Python - 在本地函数调用中解压缩kwargs

Question

I would like to pass a dictionary :

items = {"artist": "Radiohead", "track": "Karma Police"}

as a parameter to this function :

def lastfm_similar_tracks(**kwargs):

    result = last.get_track(kwargs)
    st = dict(str(item[0]).split(" - ") for item in result.get_similar())
    print (st)

where last.get_track("Radiohead", "Karma Police") is the correct way of calling the local function .

and then call it like this:

lastfm_similar_tracks(items)

I'm getting this error:

TypeError: lastfm_similar_tracks() takes exactly 0 arguments (1 given)

how should I correct this?

Answer 1

Basically, **kwargs in function definition means named arguments should not be unpacked to variables.

To call some function with dictionary of arguments, these still need to be represented as named parameters. Else python "considers" supplied argument to be positional

Thus, you should call you function as:

items = {"artist": "Radiohead", "track": "Karma Police"}
lastfm_similar_tracks(**items)

Answer 2

A few items of confusion:

You are passing the dictionary items as a parameter without the double star. This means that items is treated as the first positional argument, whereas your function only has **kwargs defined.

Here's a simple function:

>>> def f(**kwargs):
...     print (kwargs)

Let's pass it items :

>>> items = {"artist": "Radiohead", "track": "Karma Police"}
>>> f(items)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: f() takes 0 positional arguments but 1 was given

Oops: no positional arguments are allowed. You need to use the double star so that it will print:

>>> f(**items)
{'artist': 'Radiohead', 'track': 'Karma Police'}

This leads us to the next issue: kwargs inside the function is a dictionary , so you can't just pass it to last.get_track , which has two positional arguments according to your example. Assuming that order matters (it almost certainly does), you need to get the correct values from the dictionary to be passed:

result = last.get_track(kwargs['artist'], kwargs['track'])

Answer 3

First problem: you need to pass items in like this:

lastfm_similar_tracks(**items)

Second problem, inside lastfm_similar_tracks , kwargs is a dictionary, in which the keys are of no particular order, therefore you cannot guarantee the order when passing into get_track . The best that you can do is:

result = last.get_track(kwargs['artist'], kwargs['track'])

At this point, the code should work the way you intended. However, I question the use of **kwargs in lastfm_similar_tracks : do you really need to define it that way? How about:

def lastfm_similar_tracks(artist, track, **kwargs):
    result = last.get_track(artist, track)
    # The rest of your code here

Then you can call it like so:

lastfm_similar_tracks(**items)

This way, you ensure that the caller must pass in artist and track or the function call will fail.