Dispatch a Python method call with variable number of arguments by using an array slice

Question

I have a small Python tracking script with a couple of possible modes (e.g. "add" and "show") that correspond to methods that take different numbers of arguments.

I want to check the mode argument is valid, and if so, then call the appropriate method with all remaining command line arguments passed as individual parameters.

The code I have looks like this:

"""python tracking.py add <person> <date> <score> 
   python tracking.py show <person> <date>
"""
if __name__ == "__main__":
    tracking = TrackingData()
    modes = {'add': tracking.add_data, 'show': tracking.print_data }

    mode = sys.argv[1]

    # If mode is valid
    modes[mode](sys.argv[2:])

    # Original code looks a bit like this: 
    #
    # person = sys.argv[2]
    # date = sys.argv[3]
    # score = sys.argv[4]

    # tracking.add_data(person, date, score)
    # tracking.print_data(person, date)

However, the code gives a TypeError: print_data() missing 1 required positional argument: 'date' - so presumably the array slice is being passed as a single parameter, whereas I need it to be flattened into its component entries.

What's the best way to do this? Are there any better patterns for this type of dispatch table / modal action approach?

To pass on keyword arguments using dictionaries you can use the **. This requires a dictionary creation — MYousefi
– MYousefi, Commented Jan 30, 2022 at 18:35
Alternatively, I think it may be possible to use a wrapper that takes the list and places the variables properly into the original function call. I am not aware of any method that automatically places list positions as positional arguments. — MYousefi
– MYousefi, Commented Jan 30, 2022 at 18:41

Chris Sears · Accepted Answer · 2022-01-30 18:47:34Z

0

Using * will unpack values from a list into positional arguments. So you would just need to change your dispatch line to look like this:

modes[mode](*sys.argv[2:])

For example:

def add3(a, b, c):
    return a + b + c

nums = [1, 2, 3]

add3(*nums) # 6

answered Jan 30, 2022 at 18:47

Chris Sears

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