[Python-Dev] The iterator story

[Andrew Koenig]

Ping>     - Iterators provide just one method, __next__().

Ping>     - The built-in next() calls tp_iternext.  For instances,
Ping>       tp_iternext calls __next__.

Ping>     - Objects wanting to be iterated over provide just one method,
Ping>       __iter__().  Some of these are containers, but not all.

Ping>     - The built-in iter(foo) calls tp_iter.  For instances,
Ping>       tp_iter calls __iter__.

Ping>     - "for x in y" gets iter(y) and uses it as an iterator.

Ping>     - "for x from y" just uses y as the iterator.

+1.

Ping>     - We have a nice clean division between containers and iterators.

Ping>     - When you see "for x in y" you know that y is a container.

What if y is a file?  You already said that files are not containers.

Ping>     - When you see "for x from y" you know that y is an iterator.

Ping>     - "for x in y" never destroys y.

Ping>     - "if x in y" never destroys y.

What if y is a file?

Ping> Other notes:

Ping>     - The file problem has a consistent solution.  Instead of writing
Ping>       "for line in file" you write

Ping>         for line from file:
Ping>             print line

Ping>       Being forced to write "from" signals to you that the file is
Ping>       eaten up.  There is no expectation that "for line from file"
Ping>       will work again.

Ah.  So you want to break "for line in file:", which works now?

I'm still +1 as long as there is a transition scheme.

Ping> My Not-So-Ideal Protocol
Ping> ------------------------

Ping> All right.  So new syntax may be hard to swallow.  An alternative
Ping> is to introduce an adapter that turns an iterator into something
Ping> that "for" will accept -- that is, the opposite of iter().

Ping>     - The built-in seq(it) returns x such that iter(x) yields it.

Ping> Then instead of writing

Ping>     for x from it:

Ping> you would write

Ping>     for x in seq(it):

Ping> and the rest would be the same.  The use of "seq" here is what
Ping> would flag the fact that "it" will be destroyed.

I prefer "for x from it:

-- 
Andrew Koenig, ark@research.att.com, http://www.research.att.com/info/ark