Simple multiple variable loop on in Python fails on slice size, when it should be working

Question

See code below:

#!/opt/rh/python33/root/usr/bin/python
import sys
print( sys.argv[1:-1] )
if ( len( sys.argv ) < 5 or len( sys.argv ) % 3 != 2 ) :
    print( '''Oy vey! You need to input: <1lib name> <1schematic name > <nview name> .. <nlib name> <nschematic name > <nview name>  <output fname>''' )
    exit ( 1 )
print( len( sys.argv[1:-1] ) )
for lib_name,cell_name,view_name in sys.argv[1:-1] :
    print( "lib is:" + lib_name )
    print( "cell is:" + cell_name )
    print( "view is:" + view_name )

This code should be very simple. I am running the code, and I get an error:

~]./run_automated_block.py ko lo pm do
['ko', 'lo', 'pm']
3
Traceback (most recent call last):
   File "~/run_automated_block.py", line 8, in <module>
     for lib_name,cell_name,view_name in sys.argv[1:-1] :
ValueError: need more than 2 values to unpack
[~]$

I don't get it. The length of the slice is 3. There are 3 variables in the for loop.

That's an awful way to handle cli args. Why don't you use argparse ? — DeepSpace
– DeepSpace, Commented Apr 3, 2018 at 10:27

zwer · Accepted Answer · 2018-04-03 11:10:40Z

1

When you're iterating over the sys.argv[1:-1] list you're iterating one element at the time, not three as you might think, so it naturally complains that it cannot unpack ko (lo, pm...) into three variables - it would if you had the arguments themselves to be of length 3 (e.g. ko1 lo1 po1) but then it would unpack individual characters.

If you insist doing it this way, you can zip the values together into a tuple of three:

for lib_name, cell_name, view_name in zip(*[iter(sys.argv[1:-1])]*3):
    print("lib is:" + lib_name)
    print("cell is:" + cell_name)
    print("view is:" + view_name)

edited Apr 3, 2018 at 11:10

answered Apr 3, 2018 at 10:27

zwer

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