[Tutor] "x and y" means "if x is false, then x, else y"??

[Peter Otten]

Lie Ryan wrote:

> On 07/05/10 22:23, Adam Bark wrote:
> 
>> 
>> I should add that this is how something like:
>> 
>> if x != y:
>>     do_something()
>> 
>> works, if expects a True or False (this isn't always true but works for
>> comparison operators expressions such as this).
>> 
> 
> <nit> "if" expects an expression that can be converted to True or False
> by calling its __bool__()/__nonzero__(); in case of missing
> __bool__/__nonzero__, then the object is considered True. </nit>

<meta:nit>Don't forget about __len__()</meta:nit>

>>> class A:
...     def __init__(self, n): self.n = n
...     def __len__(self): return self.n
...
>>> "yes" if A(1) else "no"
'yes'
>>> "yes" if A(0) else "no"
'no'

Bonus:

>>> "yes" if A(-1) else "no"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: __nonzero__ should return >= 0

Peter