[Matplotlib-users] Wind barb bug

[Jesper L. <jes...@gm...>]

Hi Matplotlib Users,

When I make wind barbs with rounding enabled and custom barb increments I
noticed that there were no wind barbs with half barbs above 2 full barbs.
The reason seems to be a bug in the _find_tails method. The bug is
illustrated by this small script (_find_tails is a copy of the one in
matplotlib):

import numpy as np

def _find_tails(self, mag, rounding=True, half=5, full=10, flag=50):
    '''
    Find how many of each of the tail pieces is necessary.  Flag
    specifies the increment for a flag, barb for a full barb, and half for
    half a barb. Mag should be the magnitude of a vector (ie. >= 0).

    This returns a tuple of:

        (*number of flags*, *number of barbs*, *half_flag*, *empty_flag*)

    *half_flag* is a boolean whether half of a barb is needed,
    since there should only ever be one half on a given
    barb. *empty_flag* flag is an array of flags to easily tell if
    a barb is empty (too low to plot any barbs/flags.
    '''

    #If rounding, round to the nearest multiple of half, the smallest
    #increment
    if rounding:
        mag = half * (mag / half + 0.5).astype(np.int)

    num_flags = np.floor(mag / flag).astype(np.int)
    mag = np.mod(mag, flag)

    num_barb = np.floor(mag / full).astype(np.int)
    mag = np.mod(mag, full)

    half_flag = mag >= half
    empty_flag = ~(half_flag | (num_flags > 0) | (num_barb > 0))

    return num_flags, num_barb, half_flag, empty_flag

def main():
    mag = np.arange(0,21,1)
    barb_incs = {'half': 2.57222,
                 'full': 5.14444,
                 'flag': 25.7222}
    print 'With rounding'
    num_flags, num_barb, half_flag, empty_flag =  _find_tails(None, mag,
rounding=True, **barb_incs)
    for i in range(len(mag)):
        print mag[i], num_flags[i], num_barb[i], half_flag[i], empty_flag[i]
    print 'Without rounding'
    num_flags, num_barb, half_flag, empty_flag =  _find_tails(None, mag,
rounding=False, **barb_incs)
    for i in range(len(mag)):
        print mag[i], num_flags[i], num_barb[i], half_flag[i], empty_flag[i]

if __name__ == '__main__':
    exit(main())

It seems like the error is not present when the barb increments are not
set. I believe the reason for the bug is the float comparison (half_flag =
mag >= half) where the value is rounded to a value very close to/identical
to the 'half' increment. And it seems like python does the right thing when
the "half" increment is a whole number but not always when it is not.

But in any case the code should probably not depend two floats being equal.

Best regards,
Jesper