I don't get this....

[wavic]

Zero, None, an empty list, dict or whatever, an empty string evaluates to False.

In [1]: bool(0)
Out[1]: False

In [2]: bool([])
Out[2]: False

In [3]: bool(None)
Out[3]: False

In [4]: bool("")
Out[4]: False

Everything else evaluates to True. For example 20.