Question

I do not understand the following example, let’s say I have these functions:

# python likes
def save(filename, data, **kwargs):
    fo = openX(filename, "w", **kwargs) # <- #1
    fo.write(data)
    fo.close()
# python doesnt like
def save2(filename, data, **kwargs):
    fo = openX(filename, "w", kwargs) # <- #2
    fo.write(data)
    fo.close()

def openX(filename, mode, **kwargs):
    #doing something fancy and returning a file object

Why is #1 the right solution and #2 the wrong one? **kwargs is basically a dict, so if I want to pass down the argument to openX I think the correct way would be without ** and just giving the dict. But Python obviously doesn’t like the second one and tells me I gave 3 instead of 2 arguments.

So what’s the reason behind this?

Answer 1

In the second example you provide 3 arguments: filename, mode and a dictionary (kwargs). But Python expects: 2 formal arguments plus keyword arguments.

By prefixing the dictionary by ‘**’ you unpack the dictionary kwargs to keywords arguments.

A dictionary (type dict) is a single variable containing key-value pairs.

“Keyword arguments” are key-value method-parameters.

Any dictionary can by unpacked to keyword arguments by prefixing it with ** during function call.