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| For 2023 (and 2024 ...) - we are now fully retired from IT training. We have made many, many friends over 25 years of teaching about Python, Tcl, Perl, PHP, Lua, Java, C and C++ - and MySQL, Linux and Solaris/SunOS too. Our training notes are now very much out of date, but due to upward compatability most of our examples remain operational and even relevant ad you are welcome to make us if them "as seen" and at your own risk. Lisa and I (Graham) now live in what was our training centre in Melksham - happy to meet with former delegates here - but do check ahead before coming round. We are far from inactive - rather, enjoying the times that we are retired but still healthy enough in mind and body to be active! I am also active in many other area and still look after a lot of web sites - you can find an index ((here)) |
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copying an object - copy the reference
I call my father "Dad" but everyone else calls him "Norman". Same person, different name. If Dad puts on the hat that he's got from the Christmas cracker before he eats his Christmas pudding, then everyone else sees that Norman has put on his hat before eating his Christmas pudding. Two names for one and the same person. PYTHON - COPY BY REFERENCE The same principle applies to the copying of objects in Python (and Java and PHP 5 and other object oriented languages); an assignment statement is just the addition of an extra name to the same object. Let's see an example - in Python: pie = ["apple","pear","strawberry"] pudding = pie pudding[1] = "apricot" print pudding print pie When I run that: earth-wind-and-fire:~/feb05 grahamellis$ python obcop.py ['apple', 'apricot', 'strawberry'] ['apple', 'apricot', 'strawberry'] earth-wind-and-fire:~/feb05 grahamellis$ By changing "pudding" I've also changed "pie". PERL - COPY BY CONTENTS In contrast, if I copy a list in Perl (and in PHP 4, Tcl and other languages which are not 100% object oriented), I duplicate the data. Example in Perl: @pie = ("apple","pear","strawberry"); @pudding = @pie; $pudding[1] = "apricot"; print "@pudding\n"; print "@pie\n"; Which runs: earth-wind-and-fire:~/feb05 grahamellis$ perl obcop.pl apple apricot strawberry apple pear strawberry earth-wind-and-fire:~/feb05 grahamellis$ See also Python programming course Please note that articles in this section of our
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