when is filter test applied?

[Peter Otten]

Neal Becker wrote:

> In the following code (python3):
> 
> for rb in filter (lambda b : b in some_seq, seq):
>   ... some code that might modify some_seq
> 
> I'm assuming that the test 'b in some_seq' is applied late, at the start
> of each iteration (but it doesn't seem to be working that way in my real
> code), so that if 'some_seq' is modified during a previous iteration the
> test is correctly performed on the latest version of 'some_seq' at the
> start of each
> iteration.  Is this correct, and is this guaranteed?

Yes. filter() is lazy in Python 3, and otherwise the usual rules for 
binding, mutating, and lookup of some_seq apply. Example:

>>> some_seq = set("abc")
>>> for c in filter(lambda x: x in some_seq, "abbbcccCBBBBAAA"):
...     some_seq.remove(c)
... 
>>> some_seq = set("abc")
>>> for c in filter(lambda x: x in some_seq, "abbbcccCBBBBAAA"):
...     print(c)
...     some_seq.remove(c)
...     if not some_seq: some_seq = set("ABC")
... 
a
b
c  # at that point some_seq will be bound to a new set("ABC")
C
B
A  # as above
A