On 11/28/13 5:14 PM, Valentin Zahnd wrote:
2013/11/28 Ned Batchelder <n...@nedbatchelder.com>:
On 11/28/13 10:49 AM, Valentin Zahnd wrote:
Hello
For-each does not iterate ober all entries of collection, if one
removes elements during the iteration.
Example code:
def keepByValue(self, key=None, value=[]):
for row in self.flows:
if not row[key] in value:
self.flows.remove(row)
It is clear why it behaves on that way. Every time one removes an
element, the length of the colleciton decreases by one while the
counter of the for each statement is not.
The questions are:
1. Why does the interprete not uses a copy of the collection to
iterate over it? Are there performance reasons?
Because implicit copying would be pointless in most cases. Most loops don't
even want to modify the collection, why copy all iterables just in case your
loop might be one of the tiny few that might change the collection?
Okay, I get this point.
But how is the for-each parsed. Is it realised with a iterator or ist
done with a for loop where a counter is incremented whilst it is less
than the length of the list and reads the element at the index of the
counter?
The for loop creates an iterator over the expression, and pulls values
from the iterator, running the body once for each value.
This presentation explains the mechanics of loops, along with some
examples of more exotic uses, and (at least) one bad joke:
http://nedbatchelder.com/text/iter.html
Of course, if that prices is acceptable to you, you could do the copy
yourself:
for row in list(self.flows):
if row[key] not in value:
self.flows.remove(row)
2. Why is the counter for the iteration not modified?
Because the list and the iterator over the list are different objects. I
suppose the list and the iterator could have been written to update when the
list is modified, but it could get pretty complicated, even more so if you
want to do the same for other collections like dictionaries.
The best advice is: don't modify the list, instead make a new list:
self.flows = [r for r in self.flows if r[key] not in value]
How is the list comprehension done by the interpreter?
The list I'm have to work with is not that small. So to avoid
duplicated parts in memory the function looks currently like this:
def keepByValue(self, key=None, value=[]):
tmpFlows = []
while len(self.flows) > 0:
row = self.flows.pop()
if row[key] in value:
tmpFlows.append(row)
self.flows = tmpFlows
If there is no duplication in memory, the list comprehension would be
much more elegant.
The list comprehension won't copy the elements of the list, if that is
what you're worried about. And it won't reverse the order of your list
the way this loop does. BTW, my list comprehension got the sense of the
condition wrong, it should be:
self.flows = [r for r in self.flows if r[key] in value]
--Ned.
Be careful though, since there might be other references to the list, and
now you have two.
--Ned.
Valentin
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