[Tutor] true and false

[Bob Gailer]

Alan Gauld wrote:
>> What values represent true in Python?
>>     
>
> True
>
> Its a built-in boolean type value.
>
> But empty strings, lists, tuples etc are considered False
> Similarly zero and None are considered False
>   
Let us be precise here. Empty things are considered False when used 
where a boolean value is expected.
In the language reference, under the if statement I quote:"It selects 
exactly one of the suites by evaluating the expressions one by one until 
one is found to be true (see section 5.10 <Booleans.html#Booleans> for 
the definition of true and false)"

5:10: "In the context of Boolean operations, and also when expressions 
are used by control flow statements, the following values are 
interpreted as false: |False|, |None|, numeric zero of all types, and 
empty strings and containers (including strings, tuples, lists, 
dictionaries, sets and frozensets). All other values are interpreted as 
true."

The 2 flow control statements that use expressions in this way are if 
and while. Boolean operations are or, and, not.

So in "if [1]: print 1" [1] will be interpreted as True. However in  
"[1]  == True" it will NOT be interpreted as True.

-- 
Bob Gailer
510-978-4454