Here's my favorite way to get factors in your R program without wanting them: load an HDF5 file. Python's excellent h5py module and the official HDF5 tools lik

Here's my favorite way to get factors in your R program without wanting them: load an HDF5 file. Python's excellent h5py module and the official HDF5 tools like "h5dump" understand that a dataset is boolean if it has the type H5T_ENUM with two values "FALSE" (0) and "TRUE" (1). But R works a different way: if you save a boolean vector from R to HDF5 (using the rhdf5 package), it will create a dataset of type H5T_STD_I32LE, which takes 4x the storage space. And if you read the boolean vector as understood by other tools, you will get factors. Then, the most amazing thing happens: R uses the string names of the enum ("FALSE" and "TRUE"), but remaps them to factors with 1 and 2. So if you do as.integer(h5read(...)) you will get a vector of ones and twos instead of zeros and ones. And of course R itself treats 1 and 2 as truthy values, so now your data is well and truly corrupted: all the elements are truthy! That is: as.logical(as.integer(h5read(...))) always produces all TRUE values when the input is boolean written by a program other than R. This is a lovely source of bugs in real software.