[Django Code] #7726: DecimalField: Semantics of max_digits in combination with decimal_places confusing and perhaps wrong
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Django Code
Jul 11, 2008, 11:41:23 PM7/11/08
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#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
--------------------------+-------------------------------------------------
Reporter: theevilgeek | Owner: nobody
Status: new | Milestone:
Component: Validators | Version: SVN
Keywords: DecimalField | Stage: Unreviewed
Has_patch: 0 |
--------------------------+-------------------------------------------------
'''"Problem"'''
In attempting to capture fixed point decimal values in the range of .0000
to .9999, I created a DecimalField as follows:
value = forms.DecimalField(max_digits=4, decimal_places=4)
However, it seems that any decimal submitted is first normalized to
include a leading "0." should no other non-fractional
digits be provided.
The presence of this additional "0." causes the input to instead resemble
0.XXXX. However, since decimal_places = 4
and max_digits = 4, no input will successfully validate-- the leading zero
simply hasn't been accounted for.
'''Possible solutions'''
1. irc:Gulopine and irc:Magus suggest that this behavior is not a problem:
a properly formatted decimal lacking a whole component incorporates a
leading "0."; thus, the user should anticipate this extra digit and plan
accordingly.
However, the documentation for the DecimalField
(http://www.djangoproject.com/documentation/model-api/#decimalfield
[treating the model and newforms fields as being interchangeable])
indicates that the field stores a "A fixed-precision decimal number". As
fixed-precision (or fixed-point) decimal notation usually implies exacting
control over the number of positions before and after the decimal point
that will be incorporated in a representation of the data
(http://en.wikipedia.org/wiki/Fixed-point_arithmetic), it seems reasonable
to expect that the DecimalField will exactingly respect the max_digits and
decimal_places arguments to the same effect. Thus, I feel that the
exhibited behavior is, at the least, somewhat surprising.
2. irc:Gulopine suggests that DecimalField might be altered to raise an
ImproperlyConfigured error if max_digits <= decimal_places.
3. The DecimalField documentation might be enhanced to explicitly state
that a leading "0." will be tacked onto any decimal numbers lacking whole
digits and that this leading zero must be accounted for when specifying
max_digits and decimal_places.
4. The behavior of DecimalField is altered such that max_digits and
decimal_places are strictly enforced.
Finally, it is worth noting that the examples in the documentation for
DecimalField only reinforce the expectation of strict enforcement:
"For example, to store numbers up to 999 with a resolution of 2 decimal
places, you’d use: models.DecimalField(..., max_digits=5,
decimal_places=2)"
--
Ticket URL: <http://code.djangoproject.com/ticket/7726>
Django Code <http://code.djangoproject.com/>
The web framework for perfectionists with deadlines
confusing and perhaps wrong
--------------------------+-------------------------------------------------
Reporter: theevilgeek | Owner: nobody
Status: new | Milestone:
Component: Validators | Version: SVN
Keywords: DecimalField | Stage: Unreviewed
Has_patch: 0 |
--------------------------+-------------------------------------------------
'''"Problem"'''
In attempting to capture fixed point decimal values in the range of .0000
to .9999, I created a DecimalField as follows:
value = forms.DecimalField(max_digits=4, decimal_places=4)
However, it seems that any decimal submitted is first normalized to
include a leading "0." should no other non-fractional
digits be provided.
The presence of this additional "0." causes the input to instead resemble
0.XXXX. However, since decimal_places = 4
and max_digits = 4, no input will successfully validate-- the leading zero
simply hasn't been accounted for.
'''Possible solutions'''
1. irc:Gulopine and irc:Magus suggest that this behavior is not a problem:
a properly formatted decimal lacking a whole component incorporates a
leading "0."; thus, the user should anticipate this extra digit and plan
accordingly.
However, the documentation for the DecimalField
(http://www.djangoproject.com/documentation/model-api/#decimalfield
[treating the model and newforms fields as being interchangeable])
indicates that the field stores a "A fixed-precision decimal number". As
fixed-precision (or fixed-point) decimal notation usually implies exacting
control over the number of positions before and after the decimal point
that will be incorporated in a representation of the data
(http://en.wikipedia.org/wiki/Fixed-point_arithmetic), it seems reasonable
to expect that the DecimalField will exactingly respect the max_digits and
decimal_places arguments to the same effect. Thus, I feel that the
exhibited behavior is, at the least, somewhat surprising.
2. irc:Gulopine suggests that DecimalField might be altered to raise an
ImproperlyConfigured error if max_digits <= decimal_places.
3. The DecimalField documentation might be enhanced to explicitly state
that a leading "0." will be tacked onto any decimal numbers lacking whole
digits and that this leading zero must be accounted for when specifying
max_digits and decimal_places.
