It's not actually a great solution because it requires a big dependency and uses floating point.

This is poorly specified. You can't tell exactly what it wants you to do for many types of strings without trying to run it.

i think the Python runner has been changed in the meantime and issues like this cannot happen anymore. i've tried declaring local and global variables named chars and they did not get shadowed. i'm closing as this is 4 years old but if someone knows better feel free to step in

In python, if you have a global variable named 'chars' it will get set to 'abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789+/' right before the random tests start (but after the basic tests run.) This was insanely frustrating as I had a variable with this name and similar (but not identical) contents and it took me a very long time to figure out what was going on.