Indexing Keyword Arguments Using a String in Python - python-3.4

Question

I'm confused with the statement " print(kw,":",keywords[kw])" in the following program, in python.
def cheeseshop(kind,*arguments,**keywords):
print("--Do you have any",kind,"?")
print("--I'm sorry, we're all out of",kind)
for arg in arguments:
print(arg)
print("-"*40)
print(keywords)
keys=sorted(keywords)
print(keys)
for kw in keys:
print(kw,":",keywords[kw])
cheeseshop("Limburger", "It's very runny, sir.",
"It's really very, VERY runny, sir.",
shopkeeper="Michael Palin",
client="John Cleese",
sketch="Cheese Shop Sketch")
The result is below:
--Do you have any Limburger ?
--I'm sorry, we're all out of Limburger
It's very runny, sir.
It's really very, VERY runny, sir.
----------------------------------------
{'client': 'John Cleese', 'sketch': 'Cheese Shop Sketch', 'shopkeeper': 'Michael Palin'}
['client', 'shopkeeper', 'sketch']
client : John Cleese
shopkeeper : Michael Palin
sketch : Cheese Shop Sketch
In my idea, "kw" should be 'client', 'sketch' and 'shopkeeper', not numbers, then how can "kw" be the index of keywords in the statement " print(kw,":",keywords[kw])"?
To verify my idea, I also tried another program:
letters=['a','b']
for kw in letters:
print(letters[kw])
And a reasonable reply pops up:
Traceback (most recent call last):
File "<stdin>", line 2, in <module>
TypeError: list indices must be integers, not str
That furthermore confuses me for the problem I got in the first piece of program.I think it should pop up the same error to me.

Answer 1

Function arguments preceded with ** are called "keyword arguments", which take named parameters when calling the function, eg: client="John Cleese" in your example. In this case, "client" is the name and "John Cleese" is the value. Arguments passed this way are placed in a dict, which is a key-value store rather than a list, which you might be familiar with in the form
x = {
"foo": "bar"
}
print x["foo"] # prints "bar"

Answer 2

It use the keywords utilities, it's a kind of array with a key name.
You can see the lib description, in the python documentation HERE
In fact there is a special properties of **keywords arguments that provides the right to do this.
HERE is a tutorial to use it (and also to understand it) and HERE is the stackoverflow related question.