Fix ProgressBar item_show_func so it displays the current item

[JoelEager]

Per @davidism's request this PR contains my suggested fix for ProgressBar.generator(). In short, this change makes the item_show_func show the item currently being processed as opposed to the item previously processed (like it currently does). I've explained the bug more fully in #1353.

You can use this example to see the difference when run against current master vs this PR.

from time import sleep
from click import progressbar


def format_item(item):
    if item:
        return "processing {}".format(item)


with progressbar(range(5), item_show_func=format_item) as pb:
    for item in pb:
        print(" actually on", item)
        sleep(1)

Thanks in advance for taking the time to look into this.

[davidism]

This seems incompatible the changes from #1698 which added an update interval. That PR relies on update(), but this one bypasses that and unconditionally calls make_step() and render_progress(). However, moving update() before yield means that the ETA calculation is wrong. Incidentally, that PR is when I decided that the progress bar was getting too complex without enough design oversight and started recommending tqdm.

That said, I do want to get this fix in, I just now need to figure out how to reconcile the two changes.

[davidism]

Regarding the ETA issue with this fix, this fix goes against the docs, which causes the ETA and percent complete to be incorrect:

Note that progressbar() updates the bar after each iteration of the loop. So code like this will render correctly:

import time

with click.progressbar([1, 2, 3]) as bar:
    for x in bar:
        print('sleep({})...'.format(x))
        time.sleep(x)

[davidism]

In order to reconcile the update interval behavior, I changed this back to using self.update(1) after yielding. Before yielding, the progress bar is rendered with the new current item, but only at the start of the interval.