Throw an error if *args is missing comma in function argument

Question

I have a function with unknown length of inputs for a *args argument/parameter (I know I can use a list input instead but I want to know how to use *args). All works well but if I pass in scalar inputs without a comma separation it doesn't throw an error. I'd like to detect this circumstance and throw an error. How can I detect missing comma separator and throw an error for *args?

def my_fun(*args):
    
    return args

## Has comma
my_fun('cat', 'fish')
## ('cat', 'fish')

## Missing comma (Want to detect and throw an error)
my_fun('cat' 'fish')
## ('catfish',)

len(my_fun('cat' 'fish')) 
## 1


## Missing comma (Want to detect and throw an error)
my_fun('cat' 'fish', 'dog')
## ('catfish', 'dog')