pitfall for your amusement

[Bernhard Herzog]

Erik Max Francis <max at alcyone.com> writes:

> 
> 	L = [1, 2, 3]
> 	L += [4, 5, 6]
> 
> is the equivalent of
> 
> 	L = [1, 2, 3]
> 	L.extend([4, 5, 6])

Not quite. It's more like

      L = [1, 2, 3]
      L = L.__iadd__([4, 5, 6])

The rebinding as always done. So += can sometimes fail for a list:

>>> t = ([1,2,3],)
>>> t[0] += [4, 5, 6]
Traceback (most recent call last):
  File "<stdin>", line 1, in ?
TypeError: object doesn't support item assignment

Of course, the list already has been modified in place:
>>> t
([1, 2, 3, 4, 5, 6],)


   Bernhard

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