How to use the pass statement?

Question

I am in the process of learning Python and I have reached the section about the pass statement. The guide I'm using defines it as being a Null statement that is commonly used as a placeholder.

I still don't fully understand what that means though. Can someone show me a simple/basic situation where the pass statement would be used and why it is needed?

Answer 1

The pass statement does nothing. It can be used when a statement is required syntactically but the program requires no action.

Answer 2

Here's an example where I was extracting particular data from a list where I had multiple data types (that's what I'd call it in R-- sorry if it's the wrong nomenclature) and I wanted to extract only integers/numeric and NOT character data.

The data looked like:

>>> a = ['1', 'env', '2', 'gag', '1.234', 'nef']
>>> data = []
>>> type(a)
<class 'list'>
>>> type(a[1])
<class 'str'>
>>> type(a[0])
<class 'str'>

I wanted to remove all alphabetical characters, so I had the machine do it by subsetting the data, and "passing" over the alphabetical data:

a = ['1', 'env', '2', 'gag', '1.234', 'nef']
data = []
for i in range(0, len(a)):
    if a[i].isalpha():
        pass
    else:
        data.append(a[i])
print(data)
['1', '2', '1.234']

Answer 3

Python has the syntactical requirement that code blocks (after if, except, def, class etc.) cannot be empty. Empty code blocks are however useful in a variety of different contexts, such as in examples below, which are the most frequent use cases I have seen.

Therefore, if nothing is supposed to happen in a code block, a pass is needed for such a block to not produce an IndentationError. Alternatively, any statement (including just a term to be evaluated, like the Ellipsis literal ... or a string, most often a docstring) can be used, but the pass makes clear that indeed nothing is supposed to happen, and does not need to be actually evaluated and (at least temporarily) stored in memory.

• 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. 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, there is no 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”)

Answer 4

The best and most accurate way to think of pass is as a way to explicitly tell the interpreter to do nothing. In the same way the following code:

def foo(x,y):
    return x+y

means "if I call the function foo(x, y), sum the two numbers the labels x and y represent and hand back the result",

def bar():
    pass

means "If I call the function bar(), do absolutely nothing."

The other answers are quite correct, but it's also useful for a few things that don't involve place-holding.

For example, in a bit of code I worked on just recently, it was necessary to divide two variables, and it was possible for the divisor to be zero.

c = a / b

will, obviously, produce a ZeroDivisionError if b is zero. In this particular situation, leaving c as zero was the desired behavior in the case that b was zero, so I used the following code:

try:
    c = a / b
except ZeroDivisionError:
    pass

Another, less standard usage is as a handy place to put a breakpoint for your debugger. For example, I wanted a bit of code to break into the debugger on the 20th iteration of a for... in statement. So:

for t in range(25):
    do_a_thing(t)
    if t == 20:
        pass

with the breakpoint on pass.

Answer 5

pass in Python basically does nothing, but unlike a comment it is not ignored by interpreter. So you can take advantage of it in a lot of places by making it a place holder:

1: Can be used in class

   class TestClass: 
      pass

2: Can be use in loop and conditional statements:

   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: Can be used in function :

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

pass is mostly used when programmer does not want to give implementation at the moment but still wants to create a certain class/function/conditional statement which can be used later on. Since the Python interpreter does not allow for blank or unimplemented class/function/conditional statement it gives an error:

IndentationError: expected an indented block

pass can be used in such scenarios.

Answer 6

Suppose you are designing a new class with some methods that you don't want to implement, yet.

class MyClass(object):
    def meth_a(self):
        pass

    def meth_b(self):
        print "I'm meth_b"

If you were to leave out the pass, the code wouldn't run.

You would then get an:

IndentationError: expected an indented block

To summarize, the pass statement does nothing particular, but it can act as a placeholder, as demonstrated here.