 :
 ...         break
 ...  else :
...     print ( 'Loop done.' )
...
4
3
2

This loop is terminated prematurely with break, so the else clause isn’t executed.

该循环以break提前终止，因此else子句不执行。

It may seem as if the meaning of the word else doesn’t quite fit the while loop as well as it does the if statement. Guido van Rossum, the creator of Python, has actually said that, if he had it to do over again, he’d leave the while loop’s else clause out of the language.

看起来单词else的含义似乎不太适合while循环，也似乎不适合if语句。 实际上，Python的创建者Guido van Rossum曾说过，如果要让它再做一遍，他会把while循环的else子句放在语言之外。

One of the following interpretations might help to make it more intuitive:

以下解释之一可能有助于使其更直观：

• Think of the header of the loop (while n > 0) as an if statement (if n > 0) that gets executed over and over, with the else clause finally being executed when the condition becomes false.

• Think of else as though it were nobreak, in that the block that follows gets executed if there wasn’t a break.

• 将循环的标头（ while n > 0 ）视为一个if语句（ if n > 0 ），它会一遍又一遍地执行，当条件变为false时， else子句最终会执行。

• 想想else好像nobreak ，因为如果没有break ，紧随其后的程序段将被执行。

If you don’t find either of these interpretations helpful, then feel free to ignore them.

如果您对以上两种解释都没有帮助，请随时忽略它们。

When might an else clause on a while loop be useful? One common situation is if you are searching a list for a specific item. You can use break to exit the loop if the item is found, and the else clause can contain code that is meant to be executed if the item isn’t found:

while循环上的else子句何时有用？ 一种常见情况是在列表中搜索特定项目。 如果找到该项目，则可以使用break退出循环，如果没有找到该项目，则else子句可以包含要执行的代码：

 >>>  a = [ 'foo' , 'bar' , 'baz' , 'qux' ]
>>>  s = 'corge'

>>>  i = 0
>>>  while i < len ( a ):
...     if a [ i ] == s :
...         # Processing for item found
...         break
...     i += 1
...  else :
...     # Processing for item not found
...     print ( s , 'not found in list.' )
...
corge not found in list.

 >>>  if s in a :
...     print ( s , 'found in list.' )
...  else :
...     print ( s , 'not found in list.' )
...
corge not found in list.

 >>>  try :
...     print ( a . index ( 'corge' ))
...  except ValueError :
...     print ( s , 'not found in list.' )
...
corge not found in list.

An else clause with a while loop is a bit of an oddity, not often seen. But don’t shy away from it if you find a situation in which you feel it adds clarity to your code!

带有while循环的else子句有点奇怪，不经常看到。 但是，如果发现这种情况会增加代码的清晰度，请不要回避它！

无限循环 (Infinite Loops)

Suppose you write a while loop that theoretically never ends. Sounds weird, right?

假设您编写了一个理论上永远不会结束的while循环。 听起来很怪吧？

Consider this example:

考虑以下示例：

 >>>  while True :
...     print ( 'foo' )
...
foo
foo
foo
  .
  .
  .
foo
foo
foo
KeyboardInterrupt
Traceback (most recent call last):
  File "<pyshell#2>" , line 2 , in <module>
    print ( 'foo' )

This code was terminated by Ctrl+C, which generates an interrupt from the keyboard. Otherwise, it would have gone on unendingly. Many foo output lines have been removed and replaced by the vertical ellipsis in the output shown.

该代码由Ctrl + C终止，这会从键盘产生中断。 否则，它会一直持续下去。 许多foo输出行已删除，并由所示输出中的垂直省略号代替。

Clearly, True will never be false, or we’re all in very big trouble. Thus, while True: initiates an infinite loop that will theoretically run forever.

显然， True永远不会为假，否则我们都将面临巨大麻烦。 因此， while True:启动一个无限循环，理论上它将永远运行。

Maybe that doesn’t sound like something you’d want to do, but this pattern is actually quite common. For example, you might write code for a service that starts up and runs forever accepting service requests. “Forever” in this context means until you shut it down, or until the heat death of the universe, whichever comes first.

也许这听起来不像您想做的事情，但是这种模式实际上很普遍。 例如，您可能会为启动并永远运行以接受服务请求的服务编写代码。 在这种情况下，“永远”是指直到您将其关闭或直到宇宙热死为止，以先到者为准。

More prosaically, remember that loops can be broken out of with the break statement. It may be more straightforward to terminate a loop based on conditions recognized within the loop body, rather than on a condition evaluated at the top.

更一般地说，请记住可以使用break语句打破循环。 根据在循环体内识别的条件而不是在顶部评估的条件终止循环可能更直接。

Here’s another variant of the loop shown above that successively removes items from a list using .pop() until it is empty:

这是上面显示的循环的另一个变体，它使用.pop()连续从列表中删除项目，直到其为空：

 >>>  a = [ 'foo' , 'bar' , 'baz' ]
>>>  while True :
...     if not a :
...         break
...     print ( a . pop ( - 1 ))
...
baz
bar
foo

When a becomes empty, not a becomes true, and the break statement exits the loop.

当a变为空时， not a变为true， break语句退出循环。

You can also specify multiple break statements in a loop:

您还可以在循环中指定多个break语句：

In cases like this, where there are multiple reasons to end the loop, it is often cleaner to break out from several different locations, rather than try to specify all the termination conditions in the loop header.

