The Paradox of Function Header Comments

Perhaps the most widely used form of code documentation is function header comments. We performed a large-scale survey of 367 developers to catalog their expectations from such documentation and to chronicle actual practice. Paradoxically, we found that developers appreciate the value of header comments and estimate that they are worth the investment in time, but nevertheless they tend not to write such documentation in their own code. Reasons for not writing header comments vary from the belief that code should be self-documenting to concern that documentation will not be kept up-to-date. A possible outcome of this situation is that developers may evade requirements to write documentation by using templates to generate worthless comments that do not provide any real information. We define a simple metric for information-less documentation based on its similarity to the function signature. Applying this to 21,140 files in GitHub Python projects shows that most functions are undocumented, but when header comments are written they typically do contain additional information beyond the function signature.