On 5 July 2010 08:27, Richard D. Moores <rdmoo...@gmail.com> wrote: > See < > http://docs.python.org/py3k/library/stdtypes.html#boolean-operations-and-or-not > >. > I am quite familiar with the meaning of "x and y" in Python, and how > it is evaluated -- first x, and only if x is False, then evaluate y. >
Sorry if this is being overly pedantic, but I thought I'd point out the above isn't right as stated, although I understand what you're getting at (re short circuit boolean evaluation) in general. To be correct, I presume you meant "OR" where you wrote "AND", as it would be correct in that case e.g: x AND y: Will only evaluate y if x is TRUE. (If x is FALSE then you don't need to evaluate y since the resultant expression will be FALSE regardless, see footnote 2 in the page you referenced.) x OR y: Will only evaluate y if x is FALSE. (If x is TRUE then you don't need to evaluate y since the resultant expression will be TRUE regardless, see footnote 1 in the page you referenced.) See e.g. output of this. <http://pastebin.com/6c6UWcTQ> So then, to explain this line from the page you reference: x and y: "if *x*is false, then *x*, else *y" *Think about it: As per the above, if x is false, then because it's false, Python need only and will only evaluate x, and will therefore essentially return whatever "x" is when evaluating the expression. If x is true on the other hand, then by the above rules, it has to *also* evaluate y as well, and so will end up effectively returning whatever y returns as it determines what the truth value of the overall expression is. Shortening that reasoning, you can say, "if x is false, then x, else y". See? (The same sory of reasoning applies for the "or" case if you think it out.) * *Hope that helps. Walter
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