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Error messages from format()

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  Error messages from format() "Colin J. Williams" <cjw@ncf.ca> - 2012-11-13 10:08 -0500
    Re: Error messages from format() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-13 18:38 +0000
      Re: Error messages from format() "Colin J. Williams" <cjw@ncf.ca> - 2012-11-13 15:24 -0500
        Re: Error messages from format() Dave Angel <d@davea.name> - 2012-11-13 16:18 -0500
        Re: Error messages from format() Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-11-13 22:54 +0000

#33244 — Error messages from format()

Is there some way to get more informative error messages from the 
builtin format?

Most messages are such as:
Traceback (most recent call last):
   File "<interactive input>", line 1, in <module>
ValueError: Invalid conversion specification

This example doesn't point to the first invalid case.

[Dbg]>>> format((25, 31),'{0^9o} a(1:9x}')
Traceback (most recent call last):
   File "<interactive input>", line 1, in <module>
ValueError: Invalid conversion specification

Basically, I'm trying to make use of the format function with Python 
3.2, but find little in the way of examples in the docs.

Colin W.

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#33257

On Tue, 13 Nov 2012 10:08:59 -0500, Colin J. Williams wrote:

> Is there some way to get more informative error messages from the
> builtin format?

Yes -- post a feature request on the Python bug tracker, then wait until 
Python 3.4 comes out in about 16 months.

:(


> Most messages are such as:
> Traceback (most recent call last):
>    File "<interactive input>", line 1, in <module>
> ValueError: Invalid conversion specification
> 
> This example doesn't point to the first invalid case.

Better error messages would be valuable.


> [Dbg]>>> format((25, 31),'{0^9o} a(1:9x}')
> Traceback (most recent call last):
>    File "<interactive input>", line 1, in <module>
> ValueError: Invalid conversion specification

I see at least three problems.

(1) The first brace substitution is missing the colon between the 
argument selector "0" and the format spec "^9o": should be "{0:^9o}".

(2) The second format string has an opening round bracket instead of 
brace: (1:9x}

(3) But you are confusing the str.format method with the format function. 
The format function doesn't take brace substitutions!

The string format method takes a template including brace substitutions, 
plus multiple "objects to be substituted", like this:

py> '{0:^9o} a{1:9x}'.format(25, 31)
'   31     a       1f'

In this case, the template '{0:^9o} a{1:9x}' requires two arguments since 
it has two substitutions, {0} and {1}. Each substitution has a format 
spec following the colon: {0:^9o} and {1:9x}

But the format function only takes a single "object to be substituted", 
and so doesn't take a brace substitution. Instead, it just takes the 
format spec part:


py> format(25, '^9o')
'   31    '
py> format(31, '^9o')
'   37    '

format will not split a tuple into multiple arguments for you, since the 
tuple is considered a single object.



-- 
Steven

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#33262

On 13/11/2012 1:38 PM, Steven D'Aprano wrote:
> On Tue, 13 Nov 2012 10:08:59 -0500, Colin J. Williams wrote:
>
>> Is there some way to get more informative error messages from the
>> builtin format?
>
> Yes -- post a feature request on the Python bug tracker, then wait until
> Python 3.4 comes out in about 16 months.
>
> :(
>
Many thanks :)

I am working on the assumption that the first argument of the format 
builtin function and be a sequence of values, which can be selected
with {1:}, {2:}, {0:} etc.

The docs don't make this clear.  I would appreciate advice.

Colin W.
> [snip]

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#33265

On 11/13/2012 03:24 PM, Colin J. Williams wrote:
> <SNIP>
>
> I am working on the assumption that the first argument of the format
> builtin function and be a sequence of values, which can be selected
> with {1:}, {2:}, {0:} etc.
>
> The docs don't make this clear.  I would appreciate advice.
>

The built-in function format():

http://docs.python.org/3.3/library/functions.html?highlight=format%20builtin#format

The first parameter is a single object, NOT a sequence.  One object, one
format.  If you want more generality, use the str.format() method:

http://docs.python.org/3.3/library/stdtypes.html?highlight=format#str.format

where you can supply a list or a dictionary of multiple items to be
formatted into a single string.  That's the one where you supply the
curly braces.


-- 

DaveA

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#33270

On Tue, 13 Nov 2012 15:24:53 -0500, Colin J. Williams wrote:

> On 13/11/2012 1:38 PM, Steven D'Aprano wrote:
>> On Tue, 13 Nov 2012 10:08:59 -0500, Colin J. Williams wrote:
>>
>>> Is there some way to get more informative error messages from the
>>> builtin format?
>>
>> Yes -- post a feature request on the Python bug tracker, then wait
>> until Python 3.4 comes out in about 16 months.
>>
>> :(
>>
> Many thanks :)
> 
> I am working on the assumption that the first argument of the format
> builtin function and be a sequence of values, which can be selected with
> {1:}, {2:}, {0:} etc.

Um, did you read the rest of my post? I already told you that this is 
incorrect.



-- 
Steven

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