How to overide "for line in file:"

[Terry Reedy]

"John Roth" <newsgroups at jhrothjr.com> wrote in message
news:103g0mhgmfups16 at news.supernews.com...
>
> <David Morgenthaler> wrote in message
> news:tisf301qf76c8lh0jkq9086gpqoep1bb1u at 4ax.com...
> > How does one overide the iterator implied by the construct "for line
> > in file:"?

Wrap it, don't replace it.  See below.

> > For example, suppose I have a file containing row,col pairs on each
> > line, and I wish to write a subclass of file that will transform these
> > to x,y coordinates using an affine transform. I'd like it to look
> > something like this (but this clearly doesn't work):

Your class needs an __iter__ function.  However, unless you have some other
reason for the heavy duty class option, much easier is something like:

def xycoords(lines):
    # lines is an iterable that yields text lines of appropriate format
    for line in lines:
        <extract numbers and transform>
        yield x,y

You can feed xycoords a literal list of lines for testing and then an open
file for production use.
Chaining iterators like this is part of their intended use.

Terry J. Reedy