as I understand things... there's a writeup somewhere that uses the term "bind" instead of "assign" for the operation a = b
for example, in Python a = 1 the value 1 now has a name of a associated with it. b = a the value 1 now has two names, a and b, associated with it the value 1 exists as an object independent of the storage for the names a or b, which are bound to the integer object 1 in C a = 1; creates an integer-sized area in RAM, names it a, and puts the value 1 in that area. b = a; copies the value in the area named a to a new, integer-sized area named b in either language (pseudocode below): a = 1 print a <output> a b = 1 print b <output> 1 a = 2 print a <output> 2 print b <output> 1 I'm curious to know the practical value of knowing the difference: where can one get tripped up in this case? On Jul 17, 2007, at 7:38 AM, Kent Johnson wrote: > Dick Moores wrote: >> At 04:57 AM 7/17/2007, you wrote: >>> A recent comp.lang.python thread has a good explanation of Python's >>> assignment semantics: >>> http://groups.google.com/group/comp.lang.python/browse_thread/ >>> thread/56e7d62bf66a435c/ >> >> >> Kent, >> >> Yes, interesting. But could you explain what you mean by "assignment >> semantics"? Semantics: < >> http://dictionary.reference.com/search?q=semantics> >> I have trouble relating semantics to programming. > > Semantics: The study or science of meaning in language. > > In other words, what does it mean to say > a = b > in Python? > > Syntax is like spelling and grammar rules - if you understand syntax, > you can write a program that will compile and run. But without > understanding what the various programming constructs actually mean, > you > will have trouble writing a program that does something useful. > > I think to write correct programs in any language, you must have a > correct mental model of what the program is doing, where by 'correct' I > mean a model that is consistent with the actual behaviour of the > program. > > Programmers coming from a background in C and C++ have a mental model > of > variables as containers for values. This model works in those > languages, > where variables correspond to memory locations and assignment copies a > value from one location to another. It doesn't work in Python, where > variables are names for values and assignment creates another name for > the same value without copying. To write correct Python programs you > have to have a different model for what variables are and how they > behave. We regularly see questions on this list from people who are > confused by assignment because they are using a faulty model. > > So "what does it mean?" might be interpreted as "what is a useful model > of this operation that allows me to understand and predict its > behaviour?", and "assignment semantics" is a shortcut for saying "a > useful model of assignment that allows me to understand and predict its > behaviour." > > Kent > _______________________________________________ > Tutor maillist - Tutor@python.org > http://mail.python.org/mailman/listinfo/tutor > _______________________________________________ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
