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Python

Posted by Sarah Toukan
Sarah Toukan
1,079 Points

Understanding use of the double asterisks (**)

In the Packing and Unpacking lecture, Kenneth defines the following function:

def unpacker(first_name=None, last_name=None):
    if first_name and last_name:
        print("Hi {} {}!".format(first_name, last_name))
    else:
        print("Hi no name!")

and then he uses this to call it:

unpacker(**{"last_name": "Love", "first_name":"Kenneth"})

Firstly, I'm trying to understand the reasoning behind the way he calls the function. If I simply try

unpacker("Kenneth", "Love")

it gives me the same result. So what benefit is there with the longer version?

Secondly I thought the purpose of using the double asterisks was to allow a variable number of arguments, but when I try I'm not allowed to pass in a dictionary with anything other than "last_name", and "first_name".

1 Answer

Steven Parker
Steven Parker
243,160 Points
Steven Parker
Steven Parker
243,160 Points

You didn't provide a link to a course page, but I'd guess that this unusual argument passing is simply done as a demonstration of how keyword arguments don't have to be provided any any particular order. Your use of positional arguments is much more likely to be the way that function would be called in actual practice.

And this function is defined to specifically take only those two arguments, so that would explain getting an error if anything other (or additional) were supplied.

Sarah Toukan
Sarah Toukan
1,079 Points

Thank you Steven, I'll edit the post to add the link.

I still don't totally understand the use of the double asterisks when calling the function. If I remove them but write the arguments in the correct order:

unpacker({"first_name": "Kenneth", "last_name":"Love"})

the resulting output is this:

Hi no name!

It's as if I passed nothing into the function?

Sarah Toukan
Sarah Toukan
1,079 Points

If I remove the quotes I get a NameError

Steven Parker
Steven Parker
243,160 Points
Steven Parker
Steven Parker
243,160 Points

You also need to remove the braces and convert the :'s to ='s:

unpacker(first_name="Kenneth", last_name="Love")  # the unpacked equivalent

The function is expecting individual string arguments, but without the "**" operator to unpack the dictionary, you're just passing it a dictionary as a single argument (which is ignored).

Sarah Toukan
Sarah Toukan
1,079 Points

Thank you Steven, the explanation is very helpful

Steven Parker
Steven Parker
243,160 Points
Steven Parker
Steven Parker
243,160 Points

Sarah Toukan — Glad to help. You can mark a question solved by choosing a "best answer".
And happy coding!