[nanis]

Nothing surprising about this if you know grade school math. Nor is it really about Python at all.

The empty set is a subset of every set.

[billowycoat]

Are strings sets?

[nanis]

Analogously, the empty net is a subnet of every net, the empty sequence is a subsequence of every sequence etc etc.

Also, [p implies q] is vacuously true if p is false.

Strings are maps from nonnegative integers to the space of characters. You can think of them as sets of ordered pairs or sequences.