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[Python] Call by value eller call by reference?

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Det her er et lidt hypotetisk problem, for jeg har ikke tænkt mig
at benytte mig af mystiske effekter, men jeg faldt over noget i
en tutorial som jeg ikke forstod idet jeg altid er gået ud fra at
funktioner i Python kun arbejder med en kopi af de overførte
parametre. Derfor forstår jeg ikke det følgende eksempel:

  	def changeme_once (mylist):
  	    "This changes a passed list into this function"
  	    mylist.append([1,2,3,4]);
  	    print "Values inside the function: ", mylist
  	    return

  	def changeme_twice (mylist):
  	   "This changes a passed list into this function"
  	   mylist = [1,2,3,4];
  	   print "Values inside the function: ", mylist
  	   return

  	def changeme_thrice (mylist):
  	   "This changes a passed list into this function"
  	   mylist = [5,6,7,8];
  	   print "Values inside the function: ", mylist
  	   mylist.append([9,10,11,12]);
  	   print "Values inside the function: ", mylist
  	   return

  	mylist = [10,20,30];
  	print "Values before change: ", mylist
  	changeme_once(mylist);
  	print "Values outside the function: ", mylist
  	print

  	mylist = [10,20,30];
  	print "Values before change: ", mylist
  	changeme_twice( mylist );
  	print "Values outside the function: ", mylist
  	print

  	mylist = [1,2,3,4];
  	print "Values before change: ", mylist
  	changeme_thrice( mylist );
  	print "Values outside the function: ", mylist


Udskriften ved en kørsel giver:

  	Values before change:  [10, 20, 30]
  	Values inside the function:  [10, 20, 30, [1, 2, 3, 4]]
  	Values outside the function:  [10, 20, 30, [1, 2, 3, 4]]
  	
  	Values before change:  [10, 20, 30]
  	Values inside the function:  [1, 2, 3, 4]
  	Values outside the function:  [10, 20, 30]
  	
  	Values before change:  [1, 2, 3, 4]
  	Values inside the function:  [5, 6, 7, 8]
  	Values inside the function:  [5, 6, 7, 8, [9, 10, 11, 12]]
  	Values outside the function:  [1, 2, 3, 4]

Hvorfor er det kun i changeme_once() at ændringen slår igennem i
den ydre variabel?

-- 
Bertel
bertel.lundhansen.dk	fiduso.dk	obese.dk

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[Python] Call by value eller call by reference? Bertel Lund Hansen <gadekryds@lundhansen.dk> - 2016-11-26 13:07 +0100

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