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problems when unpacking tuple ...

Dear all,

Maybe I stared on the monitor for too long, because I cannot find the
bug ...
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']
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??

Thanks,

- harold -

Apr 22 '06 #1
11 1506
> for a,b,c,d in line.split() :
[snip]
The output (when given the data I want to parse) is:
['0.0','1','0.04','0']


You'll notice that you're not passing any parameters to
split(). By default, it splits on whitespace, and your
input doesn't have any whitespace in it. Thus, you're
actually only getting *one* (not three) elements back. Try
using split(",") instead.

-tkc


Apr 22 '06 #2
harold:
The output (when given the data I want to parse) is:
If you'd told us that data, and told us what version of Python you're
using, we could have reproduced the problem to look into it.
ValueError: need more than 3 values to unpack

Why does python think that I want to unpack the outcome of
line.split() into three values instead of four?


That's not what it says. It says there are only 3 values in the outcome,
and it needs more (4 to be precise).

--
Ren Pijlman
Apr 22 '06 #3

harold wrote:
Dear all,

Maybe I stared on the monitor for too long, because I cannot find the
bug ...
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']
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??


The 3 values are coming from the first element in the list '0.0' -
change it to '0' and you should get: ValueError: need more than 1 value
to unpack.
See? it's trying to get a,b,c,d from each element of the list not the
whole list.

Gerard

Apr 22 '06 #4
Rene Pijlman schrieb:
harold:
The output (when given the data I want to parse) is:


If you'd told us that data, and told us what version of Python you're
using, we could have reproduced the problem to look into it.


Thank you for the answers and sorry that I did not provide more
information in the first place.
My data file is white space seperated data (space seperated data to be
precise) and I am
using python 2.4.2
As can be seen, the output of the print statement in the lines

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

has exactly four values...

ValueError: need more than 3 values to unpack

Why does python think that I want to unpack the outcome of
line.split() into three values instead of four?


That's not what it says. It says there are only 3 values in the outcome,
and it needs more (4 to be precise).

A similar error happens in an interpreter session, when typing
for line in ["1 2 3 4"] :

.... for a,b,c,d in line.split() :
.... pass
....
Traceback (most recent call last):
File "<stdin>", line 2, in ?
ValueError: need more than 1 value tyo unpack

maybe this might help to track down the error.
Thanks!

- harold -

Apr 22 '06 #5
Thank you Gerard.
This must be the problem. Now I can get it working.

Apr 22 '06 #6
Em Sáb, 2006-04-22 *s 09:21 -0700, harold escreveu:
for line in sys.stdin :
try :
for a,b,c,d in line.split() :
pass

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


Try this:

for a, b, c, d in sys.stdin:
print a, b, c, d

--
Felipe.

Apr 22 '06 #7
harold:
A similar error happens in an interpreter session, when typing
for line in ["1 2 3 4"] :

... for a,b,c,d in line.split() :
... pass
...
Traceback (most recent call last):
File "<stdin>", line 2, in ?
ValueError: need more than 1 value tyo unpack

maybe this might help to track down the error.


Suppose the code was:

for x in line.split():

line.split() yields one list with 4 items. The loop will be performed 4
times, assigning one value of the list to x with every iteration. In your
code, x is a tuple with 4 elements: a,b,c,d. So with every iteration one
value is assigned to that tuple. Since the value is not a sequence of 4
items, this fails.

There's two sensible things you can do:

for line in ["1 2 3 4"]:
a,b,c,d = line.split()

for line in ["1 2 3 4"]:
for a in line.split():

--
Ren Pijlman
Apr 22 '06 #8
Em Sáb, 2006-04-22 *s 14:25 -0300, Felipe Almeida Lessa escreveu:
Em Sáb, 2006-04-22 *s 09:21 -0700, harold escreveu:
for line in sys.stdin :
try :
for a,b,c,d in line.split() :
pass

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


Try this:

for a, b, c, d in sys.stdin:
print a, b, c, d


Forget that. It was stupid. You should try this instead:

for line in sys.stdin:
a, b, c, d = line.split()

--
Felipe.

Apr 22 '06 #9

harold wrote:
Thank you Gerard.
This must be the problem. Now I can get it working.


Good! I got confused thinking about it too, but I think you just had
one loop too many.

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

not:

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

Gerard

Apr 22 '06 #10
Thanks for all your answer!
Of course, I wanted to assign the outcome of the split(), not to
iterate
over them. Thinks are done so easy in python that, sometimes, one
does not even notice that one actually does them ;-)
Cheers,

- harold -

Apr 22 '06 #11
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
Apr 22 '06 #12

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