Question

Consider:

args = ['-sdfkj']
print args
for arg in args:
    print arg.replace("-", '')
    arg = arg.replace("-", '')
print args

This yields:

['-sdfkj']
sdfkj
['-sdfkj']

Where I expected it to be ['sdfkj'].

Is arg in the loop a copy?

It behaves as if it is a copy (or perhaps an immutable thingie, but then I expect an error would be thrown…)

Note: I can get the right behavior with a list comprehension. I am curious as to the cause of the above behavior.

Answer 1

Is arg in the loop a copy?

Yes, it contains a copy of the reference.

When you reassign arg you aren’t modifying the original array, nor the string inside it (strings are immutable). You modify only what the local variable arg points to.

Before assignment         After assignment

args          arg          args          arg
  |            |            |            |
  |            |            |            |
(array)        /          (array)       'sdfkj'
  |[0]        /             |[0]        
   \         /              |
    \       /               |
     '-sdfkj'            '-sdfkj'