Spot the subtle error

[ckd]

#!/usr/bin/env python

# set up items, define is_type_x functions, etc

list_a = list_b = list_c = []

for item in items:
    if is_type_a(item):
        list_a.append(item)
    elif is_type_b(item):
        list_b.append(item)
    elif is_type_c(item):
        list_c.append(item)
    else:
        print 'wtf is this?',item

The a = b = c = [] construct makes all three variables references to the same empty list. list.append() modifies the list in place. After this loop runs, all three lists will contain all of the items except those that aren't type a, b, or c.

The most concise fix is to use list_a, list_b, list_c = [], [], [] as a replacement.

Don't ask how long I was beating my head against this one.

Comments

[dd-b.livejournal.com]

I'm glad somebody covered this before I got to it :-).

Of course, one might claim that that is not a *subtle* error.

My particular forehead-basher this afternoon involves tcl and expect. I think I'm developing a good-sized lump.