Of course, in this case, one would probably use return instead of assignment, but in cases where mutation is desired, this works best. # Otherwise, number should remain unchanged # Negative odd numbers should return their absolute value 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”)īesides its use as a placeholder for unimplemented functions, pass can be useful in filling out an if-else statement ("Explicit is better than implicit.") def some_silly_transform(n): ![]() (A third option for this case is raise NotImplementedError. Where others might have def update_agent(agent):Īs 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. ( Note that the Ellipsis literal is a valid expression only in Python 3)įor example, if I write a model in broad strokes, I might write def update_agent(agent): in order to strictly differentiate between this and the intentional “no-op” in the previous example. has not been implemented yet, but this will be the place to do it”, although I personally prefer the Ellipsis literal. In some cases, pass is used as a placeholder to say “This method/class/if-block/. """Error encountered while parsing an ill-formed datafile.""" 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): 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. 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, 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):ĭef retrieve(self, deferred_results): pass ![]() : pass statements catch all exceptions, so it's still a frequent pattern in Python programming.ĭeriving an exception class that does not add new behaviour (e.g., in SciPy): class CompileError(Exception): A quick analysis of all Python modules I have installed gave me that more than 10% of all except. ![]() Instead using at least except Error: or in this case preferably except OSError: is considered much better practice. 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?). 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. Self.version = "Expat %d.%d.%d" % expat.version_info Ignoring (all or) a certain type of Exception (example from xml): try: 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. Alternatively, any statement (including just a term to be evaluated, like the Ellipsis literal. Therefore, if nothing is supposed to happen in a code block, a pass is needed for such a block to not produce an IndentationError. ![]() 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. Python has the syntactical requirement that code blocks (after if, except, def, class etc.) cannot be empty.
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