Thanks for the responses. The distinction between replacement and modification seems to capture the essential aspect and helps to clarify the issue for me.
spir: Quite the opposite, in python "symbolic assignment" (where the right side also is a symbol) never copies, in fact never creates a new value, but bind the left symbol to the same, unique value, as the right symbol. If this is accurate, I can see crazy bugs in my future until I internalize it. Python 2.7.3 (default, Sep 26 2013, 20:03:06) [GCC 4.6.3] on linux2 Type "help", "copyright", "credits" or "license" for more information. >>> a=[2,3] >>> b=[10,20] >>> a[0]=b[0] >>> a [10, 3] >>> b[0]=100 >>> a [10, 3] Apparently a[0]=b[0] does not qualify as "symbolic assignment" in this case. a[0] is not a reference to b[0]. I think I see the essential distinction. Experience will complete the picture for me. -DH
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