[Python-Dev] [Python-checkins] cpython (3.2): Nudge readers towards a more accurate mental model for loop else clauses (original) (raw)

Terry Reedy tjreedy at udel.edu
Thu Jun 7 20:08:47 CEST 2012

On 6/7/2012 8:42 AM, nick.coghlan wrote:

http://hg.python.org/cpython/rev/6e4ec47fba6a changeset: 77369:6e4ec47fba6a branch: 3.2 parent: 77363:aa9cfeea07ad user: Nick Coghlan<ncoghlan at gmail.com> date: Thu Jun 07 22:41:34 2012 +1000 summary: Nudge readers towards a more accurate mental model for loop else clauses

files: Doc/tutorial/controlflow.rst | 7 +++++++ 1 files changed, 7 insertions(+), 0 deletions(-)

diff --git a/Doc/tutorial/controlflow.rst b/Doc/tutorial/controlflow.rst --- a/Doc/tutorial/controlflow.rst +++ b/Doc/tutorial/controlflow.rst @@ -187,6 +187,13 @@ (Yes, this is the correct code. Look closely: the else clause belongs to the :keyword:for loop, not the :keyword:if statement.) +When used with a loop, the else clause has more in common with the +else clause of a :keyword:try statement than it does that of +:keyword:if statements: a :keyword:try statement's else clause runs +when no exception occurs,

(And there no return. But that is true of of every statement.)

I think the above is wrong.

I claim that try-statement else: is essentially identical to if-statement else:. The try-statement else: clause is subordinate to the except: clauses, not the try: part itself. It must follow at least one except condition just as an if-statement must follow at least one if condition. So it is really an except else, not a try else.

Furthermore, 'except SomeError' is an abbreviation for 'if/elif SomeError was raised since the corresponding try' and more particularly 'if/elif isinstance(exception, SomeError). I use exception here as the equivalent of C's errno, except that it is hidden. (If it were not hidden, we would not need 'as e', and indeed, we would not really need 'except' either.) The else clause runs when all the implied conditionals of the excepts are false. Just as an if-statement else clause runs when all the explicit conditionals of the if/elifs are false.

The real contrast is between if/except else and loop else. The latter is subordinate to exactly one condition instead of possibly many, but that one condition may be evaluated multiple times instead of just once. It is the latter fact that seems to confuse people.

and a loop's else clause runs when no break occurs.

As I explained on Python-ideas, the else clause runs when the loop condition is false. Period. This sentence is an incomplete equivalent to 'the loop condition is false'. The else clause runs when no 'break', no 'return', no 'raise' (explicit or implicit), AND no infinite loop occurs.

I think your addition should be reverted. Here is a possible alternative if you think something more is needed.

"An else clause used with a loop has two differences from an else clause used with an if statement. It is subordinate to just one condition instead of possibly many. (In for statements, the condition is implicit but still there.) That one condition is tested repeatedly instead of just once. A loop else is the same in that it triggers when that one condition is false."

Terry Jan Reedy

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