Can't unpack values of dictionary with **

Can't unpack values of dictionary with ** - Printable Version

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Can't unpack values of dictionary with ** - Snake - Mar-11-2020

I've seen similar examples in 4.7.5 Unpacking argument lists, but my somehow don't work.

>>> d={'USA':'Washington','France':'Paris','China':'Beijing'}
	  
>>> **d
	  SyntaxError: invalid syntax

Also:

>>> def f(a,b,c):
	  print(a,b,c)
	  
>>> f(**d)
	  
Traceback (most recent call last):
  File "<pyshell#33>", line 1, in <module>
    f(**d)
TypeError: f() got an unexpected keyword argument 'USA'

RE: Can't unpack values of dictionary with ** - ndc85430 - Mar-11-2020

The keys in the dict need to match the parameter names of the function.

RE: Can't unpack values of dictionary with ** - buran - Mar-11-2020

your function expects only positional arguments and you try to pass keyword arguments it doesn't expect, thus the error. If you pass keyword arguments they need to be present in the function signature. However if keyword arguments present in the function signature you can [almost ever] pass them also as positional arguments (it's possible to force that function take only positional or only keyword arguments). e.g.

def greet(name='John', familly='Doe'):
    print(f'Hello, {name} {familly}')

user = {'name': 'Harry', 'familly': 'Potter'}

greet(**user)
greet(name='Hermione', familly='Granger')
greet('Ron', 'Weasley')
greet()

Output:Hello, Harry Potter
Hello, Hermione Granger
Hello, Ron Weasley
Hello, John Doe

In addition, your use case does not really warrant use of multiple keyword arguments. Do you really want to have close to 200 arguments? It would be better to pass the dict as single argument

def print_capitals(capitals=None):
    if capitals:
        for country, capital in capitals.items():
            print(f'Capital of {country} is {capital}')
    else:
        print("Youd didn't supply capitals")

my_dict = {'USA':'Washington','France':'Paris','China':'Beijing'}
print_capitals()
print_capitals(capitals=my_dict)

Output:Youd didn't supply capitals
Capital of USA is Washington
Capital of France is Paris
Capital of China is Beijing

Now, there is a way to take arbitrary number of positional and keyword arguments.

def some_func(*args, **kwargs):
    print(f'Positional arguments: {args}')
    print(f'Keyword arguments: {kwargs}')
    print('--------------\n')


pos = {'foo', 'bar'}
keyw = {'spam':1, 'eggs':2}

some_func('foo', spam=1)
some_func('foo', 'bar', spam=1, eggs=2)
some_func(*pos)
some_func(**keyw)
some_func('foo', **keyw)
some_func(*pos, **keyw)

Output:Positional arguments: ('foo',)
Keyword arguments: {'spam': 1}
--------------

Positional arguments: ('foo', 'bar')
Keyword arguments: {'spam': 1, 'eggs': 2}
--------------

Positional arguments: ('foo', 'bar')
Keyword arguments: {}
--------------

Positional arguments: ()
Keyword arguments: {'spam': 1, 'eggs': 2}
--------------

Positional arguments: ('foo',)
Keyword arguments: {'spam': 1, 'eggs': 2}
--------------

Positional arguments: ('foo', 'bar')
Keyword arguments: {'spam': 1, 'eggs': 2}
--------------

Note, args and kwargs are just names used by convention, they can be different

RE: Can't unpack values of dictionary with ** - Snake - Mar-11-2020

Got it. Thanks guys.