# Misunderstanding why when i put "0" in a float field it doesn't work normally
**URL:** https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332
**Category:** Developer
**Created:** [June 18, 2021, 10:44am UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332 "2021-06-18T10:44:39Z")
**Posts on this page:** 6
**Page:** 1
### Author:  [@aso](https://discuss.tryton.org/u/aso)
#### Post date: [June 18, 2021, 10:44am UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/1 "2021-06-18T10:44:39Z")
Hi am trying to understand something , i have a field type float and i want to display a sentence if i put “0” , it works with all numbers under 0 and after 0 but not with “0”
is it normal with tryton , is their a problem with using 0 with tryton
```auto
diameter = fields.Float("Diameter ")
@fields.depends('diameter', 'art')
def on_change_with_diameter_arts(self, name=None):
if (self.diameter == 0 and self.art == "carpentry" ):
return " No carpentry "
```
but nothing display when i put “0” in the field “diameter” , it doesn’t work just when i make “0” in field but in other conditions and with other number it works normally !
---
### Author:  [@ced](https://discuss.tryton.org/u/ced)
#### Post date: [June 18, 2021, 12:01pm UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/2 "2021-06-18T12:01:48Z")
I guess you have to debug to see what is the value of `diameter` and why your test is not working.
---
### Author:  [@rvong](https://discuss.tryton.org/u/rvong)
#### Post date: [June 18, 2021, 1:44pm UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/3 "2021-06-18T13:44:11Z")
Something is not right with your **on\_change\_with** name function. Are you trying to have the field **art** display the value **No carpentry** based on the value of the diameter? If so then your **on\_change\_with** function should be named correctly to:
`
def on_change_with_art(self, name=None)
`
The naming convention for **on\_change\_** and **on\_change\_with\_** is very critical for it to work.
---
### Author:  [@aso](https://discuss.tryton.org/u/aso)
#### Post date: [June 18, 2021, 2:11pm UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/4 "2021-06-18T14:11:56Z")
Hi , No am trying to display in the field" ```
diameter\_arts
" the value returned by the function on\_change\_with\_diameter\_arts ,
when i tried to work in debug mode like M. @ced advice me, there was an error shown for the first time :
# float() argument must be a string or a number, not 'NoneType
---
### Author:  [@rvong](https://discuss.tryton.org/u/rvong)
#### Post date: [June 18, 2021, 2:18pm UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/5 "2021-06-18T14:18:49Z")
I see, well, I guessed wrong.
In that case, depending on how and where your form is displayed, the **NoneType** problem maybe due to the lack of a default value for the field **diameter**. Have you tried putting a
`
def default_diameter(self):
return 0
`
and see if that fixes your problem?
---
### Author:  [@ced](https://discuss.tryton.org/u/ced)
#### Post date: [June 18, 2021, 3:03pm UTC](https://discuss.tryton.org/t/misunderstanding-why-when-i-put-0-in-a-float-field-it-doesnt-work-normally/4332/6 "2021-06-18T15:03:53Z")
You must always in `on_change` functions support the possibility that the value is `None` (even if required).