Any input that your program accepts that produces a sensible output is within the set of inputs for which your program is designed to be valid, which is a subse

Any input that your program accepts that produces a sensible output is within the set of inputs for which your program is designed to be valid, which is a subset of all possible inputs. Arbitrary input means any possible input, beyond the set for which your program is valid, including those that causes your program to segfault or set your cat on fire. Expecting to handle "arbitrary input data" would mean to try to accept data outside of that set. In the case of a CSV parser, it would mean parsing /dev/urandom as a CSV and producing sensible output ("sensible" defined by the users, which in your example is the dog "Sue"). What the maxim says about being flexible in what you accept is not trying to tell you that everything should be considered valid, but instead that it doesn't hurt to allow certain rules to be broken if it cannot do any harm. This could mean supporting trailing commas in JSON arrays, which while illegal is a harmless "extension". However, it is not trying to tell you that you should accept garbage and "silently recover", trying to do something with invalid input. Doing so does more harm than good. Be flexible, not stupid.