Question

If inside a loop, is it possible to instantiate objects to be manipulated during subsequent iterations of the loop, and still available when the scope of the loop has been left?

Here is a simple example of what I thought might work:

>>> for i in range(2):
...   r = [] if r is None else r
...   r.append[i]
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
NameError: name 'r' is not defined

And here’s my second attempt using a NameError exception:

>>> for i in range(2):
...   try:
...     r = r
...   except NameError:
...     r = []
...   r.append(i)
...
>>>

I suspect that what I have been trying to do is actually prohibited, but I don’t understand why it would be.

Can someone throw some light on this for me please?

Edit:

So the second way works, but it’s very long winded. Is there a quick way?

Answer 1

You can find out if the name is in your locals like this: name in locals() Regarding your question:

>>> 'r' in locals()
False
>>> for i in range(2):
    if 'r' not in locals(): r = []
    r.append(i)


>>> r
[0, 1]
>>> 

I agree with Ignacio Vazquez-Abrams; this is not good style. Please do what you need to before looping.