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To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Odd ValueError using float
Date: Sat, 14 Mar 2015 17:08:11 +0100
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emile wrote:

> On 03/13/2015 08:09 PM, Chris Angelico wrote:
>> On Sat, Mar 14, 2015 at 1:33 PM, Paul Rubin <no.email@nospam.invalid>
>> wrote:
>>> emile <emile@fenx.com> writes:
>>>> *** NameError: name 'val' is not defined
>>>> (Pdb) l
>>>> 139         try:
>>>> 140             val = round(float(decval),1)
>>>> 141         except:
>>>> 142             import pdb; pdb.set_trace()
>>>
>>> If 'float' or 'round' throw an exception, the assignment to 'val' never
>>> happens, so 'val' is undefined.  Try examining the value of 'decval' in
>>> the debugger to see what is making the conversion fail.
>>
>> That's exactly what the OP did :) Trouble is, it didn't help, because
>> it sure looked as if decval was the string '4'. My best guess was -
>> and is - that it's not just a string. We're looking at an SQL
>> interface routine here, so it may be that there's a string subclass
>> that length-limits itself, on the assumption that it's going into a
>> fixed-length database field.
> 
> It ran almost to completion before generating the error again --
> 
> (Pdb) decval
> '4'
> (Pdb) type(decval)
> <type 'str'>
> (Pdb) len(decval)
> 1
> (Pdb) int(decval)
> *** ValueError: invalid literal for int() with base 10:
> '41.700000000000003'
> 
> So there's still something amiss.

Why are you checking

int(decval)

when the actual failing expression is

round(float(decval),1)

? Try to evaluate the latter in the debugger, and if that gives no clue 
change

>>>> 139         try:
>>>> 140             val = round(float(decval),1)
>>>> 141         except:
>>>> 142             import pdb; pdb.set_trace()

to just

val = round(float(decval), 1)

to get a meaningful traceback and post that.