ValueError too many values to unpack

Question

My code:

from sys import argv
pl_magic, pl_pet = argv
pl_enemy = raw_input("The second enchanter:")
print "The most powerful enchanter is",pl_magic
print pl_pet,"is the pet of",pl_magic
print "They hate %s" %pl_enemy

Output in powershell:

PS D:\FILE\LPHW> python ex13b.py 1 2 3
Traceback (most recent call last):
  File "ex13b.py", line 2, in <module>
    pl_magic, pl_pet = argv
ValueError: too many values to unpack

I'm wondering what's wrong with the code....

Answer 1

Argv array contains not only command-line arguments but also name of the running script so

argv[0] = "ex13b.py"

Also, by writing

pl_magic, pl_pet = argv

you are expecting argv to contain only 2 values while in command-line arguments you are giving 3 arguments. That makes argv contain a total of 4 values because the first value is always script name.

Your code should look something like this:

script_name, pl_magic, pl_pet, third_argument = argv

That being said, you shouldn't just try to unpack things blindly but also add some checks for example:

args_count = len(argv) - 1
if args_count < 3:
    #do something if not enough arguments provided

Answer 2

When you do:

pl_magic, pl_pet = argv

And enter the parameters (remember, you should take the filename into account in sys.argv ):

ex13b.py 1 2 3

You actually run the following line:

pl_magic, pl_pet = ['ex13b.py', '1', '2', '3']

When unpacking values into variables, you need the same number of variables and values. This is why your code won't work.

You can use:

_, par1, par2, par3 = sys.argv

---

In Python 3 you can also use:

_, par1, *par2 = sys.argv

Which will result in:

_ = 'ex13b.py'
par1 = '1'
par2 = ['2', '3']