4. The behavior of DecimalField is altered such that max_digits and
decimal_places are strictly enforced.
Finally, it is worth noting that the examples in the documentation for
DecimalField only reinforce the expectation of strict enforcement:
"For example, to store numbers up to 999 with a resolution of 2 decimal
places, you’d use: models.DecimalField(..., max_digits=5,
decimal_places=2)"
--
Ticket URL: <http://code.djangoproject.com/ticket/7726>
Django Code <http://code.djangoproject.com/>
The web framework for perfectionists with deadlines
Django Code
Jul 11, 2008, 11:53:00 PM7/11/08
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
----------------------------------+-----------------------------------------
confusing and perhaps wrong
Reporter: theevilgeek | Owner: nobody
Status: new | Milestone:
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Status: new | Milestone:
Component: Validators | Version: SVN
Stage: Accepted | Has_patch: 0
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by mtredinnick):
* needs_better_patch: => 0
* stage: Unreviewed => Accepted
* needs_tests: => 0
* needs_docs: => 0
Comment:
It should be a validation error if `max_digits < decimal_places +1`. That
is, this would be an error detected when you run `manage.py validate`.
That's all that's required here. There will be at least one digit
(possibly an implicit zero) to the left of the decimal point.
A patch to add that check to the validation routines in
`django.core.management` will be most welcome. Thanks for taking the time
to work through this.
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:1>
Django
Dec 17, 2010, 10:08:57 PM12/17/10
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
----------------------------------+-----------------------------------------
confusing and perhaps wrong
Reporter: theevilgeek | Owner: elbarto
Status: new | Milestone:
Component: Validators | Version: SVN
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Stage: Accepted | Has_patch: 0
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by elbarto):
Stage: Accepted | Has_patch: 0
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
* owner: nobody => elbarto
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:2>
Django <http://code.djangoproject.com/>
The Web framework for perfectionists with deadlines.
Django
Dec 17, 2010, 10:17:22 PM12/17/10
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
confusing and perhaps wrong
----------------------------------+-----------------------------------------
Reporter: theevilgeek | Owner: elbarto
Reporter: theevilgeek | Owner: elbarto
Status: new | Milestone:
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Component: Validators | Version: SVN
Stage: Accepted | Has_patch: 1
Needs_docs: 0 | Needs_tests: 0
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by elbarto):
* has_patch: 0 => 1
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by elbarto):
Comment:
I have run the validators tests and it returns OK.
However, because of it's my first patch to the code, I don't know if I
have to write new tests.
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:3>
Django
Dec 18, 2010, 8:11:22 AM12/18/10
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
confusing and perhaps wrong
----------------------------------+-----------------------------------------
Reporter: theevilgeek | Owner: elbarto
Reporter: theevilgeek | Owner: elbarto
Status: new | Milestone:
Component: Validators | Version: SVN
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Stage: Accepted | Has_patch: 1
Needs_docs: 1 | Needs_tests: 1
Stage: Accepted | Has_patch: 1
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by ramiro):
* needs_docs: 0 => 1
* needs_tests: 0 => 1
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:4>
Django
Dec 18, 2010, 6:29:16 PM12/18/10
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
confusing and perhaps wrong
----------------------------------+-----------------------------------------
Reporter: theevilgeek | Owner: elbarto
Reporter: theevilgeek | Owner: elbarto
Status: new | Milestone:
Component: Validators | Version: SVN
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Stage: Accepted | Has_patch: 1
Needs_docs: 1 | Needs_tests: 0
Stage: Accepted | Has_patch: 1
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by elbarto):
* needs_tests: 1 => 0
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:5>
Django
Dec 18, 2010, 6:46:41 PM12/18/10
to djang...@holovaty.com, django-...@googlegroups.com
#7726: DecimalField: Semantics of max_digits in combination with decimal_places
confusing and perhaps wrong
confusing and perhaps wrong
----------------------------------+-----------------------------------------
Reporter: theevilgeek | Owner: elbarto
Reporter: theevilgeek | Owner: elbarto
Status: new | Milestone:
Component: Validators | Version: SVN
Component: Validators | Version: SVN
Resolution: | Keywords: DecimalField
Stage: Accepted | Has_patch: 1
Needs_docs: 0 | Needs_tests: 0
Stage: Accepted | Has_patch: 1
Needs_better_patch: 0 |
----------------------------------+-----------------------------------------
Changes (by elbarto):
* needs_docs: 1 => 0
----------------------------------+-----------------------------------------
Changes (by elbarto):
--
Ticket URL: <http://code.djangoproject.com/ticket/7726#comment:6>
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