[GRASS-dev] Python assertion error in GRASS GIS script

[Markus Metz]

On Mon, Oct 8, 2018 at 6:43 PM Nikos Alexandris <nik at nikosalexandris.net>
wrote:
>
> Dears,
>
> I try to understand why the following assertion fails to work, as
> expected, in a Python GRASS GIS script?
>
> The following fragment of code
> ```
> fraction_categories = grass.parse_command('r.category',
>                 map=fractions,
>                 delimiter='\t')
>
> fractions_sum = sum([float(x)
>     if not math.isnan(float(x))
>     else 0
>     for x
>     in fraction_categories.values()])
> msg = "Fractions: {f}".format(f=fraction_categories)
> g.message(_(msg))
> g.message(_("Sum: {s}".format(s=fractions_sum)))
> ```
>
> gives
> ```
> Fractions: {u'24': u'nan', u'25': u'0.0159235666814', u'26':
> u'0.0891719747445', u'27': u'nan', u'20': u'0.0238853509712', u'21':
> u'0.0286624211654', u'23': u'0.143312100132', u'29': u'nan', u'40':
u'nan',
> u'41': u'0.0318471333627', u'1': u'nan', u'3': u'nan', u'2':
> u'0.0159235669187', u'5': u'nan', u'4': u'nan', u'7': u'nan', u'6':
u'nan',
> u'9': u'nan', u'8': u'nan', u'11': u'0.00159235669186', u'10': u'nan',
> u'12': u'0.162420386604', u'15': u'0.366242033672', u'16': u'nan', u'18':
> u'0.0573248423308', u'31': u'nan', u'36': u'nan', u'35':
> u'0.0636942667255', u'32': u'nan'}
> Sum: 1.0
> ```
>
> Yet the following assertion does not work as expected
> ```
> assert fractions_sum < 1.000000000000001, "Sum of fractions is > 1"
> ```
> and returns
> ```
> Traceback (most recent call last):
>   File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap",
line 3979, in <module>
>     sys.exit(main())
>   File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap",
line 3964, in main
>     **supply_parameters)
>   File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap",
line 2864, in compute_supply
>     assert fractions_sum < 1.000000000000001, "Sum of fractions is > 1"
> AssertionError: Sum of fractions is > 1
> ```

What is the sum when printed out with %.17g?

Why do you use 1.000000000000001 and not e.g. 1.000001 as threshold?
1.000000000000001 is most probably a too small difference to 1 considering
floating point precision limits.

Markus M

>
> The following _does_ work, in Python (inside a GRASS GIS session),
> ```
> import os, sys, subprocess
> import datetime, time
> import csv
> import math
> import atexit
> import grass.script as grass
> from grass.exceptions import CalledModuleError
> from grass.pygrass.modules.shortcuts import general as g
> from grass.pygrass.modules.shortcuts import raster as r
> from grass.pygrass.modules.shortcuts import vector as v
> fractions_sum = 1.
> assert fractions_sum < 1.000000000000001, "Sum of fractions is > 1"
> ```
>
> Nikos
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