Question

For example, the distance is defined by meters.
Procedure should format value on a scale:

if distance < 100:
    return '%dm' % distance
elif distance < 1000:
    return '%.0fm' % round(distance, -1)
else:
    import math
    m, km = math.modf(distance / 1000.0)
    if m < 0.01:
        return '%.0fkm' % km
    else:
        return '%.1fkm' % km + round(m, 1)

It would be great to use already existing solution.

Update: django-distanceformatting is launched, but the question remains open.

Answer 1

There are some generic unit handling modules for Python, which could be wrapped in a template tag (no specific order):

• quantities
• Buckingham
• units
• Unum

Maybe one of them gives you what you need. But I still stand to my earlier comment that if you only need so little functionality, you should not need to bother about reinventing the wheel (or your tiny part thereof).

After a closer look at your code, I found some inconsistencies:

• 1.99999 is returned as 1m (should probably be 2m)
• I assume that you want to keep two significant digits, but 1.2 is returned as 1m or 0.12 as 0m
• The last line needs parentheses around the values tuple

Here is a more consistent and concise version of your function (plus a little wrapping around to test):

from math import modf
def old_dist_format(distance):
    if distance < 100:
        return '%dm' % distance
    elif distance < 1000:
        return '%.0fm' % round(distance, -1)
    else:
        m, km = modf(distance / 1000.0)
        if m < 0.01:
            return '%.0fkm' % km
        else:
            return '%.1fkm' % (km + round(m, 1))


def dist_format(distance):
    rounded = float('%.2g' % distance)
    km = modf(rounded / 1000.0)
    if km[1]:
        return "%gkm" % (km[0] + km[1])
    else:
        return "%gm" % rounded


if __name__ == "__main__":
    while True:
        try:
            s = raw_input("> ")
            try:
                d = float(s)
            except ValueError:
                print "Enter number"
                continue
            print "%s\t%s" % (dist_format(d), old_dist_format(d))
        except KeyboardInterrupt:
            print
            break

The first line in the new dist_format() function always keeps two significant figures. %g leaves out everything from the decimal point if not needed. Caveat: My function will give you exponential notation for (astronomic) distances >= 1000000000.