In Python 3.6.8 (default, Apr 16 2020, 01:36:27) [GCC 8.3.1 20191121 (Red Hat 8.3.1-5)] on linux, when I materialize the list, it is shown and is deleted. Shouldn't the list persist in memory? Is this a bug or is it really?

```python
>>> vet_neg
['EH01', 'EH02', 'EH03']
Categories (3, object): ['EH01', 'EH02', 'EH03']
>>> cenarios
[0, 1]
>>> vet_neg_cenarios = itertools.product(vet_neg, cenarios, cenarios)
>>> vet_neg_cenarios
<itertools.product object at 0x7f6fa16a65e8>
>>> list(vet_neg_cenarios)
[('EH01', 0, 0), ('EH01', 0, 1), ('EH01', 1, 0), ('EH01', 1, 1), ('EH02', 0, 0), ('EH02', 0, 1), ('EH02', 1, 0), ('EH02', 1, 1), ('EH03', 0, 0), ('EH03', 0, 1), ('EH03', 1, 0), ('EH03', 1, 1)]
>>> list(vet_neg_cenarios)
[]
>>> 
```

This is not a bug. itertools.product returns an iterator: https://docs.python.org/3/glossary.html#term-iterator

Quoting from there:

> [...] every iterator is also iterable and may be used in most places where other iterables are accepted. One notable exception is code which attempts multiple iteration passes. A container object (such as a list) produces a fresh new iterator each time you pass it to the iter() function or use it in a for loop. Attempting this with an iterator will just return the same exhausted iterator object used in the previous iteration pass, making it appear like an empty container.

Florian's answer is correct. Thanks!