Dave, Just one point.
many things are allowed by a language even if normal people would NOT have any reason to do it NOR should use it. Although when used in one context + and - can be considered unary operators, the evaluation may result in successive unary operations being done one after another. Here are examples I just typed: +1 1 +++++1 1 +-+-+-1 -1 ---1 -1 --1 1 Luckily since Python has no ++ or -- pre or post operators, these are legal, albeit a tad weird. -----Original Message----- From: Python-list <python-list-bounces+avigross=verizon....@python.org> On Behalf Of dn via Python-list Sent: Tuesday, October 5, 2021 7:11 PM To: python-list@python.org Subject: Re: Understanding the working mechanis of python unary arithmetic operators. On 06/10/2021 10.10, Chris Angelico wrote: > On Wed, Oct 6, 2021 at 7:52 AM hongy...@gmail.com <hongyi.z...@gmail.com> wrote: >> >> On Saturday, October 2, 2021 at 4:59:54 PM UTC+8, ju...@diegidio.name wrote: This thread seems to have been very one-sided. Either I've forgotten selective use of the DEL-key, or the above contributor has been forgetting to reply to the list (or didn't intend contribution?) Please share your knowledge with all! >>> On Saturday, 2 October 2021 at 10:34:27 UTC+2, hongy...@gmail.com wrote: > If bool(a) is True, then ~bool(a) is exactly the same as writing > ~True, and a has become irrelevant. That is simply how expression > evaluation works. @Chris and I presented at a recent PUG meeting on the subject of 'operators'. (I was the 'warm-up act', @Chris the 'star'...) I covered unary operators (https://docs.python.org/3/reference/expressions.html#unary-arithmetic-and-b itwise-operations) - and very briefly, because that's proportional to their application 'in the real world'. Negation is sometimes used, binary inversion very occasionally, but the 'unary plus' almost never! Thus: >>> a = 3 >>> +a 3 >>> -a -3 Why would one use the plus operator? Particularly when it confuses some, ie if negation changes the sign, won't the unary-plus? >>> b = -3 >>> +b -3 >>> -b 3 What has the unary-plus achieved in either/both of these cases? Not much, but may not be doing something that was 'expected'! The bitwise inversion (~) is also widely misunderstood (just like the story of my life). It is not "bit inversion", as in, a single bit being inverted: (values here are expressed in base-2!) ~0 -> 1 ~1 -> 0 Instead, it involves the inversion of every bit. Thus, if we are talking about a nibble/nybble (because I'm too lazy to type more binary digits): ~0000 -> 1111 ~0001 -> 1110 which accounts for the problem discussed earlier in the thread. To make things work the way you seemed to imagine: ~1111 -> 0000 # and thus, is False ~0000 -> 1111 # and thus, is True ie a bitwise inter-change that works binary ints and booleans. Where some (other) thinking may have come-adrift from Python's model, is the mixing of objects/types, eg the 'truthiness' of any integer other than 0. Also, whilst coercion from one type to another may work satisfactorily in one 'direction', it may not be so amenable in reverse. Another bemusing (OP) point was the concept of "-+~1". I'd never, ever, think of doing that! Hey, maybe that says something about me? The term "unary operator" involves the word "one". Rather than "operator", the "one" is the number of "operands". (yes, you know this) However, having already talked about the infrequency with which unary-operators are employed, the idea of nesting (is that the word?) a series of unary-operators, almost blew my mind. Why? Please show an example scenario... Finally, there is the process of "over-loading" operators. Thus, "1 + 2" translates to "add the value 2 to 1"; whereas "a" + "b" means "concatenate" the two strings. In both cases, the result becomes a third value. When we choose the correct terminology, the words for "+" are different - as are the resultant processes! We could also code "instance_a + instance_b". What does this mean? Addition? Concatenation? Bitwise? If I tell you that both are instances of MyClass, does that answer the question? The only way to work-out what craziness I hold in store, is to find where MyClass is defined and look for the __add__() 'magic method'*. At which point, you discover that the instance-attributes are split into components, and the components merged together in a random sequence, with today's date mixed-in for (no) good measure. * or you could ask me (although neither explanation will have a +impact on your mental health) So, the fact that two object instances, eg an integer and a floating-point number, both utilise the same operator-symbol, enables us to draw analogies (and in this case, probably get it 100% correct), but this does not imply a complete equivalence across-the-board (particularly when crazy-people are let-loose with custom classes! Thus "a" + "b" should not be pronounced "add", even though the operator looks very much like the one we use to add two numbers! For fun, and quickly now, what happens here: 2 + 2 2 + 2.0 2.0 + 2 2.0 + 2.0 "2" + "2" 2 + "2" "2" + 2 (they're all 'the same', except the last two?three - aren't they???) -- Regards, =dn -- https://mail.python.org/mailman/listinfo/python-list -- https://mail.python.org/mailman/listinfo/python-list
