Testing on the Toilet: Avoid Hardcoding Values for Better Libraries

This article was adapted from a Google Testing on the Toilet (TotT) episode. You can download a printer-friendly version of this TotT episode and post it in your office.

By Adel Saoud


You may have been in a situation where you're using a value that always remains the same, so you define a constant. This can be a good practice as it removes magic values and improves code readability. But be mindful that hardcoding values can make usability and potential refactoring significantly harder.

Consider the following function that relies on hardcoded values:
// Declared in the module.
constexpr int kThumbnailSizes[] = {480, 576, 720};

// Returns thumbnails of various sizes for the given image.
std::vector<Image> GetThumbnails(const Image& image) {
  std::vector<Image> thumbnails;
  for (const int size : kThumbnailSizes) {
    thumbnails.push_back(ResizeImage(image, size));
  }
  return thumbnails;
}


Using hardcoded values can make your code:
  • Less predictable: The caller might not expect the function to be relying on hardcoded values outside its parameters; a user of the function shouldn’t need to read the function’s code to know that. Also, it is difficult to predict the product/resource/performance implications of changing these hardcoded values.
  • Less reusable: The caller is not able to call the function with different values and is stuck with the hardcoded values. If the caller doesn’t need all these sizes or needs a different size, the function has to be forked or refactored to avoid aforementioned complications with existing callers.

When designing a library, prefer to pass required values, such as through a function call or a constructor. The code above can be improved as follows:
std::vector<Image> GetThumbnails(const Image& image, absl::Span<const int> sizes) {
  std::vector<Image> thumbnails;
  for (const int size : sizes) {
    thumbnails.push_back(ResizeImage(image, size));
  }
  return thumbnails;
}


If most of the callers use the same value for a certain parameter, make your code configurable so that this value doesn't need to be duplicated by each caller. For example, you can define a public constant that contains a commonly used value, or use default arguments in languages that support this feature (e.g. C++ or Python).
// Declared in the public header.
inline constexpr int kDefaultThumbnailSizes[] = {480, 576, 720};

// Default argument allows the function to be used without specifying a size.
std::vector<Image> GetThumbnails(const Image& image,
                                 absl::Span<const int> sizes = kDefaultThumbnailSizes);