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

[Nikos Alexandris]

Nikos wrote:

>> 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
>> ```

Markus Metz:

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

Bingo!

Following is from a script run, not from the fractions posted in this
thread.

```
Sum: 1.0000000000003
Traceback (most recent call last):
  File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap", line 4207, in <module>
    sys.exit(main())
  File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap", line 4192, in main
    **supply_parameters)
  File "/osgeo/grasstrunk/dist.x86_64-pc-linux-gnu/scripts/r.estimap", line 3019, in compute_supply
    assert fractions_sum < 1.000000000000001, "Sum of fractions is > 1"
AssertionError: Sum of fractions is > 1
```

>Why do you use 1.000000000000001 and not e.g. 1.000001 as threshold?

Because of this (wrong I understand now) quick trial and error: 

In [5]: 1.00000000000001 > 1
Out[5]: True

In [6]: 1.000000000000001 > 1
Out[6]: True

In [7]: 1.0000000000000001 > 1
Out[7]: False

>1.000000000000001 is most probably a too small difference to 1 considering
>floating point precision limits.

It is. But then again, how do I decide how "loose" it can be?

1.01?
1.001?
1.0001?

N
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