# The pass statement
# Ignore an exception
try: metadata = metadata['properties'] except KeyError: pass
# Create a new Exception that can be caught
class CompileError(Exception): pass
Why would you ever want to tell the interpreter to explicitly do nothing?
Python has the syntactical requirement that code blocks (after
class etc.) cannot be empty.
But sometimes an empty code block is useful in itself. An empty
class block can definine a new, different class, such as exception that can be caught. An empty
except block can be the simplest way to express “ask for forgiveness later” if there was nothing to ask for forgiveness for. If an iterator does all the heavy lifting, an empty
for loop to just run the iterator can be useful.
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
... 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. Here is a small annotated collection of the most frequent uses of
pass that crossed my way – together with some comments on good and bad pratice.
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.
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.
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.
class _BaseSubmittingController(_BaseController): def submit(self, tasks): pass def retrieve(self, deferred_results): pass
for x, error in MDNewton(mp, f, (1,-2), verbose=0, norm=lambda x: norm(x, inf)): pass
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 `...` (NOTE: python-3 only) 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
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”)