Python keyword output interpretation

问题:

I'm going through the Python 2.7 tutorial, and I was looking at the output of the following statement:

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
    keys = sorted(keywords.keys())
    for kw in keys:
        print kw, ":", keywords[kw]

So, if I call the program as such:

cheeseshop("Cheddar", "No.", "Seriously?",
       Shopkeeper="Michael Palin",
       Client="John Cleese")

It outputs:

Do you have any Cheddar?
I'm sorry, we're all out of Cheddar
No.
Seriously?
--------------------------------------
Client: John Cleese
Shopkeeper: Michael Palin

This is correct.

If I change that print statement to print keywords, I get the following representation:

{'Shopkeeper': 'Ryan Lambert', 'Client': 'John Cleese'}

I'm a bit confused on how printing keywords[kw] just comes back with a name, and keywords does not.

回答1:

In Python, you can pass optional keyword parameters by putting a ** in front of the function parameter's list.

So the keywords variable is actually a dictionary type. Thus, if you do:

print keywords

you get back (reformatted to make the mapping more obvious)

{
    'Shopkeeper': 'Ryan Lambert', 
    'Client': 'John Cleese'
}

which is a dictionary. And if you do:

print keywords[kw]

you get back the value of the dictionary associated with the key kw. So if kw was 'Shopkeeper', then keywords[kw] becomes 'Ryan Lambert', and if kw was 'Client', then keywords[kw] becomes 'John Cleese'

回答2:

keywords is stored as a dictionary. ( See this for more)

If you print the dictionary itself it is going to output the complete set of pairs it contains (key,value).

On your program:

• keys are: 'Shopkeeper' and 'Client'
• values are respectively: 'Ryan Lambert' and 'John Cleese'

One way to access the values is to "search" for it with its key: dict[key]

So when you wrote: "keywords[kw]" you are actually passing a key and python is going to give you the corresponding value.

You can think it as similar as accessing an array value:

a = ['a', 'b', 'c']

if you:

print a #output: ['a', 'b', 'c']    
print a[0] # outputs: 'a'

just unlike arrays the data is not stored "neatly" together in memory, but using hashing

Hope it helped, Cheers

回答3:

When you call the function with

cheeseshop("Cheddar", "No.", "Seriously?",
           Shopkeeper="Michael Palin", Client="John Cleese")

the keywords parameter takes on the value {'Shopkeeper': 'Ryan Lambert', 'Client': 'John Cleese'}, i.e., it's a dictionary.

This is equivalent to (and much easier to read than) calling the function as

cheeseshop("Cheddar", *["No.", "Seriously?"],
           **{"Shopkeeper":"Michael Palin", "Client":"John Cleese"})

That is, the values in the first function call are automatically wrapped inside the *arguments and **keywords parameters -- that's what those * and ** are for.

Now, when you do this:

keys = sorted(keywords.keys())
for kw in keys:
    print kw, ":", keywords[kw]

keyword.keys() is ['Shopkeeper', 'Client'], i.e. the "keys" in the dictionary. Next, you sort those keys and for each key, you print the respective entry in the dictionary, e.g., "John Cleese" for "Client".