[Python-Dev] should list's call to __len__ swallow SystemExit?

Dino Viehland dinov at microsoft.com
Wed Jan 14 05:24:45 CET 2009

We had a bug reported that effectively boils down to we’re not swallowing exceptions when list calls __len__ (http://www.codeplex.com/WorkItem/View.aspx?ProjectName=IronPython&WorkItemId=20598).

We can obviously make the change to catch exceptions here in IronPython even if it seems like a bad idea to me ☺  But CPython seems to catch not only normal exceptions, but also SystemExit.  It seems like there’s been a move away from this so I thought I’d mention it here.  I tested it on 2.6.1 and 3.0.

import sys
class A(object):
    def __iter__(self): return iter(range(10))
    def __len__(self):
        except Exception as e:
            print('can I catch it?', e)


which prints:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

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