在这种情况下，有多种原因终止循环，从几个不同的位置break通常会比较干净，而不是尝试在循环头中指定所有终止条件。

Infinite loops can be very useful. Just remember that you must ensure the loop gets broken out of at some point, so it doesn’t truly become infinite.

无限循环可能非常有用。 只需记住，您必须确保循环在某个时刻被打破，这样才不会真正变成无限循环。

嵌套while循环 (Nested while Loops)

In general, Python control structures can be nested within one another. For example, if/elif/else conditional statements can be nested:

通常，Python控件结构可以相互嵌套。 例如， if / elif / else条件语句可以嵌套：

 if if age age < < 1818 :
    :
    if if gender gender == == 'M''M' :
        :
        printprint (( 'son''son' )
    )
    elseelse :
        :
        printprint (( 'daughter''daughter' )
)
elif elif age age >= >= 18 18 and and age age < < 6565 :
    :
    if if gender gender == == 'M''M' :
        :
        printprint (( 'father''father' )
    )
    elseelse :
        :
        printprint (( 'mother''mother' )
)
elseelse :
    :
    if if gender gender == == 'M''M' :
        :
        printprint (( 'grandfather''grandfather' )
    )
    elseelse :
        :
        printprint (( 'grandmother''grandmother' )
)

Similarly, a while loop can be contained within another while loop, as shown here:

同样， while循环可以包含在另一个while循环中，如下所示：

 >>>  a = [ 'foo' , 'bar' ]
>>>  while len ( a ):
 ...     print ( a . pop ( 0 ))
...     b = [ 'baz' , 'qux' ]
...     while len ( b ):
 ...         print ( '>' , b . pop ( 0 ))
...
foo
> baz
> qux
bar
> baz
> qux

A break or continue statement found within nested loops applies to the nearest enclosing loop:

在嵌套循环中找到的break或continue语句适用于最近的封闭循环：

Additionally, while loops can be nested inside if/elif/else statements, and vice versa:

另外， while循环可以嵌套在if / elif / else语句中，反之亦然：

 if if << exprexpr >> :
    :
    statement
    statement
    while while << exprexpr >> :
        :
        statement
        statement
        statement
statement
elseelse :
    :
    while while << exprexpr >> :
        :
        statement
        statement
        statement
    statement
    statement
statement

In fact, all the Python control structures can be intermingled with one another to whatever extent you need. That is as it should be. Imagine how frustrating it would be if there were unexpected restrictions like “A while loop can’t be contained within an if statement” or “while loops can only be nested inside one another at most four deep.” You’d have a very difficult time remembering them all.

实际上，所有Python控件结构都可以相互混合到所需的程度。 那是应该的。 想象一下，怎么折腾那将是如果有类似的限制意外“A while循环不能被包含在一个if声明”或“ while循环只能被嵌套在彼此最多四层。” 您将很难记住它们。

Seemingly arbitrary numeric or logical limitations are considered a sign of poor program language design. Happily, you won’t find many in Python.

似乎任意的数字或逻辑限制被认为是糟糕的程序语言设计的标志。 幸运的是，在Python中找不到很多。

一线while循环 (One-Line while Loops)

As with an if statement, a while loop can be specified on one line. If there are multiple statements in the block that makes up the loop body, they can be separated by semicolons (;):

与if语句一样，可以在一行上指定while循环。 如果在构成循环体的块中有多个语句，则可以用分号（ ; ）分隔它们：

>>> n = 5
>>> while n > 0: n -= 1; print(n)

4
3
2
1
0

This only works with simple statements though. You can’t combine two compound statements into one line. Thus, you can specify a while loop all on one line as above, and you write an if statement on one line:

但是，这仅适用于简单的语句。 您不能将两个复合语句合并为一行。 因此，您可以如上所述在一行上全部指定一个while循环，然后在一行上编写一个if语句：

>>> if True: print('foo')

foo

But you can’t do this:

但是您不能这样做：

>>> while n > 0: n -= 1; if True: print('foo')
SyntaxError: invalid syntax

Remember that PEP 8 discourages multiple statements on one line. So you probably shouldn’t be doing any of this very often anyhow.

请记住， PEP 8不鼓励在一行上使用多个语句。 因此，您可能不应该经常这样做。

结论 (Conclusion)

In this tutorial, you learned about indefinite iteration using the Python while loop. You’re now able to:

在本教程中，您学习了使用Python while循环进行无限迭代的方法。 您现在可以：

• Construct basic and complex while loops
• Interrupt loop execution with break and continue
• Use the else clause with a while loop
• Deal with infinite loops

• 构造基本而复杂的while循环
• 中断循环执行并break并continue
• 将else子句与while循环一起使用
• 处理无限循环

You should now have a good grasp of how to execute a piece of code repetitively.

现在，您应该掌握如何重复执行一段代码。

The next tutorial in this series covers definite iteration with for loops—recurrent execution where the number of repetitions is specified explicitly.

本系列的下一篇教程将介绍带for循环的确定迭代 -循环执行，其中明确指定重复次数。

翻译自: https://www.pybloggers.com/2018/11/python-while-loops-indefinite-iteration/