problems when unpacking tuple ...

[John Machin]

On 23/04/2006 2:21 AM, harold wrote:
> Dear all,
> 
> Maybe I stared on the monitor for too long, because I cannot find the
> bug ...

You already have your answer, but below are clues on how to solve such 
problems much faster by yourself.

> My script "transition_filter.py" starts with the following lines:
> 
> import sys
> 
> for line in sys.stdin :
>     try :
>         for a,b,c,d in line.split() :
>             pass
> 
>     except ValueError , err :
>         print line.split()
>         raise err
> 
> The output (when given the data I want to parse) is:
> ['0.0','1','0.04','0']

I doubt it. Much more likely is
['0.0', '1', '0.04', '0']
It doesn't matter in this case, but you should really get into the
habit of copy/pasting *EXACTLY* what is there, not re-typing what you 
think is there.

> Traceback (most recent call last):
>   File "transition_filter.py", line 10, in ?
>     raise err
> ValueError: need more than 3 values to unpack
> 
> What is going wrong here? Why does python think that
> I want to unpack the outcome of line.split() into three
> values instead of four? I must be really tired, but I just
> cannot see the problem. Any clues??
> 

Clues:
1. Use the print statement to show what you have.
2. Use the built-in repr() function to show *unambiguously* what you 
have -- very important when you get into Unicode and encoding/decoding 
problems; what you see after "print foo" is not necessarily what 
somebody using a different locale/codepage will see.
3. In some cases (not this one), it is also helpful to print the type() 
of the data item.

Example:

C:\junk>type harold.py
import sys

def harold1():
     for line in sys.stdin :
         try :
             for a,b,c,d in line.split() :
                 pass

         except ValueError , err :
             print line.split()
             raise err

def harold2():
     for line in sys.stdin:
         print "line =", repr(line)
         split_result = line.split()
         print "split result =", repr(split_result)
         for x in split_result:
             print "about to try to unpack the sequence", repr(x), "into 
4 items"

             a, b, c, d = x

harold2()

C:\junk>harold.py
0.0 1 0.04 0
a b c d e f
^Z
line = '0.0 1 0.04 0\n'
split result = ['0.0', '1', '0.04', '0']
about to try to unpack the sequence '0.0' into 4 items
Traceback (most recent call last):
   File "C:\junk\harold.py", line 22, in ?
     harold2()
   File "C:\junk\harold.py", line 20, in harold2
     a, b, c, d = x
ValueError: need more than 3 values to unpack

=====

Coding style: Not inventing and using your own dialect makes two-way 
communication much easier in any language (computer or human). Consider 
reading and following http://www.python.org/dev/peps/pep-0008/

Hope this helps,
John