使用 sys.argv[1] 和 numpy 数组将多个 arguments 传递给 Python 脚本。 IndexError：列表索引超出范围

Question

I'm working in a small code to receive multiple arguments from an MQTT server and use them to predict another value. I'm showing a simplified code here just to get some help. To pass the arguments to the script for executing the prediction, first part is the creation of a numpy array, then pass the arguments to the script using sys.argv[], then the indexing to position the incoming values.

import numpy as np
import sys

    # creating empty numpy array for feature values
X = np.empty(2).reshape(1, 2)

    #storing the arguments
azimuth_sin=sys.argv[1]
azimuth_cos=sys.argv[2]

    #displaying the arguments
print("azimuth_sin : " + azimuth_sin)
print("azimuth_cos : " + azimuth_cos)
print("Number of arguments : ", len(sys.argv))
   
    # set vector values
X[:,0] = sys.argv[1]
X[:,1] = sys.argv[2]

print(X)

However, I have an issue with the second argument as I get an error:

exit code: 1, Traceback (most recent call last): File "numpy-array.py", line 10, in azimuth_cos=sys.argv[2] IndexError: list index out of range

The only way to avoid that error is if I set both arguments to: sys.arg[1]

   #storing the arguments
azimuth_sin=sys.argv[1]
azimuth_cos=sys.argv[1]

   #displaying the arguments
print("azimuth_sin : " + azimuth_sin)
print("azimuth_cos : " + azimuth_cos)
print("Number of arguments : ", len(sys.argv))
   
   # set vector values
X[:,0] = sys.argv[1]
X[:,1] = sys.argv[1]

print(X)

Then I get two consecutive outputs:

azimuth_sin: -0.9152180545267792 azimuth_cos: -0.9152180545267792 Number of arguments: 2 [[-0.91521805 -0.91521805]]

and:

azimuth_sin: 0.40295894662883136 azimuth_cos: 0.40295894662883136 Number of arguments: 2 [[0.40295895 0.40295895]]

which are actually the values of the two arguments printed, but repeated twice: sin = -0.9152180545267792 and cos = 0.40295894662883136

If I put the arguments in one line:

   #storing the arguments
azimuth_sin, azimuth_cos = sys.argv[1:2] 

The error is:

exit code: 1, Traceback (most recent call last): File "numpy-array-t1.py", line 10, in azimuth_sin, azimuth_cos = sys.argv[1:2] ValueError: not enough values to unpack (expected 2, got 1)

I've tried many ways to fix this without success, I'd appreciate any help or suggestions. Thank you in advance.

Answer 1

Start with something simple to validate the data you are receiving.

Do something like:

import sys  
  
# Verify first so you don't get an error  
# This check verifies we have at least two parameters
if 1 < len(sys.argv):
    if sys.argv[1]:
        var_argv1 = sys.argv[1]
        print("var_argv1 type: %s, length: %s" % (type(var_argv1), len(var_argv1)))

        # It is pointless to continue if argv[1] has no data
        if sys.argv[2]:
            var_argv2 = sys.argv[2]
            print("var_argv2 type: %s, length: %s" % (type(var_argv2), len(var_argv2)))
        else:
            print("sys.argv[2] has no data")
    else:
        print("sys.argv[1] has no data")

It may be that you are trying to process a numpy object on the command line

Note:
Do you have access to the MQTT server?
It might be easier to pick a channel (topic) to use for this data transfer.
You could get the MQTT server to publish this data on a channel and have this script subscribe to that channel.
Then you could make sending information as easy as a function call on your MQTT system.

Answer 2

In Linux terminal window, I have a simple script that just displays sys.argv :

1619:~$ cd mypy
1619:~/mypy$ cat echo.py
import sys
print(sys.argv)

When I call it thus:

1619:~/mypy$ python3 echo.py 1.23 3.112 foo bar
['echo.py', '1.23', '3.112', 'foo', 'bar']

See that sys.argv is list of 5 strings, which come from the commandline.

If you are calling your script from a shell or windows command window, you should be able to enter and see multiple strings.

But people have problems using sys.argv (and argparse ) when running the script from something like pydev or as a Jupyter notebook. I don't know anything about the MQTT server so can't help with providing even one command line argument. As I demonstrate, sys.argv is primarily intended as a way of providing startup values and options when running a script from the operating system.