_Python in a NutShell_, p. 138 has a bit on the assert statement which I don't completely understand.

It says the syntax is

assert condition[, _expression_]

I was hoping to put some sort of explanation of failure in an assert statement. But how to do it?

In my code I have

assert(len(list(set(colors_used_this_cycle))) == len(colors_used_this_cycle), "A color has been used twice!")

But it doesn't work. Cases where a color has been used more than once go right through it, whereas

colors_used_this_cycle.append(color)
assert(len(list(set(colors_used_this_cycle))) == len(colors_used_this_cycle)

works perfectly.

So I'd like to know what that '_expression_' in the syntax can be, and how to use it.

Thanks,

Dick Moores
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