On 10 February 2013 14:32, Walter Prins <wpr...@gmail.com> wrote:
[snip
>
> This worked mostly fine, however yesterday I ran into a slightly unexpected
> problem when I found that when the list contents is iterated over and values
> retrieved that way rather than via [], then __getitem__ is in fact *not*
> called on the list to read the item values from the list, and consequently I
> get back the "not yet calculated" entries in the list, without the
> calculation routine being automatically called as is intended.
>
> Here's a test application that demonstrates the issue:
>
> class NotYetCalculated:
> pass
>
> class CalcList(list):
> def __init__(self, calcitem):
> super(CalcList, self).__init__()
> self.calcitem = calcitem
>
> def __getitem__(self, key):
> """Override __getitem__ to call self.calcitem() if needed"""
> print "CalcList.__getitem__(): Enter"
> value = super(CalcList, self).__getitem__(key)
> if value is NotYetCalculated:
> print "CalcList.__getitem__(): calculating"
> value = self.calcitem(key)
> self[key] = value
> print "CalcList.__getitem__(): return"
> return value
>
> def calcitem(key):
> # Demo: return square of index
> return key*key
>
> What's the best way to fix this problem? Do I need to maybe override
> another method, perhaps provide my own iterator implementation? For that
> matter, why doesn't iterating over the list contents fall back to calling
> __getitem__?
It would use __getitem__ if __iter__ wasn't defined. e.g.:
>>> class A(object):
... def __getitem__(self, index):
... if index > 4:
... raise IndexError
... return index ** 2
...
>>> a = A()
>>> for x in a:
... print(x)
...
0
1
4
9
16
The problem is that by subclassing list you inherit its __iter__
method. A for loop begins by calling iter() on the iterable. The iter
function is roughly like:
def iterseq(seq):
count = 0
while True:
try:
yield seq[count]
except IndexError:
return
count += 1
def iter(iterable):
if hasattr(iterable, '__iter__'):
return iterable.__iter__()
elif hasattr(iterable, '__getitem__'):
return iterseq(iterable)
else:
raise TypeError
Oscar
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