Is concurrency hard? You can certainly find people arguing both sidesof the issue if you look hard enough (mostly for rather than against).But I think the question may be blurring two things together that areworthwhile to separate: 'concurrency is difficult to learn andunderstand', and 'concurrency is tricky to implement correctly'.

For example, in his reaction to Joel Spolsky's Java Schools article ,Tim Bray gives a little taxonomy of difficulty:

My experience differs from Joel's in another respect:

Something I took from this Ian Bicking entry is that synergies and elegance don't just happen; someone usually workedhard to make it all come out neatly. Python lists have an interestingcase of this; some apparently odd decisions in other places turn out tobe needed to create useful (and error-avoiding )synergies.

Python lists (and sequences in general) are indexed from 0. Zero-basedindexing presents a problem, which can be succinctly stated as this:list element indexes run from 0 to len