README.md

Multiprocessing and multiple progress bars

Well this was a success.

I've got multiple processes running at once in parallel

What I want to do here is have a progress bar for each process and have these displayed nicely on the screen.

There is already a decent module for printing progress bars, so we're going to try and use that.

Writing stuff to specific places on the screen is a job for curses, which is messy to use. Luckily I came across blessings, which is an excellent clean wrapper around curses.

I've used blessings version 1.5 and progressbar version 2.3.

The main class in progressbar, ProgressBar, has an instantiation argument fd=sys.stderr that is an object with a write(string) method. This is how progressbar actually writes out to the screen. By default, this happens by sys.stderr.write('[----progress...]'), but we can supply our own writer.

Blessings works something like this (the documentation is excellent btw):

from blessings import Terminal

term = Terminal()

location = (0, 10)
text = 'blessings!'
print term.location(*location), text

# alternately,
with term.location(*self.location):
    print text

Progressbar works something like this:

import time

from progressbar import ProgressBar

pbar.start()
for i in range(100):
    # mimic doing some stuff
    time.sleep(0.01)
    pbar.update(i)
pbar.finish()

We need to make something to connect progressbar and blessings. Create an object that can write like this:

class Writer(object):
    """Create an object with a write method that writes to a
    specific place on the screen, defined at instantiation.

    This is the glue between blessings and progressbar.
    """
    def __init__(self, location):
        """
        Input: location - tuple of ints (x, y), the position
                        of the bar in the terminal
        """
        self.location = location

    def write(self, string):
        with term.location(*self.location):
            print(string)

Then we can put our progress bar wherever we want by feeding our writer object to progressbar:

def test_function(location):
    writer = Writer(location)
    pbar = ProgressBar(fd=writer)
    pbar.start()
    for i in range(100):
        # mimic doing some stuff
        time.sleep(0.01)
        pbar.update(i)
    pbar.finish()

x_pos = 0  # zero characters from left
y_pos = 10  # ten characters from top
location = (x_pos, y_pos)
test_function(location)

Multiprocessing

Now that we can put a progressbar where we choose it is fairly trivial to extend this to multiprocessing.

Basic multiprocessing usage, mapping a function our_function onto a list of arguments arg_list:

from multiprocessing import Pool

p = Pool()
p.map(our_function, arg_list)
p.close()

In our case the function is test_function and the list of arguments is a list of locations. For example, to have a progress bar at the start of the line on the 2nd, 7th and 8th lines:

locations = [(0, 1), (0, 6), (0, 7)]
p = Pool()
p.map(test_function, locations)
p.close()

I've only got two active progress bars here because I've only got a two core processor. Pool() defaults to making a number of worker processes equal to the number of processors. Here is the same code run on more cores with a load more locations:

Fullscreen output

You might notice that the examples above mess the screen up a bit. This is because blessings, unlike curses, does not force a fullscreen view that disappears on completion. We can tell blessings to have this behaviour like this:

with term.fullscreen():
    do_the_stuff_above()

I've made an script that demonstrates all of the above.

And there you go, multiple independent progress bars implemented in Python with not much hassle at all. This took me about 4 hours, blog post included, thanks in large part to how easy blessings makes curses.