My rule of thumb: only using single letter variables in one-liners (and never if it spills to another line), or for something that is conventionally represented as such. So for example:

    ```python
    bar = [foo(e) for e in elements]
    ```

or, using `x`, `n`, `s` and similar when they represent just that, a generic variable with a number, string, etc. I think there is a Code Complete chapter about it.