Python在语法上要求代码块(在if, except, def, class等之后)不能为空。然而，空代码块在各种不同的上下文中都很有用，比如下面的例子，这是我见过的最常见的用例。

因此，如果在代码块中不希望发生任何事情，则需要一个传递以使该代码块不产生IndentationError。或者，任何语句(包括一个要求值的项，如Ellipsis字面量…)可以使用字符串(通常是docstring)，但该传递清楚地表明，实际上不应该发生任何事情，也不需要实际计算并(至少暂时)存储在内存中。

Ignoring (all or) a certain type of Exception (example from xml): try: self.version = "Expat %d.%d.%d" % expat.version_info except AttributeError: pass # unknown Note: Ignoring all types of raises, as in the following example from pandas, is generally considered bad practice, because it also catches exceptions that should probably be passed on to the caller, e.g. KeyboardInterrupt or SystemExit (or even HardwareIsOnFireError – How do you know you aren't running on a custom box with specific errors defined, which some calling application would want to know about?). try: os.unlink(filename_larry) except: pass Instead using at least except Error: or in this case preferably except OSError: is considered much better practice. A quick analysis of all Python modules I have installed gave me that more than 10% of all except ...: pass statements catch all exceptions, so it's still a frequent pattern in Python programming. Deriving an exception class that does not add new behaviour (e.g., in SciPy): class CompileError(Exception): pass Similarly, classes intended as abstract base class often have an explicit empty __init__ or other methods that subclasses are supposed to derive (e.g., pebl): class _BaseSubmittingController(_BaseController): def submit(self, tasks): pass def retrieve(self, deferred_results): pass Testing that code runs properly for a few test values, without caring about the results (from mpmath): for x, error in MDNewton(mp, f, (1,-2), verbose=0, norm=lambda x: norm(x, inf)): pass In class or function definitions, often a docstring is already in place as the obligatory statement to be executed as the only thing in the block. In such cases, the block may contain pass in addition to the docstring in order to say “This is indeed intended to do nothing.”, for example in pebl: class ParsingError(Exception): """Error encountered while parsing an ill-formed datafile.""" pass In some cases, pass is used as a placeholder to say “This method/class/if-block/... has not been implemented yet, but this will be the place to do it”, although I personally prefer the Ellipsis literal ... in order to strictly differentiate between this and the intentional “no-op” in the previous example. (Note that the Ellipsis literal is a valid expression only in Python 3) For example, if I write a model in broad strokes, I might write def update_agent(agent): ... where others might have def update_agent(agent): pass before def time_step(agents): for agent in agents: update_agent(agent) as a reminder to fill in the update_agent function at a later point, but run some tests already to see if the rest of the code behaves as intended. (A third option for this case is raise NotImplementedError. This is useful in particular for two cases: Either “This abstract method should be implemented by every subclass, and there isn't a generic way to define it in this base class”, or “This function, with this name, is not yet implemented in this release, but this is what its signature will look like”)

在Python中，pass基本上什么都不做，但与注释不同的是，它不会被解释器忽略。所以你可以在很多地方利用它，把它变成占位符:

1:可以在课堂上使用

class TestClass:
    pass

2:可以在循环语句和条件语句中使用:

if (something == true):  # used in conditional statement
    pass

while (some condition is true):  # user is not sure about the body of the loop
    pass

3:可用于功能:

def testFunction(args): # The programmer wants to implement the body of the function later
    pass

当程序员暂时不想给出实现，但仍然想创建某个类/函数/条件语句以供以后使用时，通常使用Pass。由于Python解释器不允许出现空白或未实现的类、函数或条件语句，因此会给出一个错误:

IndentationError:期望一个缩进的块

Pass可以在这种情况下使用。

除了用作未实现函数的占位符外，pass还可以用于填充if-else语句(“显式比隐式好”)。

def some_silly_transform(n):
    # Even numbers should be divided by 2
    if n % 2 == 0:
        n /= 2
        flag = True
    # Negative odd numbers should return their absolute value
    elif n < 0:
        n = -n
        flag = True
    # Otherwise, number should remain unchanged
    else:
        pass

当然，在这种情况下，可能会使用return而不是赋值，但在需要突变的情况下，这种方法效果最好。

在这里使用pass特别有用，可以警告未来的维护者(包括您自己!)不要将多余的步骤放在条件语句之外。在上面的例子中，在特别提到的两种情况中设置了flag，但在其他情况中没有设置。如果不使用pass，未来的程序员可能会将flag = True移到条件之外，从而在所有情况下设置flag。

另一种情况是经常出现在文件底部的样板函数:

if __name__ == "__main__":
    pass

在某些文件中，最好将其与pass一起保留，以便稍后更容易地进行编辑，并明确表示当文件单独运行时不期望发生任何事情。

最后，正如在其他回答中提到的，当异常被捕获时，什么都不做是很有用的:

try:
    n[i] = 0
except IndexError:
    pass

在Python中，pass语句用于在语法上需要语句，但不希望执行任何命令或代码的情况。

pass语句是一个空操作;执行时什么也不发生。pass在你的代码最终要去的地方也很有用，但还没有写出来(例如，在存根中):

例子:

#!/usr/bin/python

for letter in 'Python':
   if letter == 'h':
      pass
      print 'This is pass block'
   print 'Current Letter :', letter

print "Good bye!"

这将产生以下结果:

Current Letter : P
Current Letter : y
Current Letter : t
This is pass block
Current Letter : h
Current Letter : o
Current Letter : n
Good bye!

如果字母的值是'h'，上面的代码不会执行任何语句或代码。当您创建了一个代码块时，pass语句很有用，但不再需要它。

然后，您可以删除块中的语句，但让块保留pass语句，这样它就不会干扰代码的其他部分。

如果你想导入一个模块，如果它存在，并且忽略导入它，如果该模块不存在，你可以使用下面的代码:

try:
    import a_module
except ImportError:
    pass
# The rest of your code

如果您避免编写pass语句并继续编写其余代码，则会引发IndentationError，因为打开except块后的行没有缩进。

一个可以“按原样”使用的常见用例是重写一个类来创建一个类型(在其他方面与超类相同)，例如。

class Error(Exception):
    pass

因此，您可以引发并捕获Error异常。这里重要的是异常的类型，而不是内容。