For-each behavior while modifying a collection

[Valentin Zahnd]

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?
2. Why is the counter for the iteration not modified?

Valentin