Why doesn't Python (error msg) tell me WHAT the actual (arg) values are ?

[Peter J. Holzer]

On 2023-02-22 15:46:09 -0800, Hen Hanna wrote:
> On Wednesday, February 22, 2023 at 12:05:34 PM UTC-8, Hen Hanna wrote:
> > > py bug.py 
> > Traceback (most recent call last): 
> > File "C:\Usenet\bug.py", line 5, in <module> 
> > print( a + 12 ) 
> > TypeError: can only concatenate str (not "int") to str 
> > 
> > 
> > Why doesn't Python (error msg) do the obvious thing and tell me 
> > WHAT the actual (offending, arg) values are ? 
> > 
> > In many cases, it'd help to know what string the var A had , when the error occurred. 
> > ------------ i wouldn't have to put print(a) just above, to see. 
> > 
> > ( pypy doesn't do that either, but Python makes programming
> > (debugging) so easy that i hardly feel any inconvenience.)

That seems like a non-sequitur to me. If you hardly feel any
inconvenience, why argue so forcefully?
And why is pypy relevant here?


> i  see that my example   would be clearER  with this one-line  change:
> 
> 
>                   >  py   bug.py
> 
>                    Traceback (most recent call last):
> 
>                   File "C:\Usenet\bug.py", line 5, in <module>
>                                          map( Func,    fooBar(  X,  Y,  X +  Y  ))
>  
>                    TypeError: can only concatenate str (not "int") to str
> 
> 
> i hope that   NOW   a few of you  can  see this as a genuine,  (reasonable)  question.

That doesn't seem a better example to me. There is still only one
subexpression (X + Y) where that error can come from, so I know that X
is a str and Y is an int.

A better example would be something like 

    x = (a + b) * (c + d)

In this case it could be either (a + b) or (c + d) which caused the
error. But what I really want to know here is the names of the involved
variables, NOT the values. If the error message told me that the values
were 'foo' and 12.3, I still wouldn't be any wiser. The problem here of
course is that the operands aren't necessarily simple variables as in
this example - they may be arbitrarily complex expressions. However, it
might be sufficient to mark the operator which caused the exception:

|   ...
|   File "/home/hjp/tmp/./foo", line 4, in f
|     return (a + b) * (c + d)
|                         ^
| TypeError: can only concatenate str (not "int") to str

would tell me that (c + d) caused the problem and therefore that c must
be a str which it obviously shouldn't be.

        hp

-- 
   _  | Peter J. Holzer    | Story must make more sense than reality.
|_|_) |                    |
| |   | hjp at hjp.at         |    -- Charles Stross, "Creative writing
__/   | http://www.hjp.at/ |       challenge!"
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