optional 'else' block for 'for' and 'while' loops

for <target_list> in <enumerable>:
   <for_block>
else:
   <else_block>

<else_block> is only executed when the loop terminates because <enumerable>.MoveNext() returns false when evaluated, i.e. whenever the <enumerable> attempts to move to the next item, but the <enumerable> is exhausted. This also means that if <enumerable> has no item at the first 'iteration', <enumerable>.MoveNext() will return false, and the <else_block> will be executed.

while <expression>:
     <while_block>
else:
     <else_block>

<else_block> is only executed when the loop terminates the <expression> is tested but resolves to a false, null, or 0 value. This also means that if <expression> is false the first time it is tested (even before entering <while_block>) <else_block> will be executed.

The effect is that for these loops, <else_block> will not be executed when execution leaves the loop due to a break, raise, or return because <expression>/<enumerable>.MoveNext() is not evaluated. Note that the cause of the <else-block> executing is the condition evaluating to false.