In Python, everything's an object.  Might seem odd at first, but the more you get to know it the more clear and logical it is.  Even methods are named objects!

Have a look at this example piece of code:

Code:

source = ["apple","orange","banana","pear"] c1cat = c1dog = [] c2cat = c2dog = [] print "Using the + Operator" for thing in source:        c1cat = c1cat + [thing]        c1dog = c1dog + [thing] print c1cat print c1dog print "Using the += assignment" for thing in source:        c2cat += [thing]        c2dog += [thing] print c2cat print c2dog

And when it runs, you get:

Code:

dolphin:/Library/WebServer/live_cgi-bin/net/py05 graham$ python obc Using the + Operator ['apple', 'orange', 'banana', 'pear'] ['apple', 'orange', 'banana', 'pear'] Using the += assignment ['apple', 'apple', 'orange', 'orange', 'banana', 'banana', 'pear', 'pear'] ['apple', 'apple', 'orange', 'orange', 'banana', 'banana', 'pear', 'pear'] dolphin:/Library/WebServer/live_cgi-bin/net/py05

Why is everything doubled when you use the += operator?  

Objects explain in all.

In the first case, a new object is created each time something is added onto the end of the previous list.  But in the second case, something is added onto the end of the existing list in the loop; since c2cat and c2dog are just two different names for the same object, that something added is added twice.

---