比较Swift和Python字典对象

Question

I'm trying to get familiar with Swift, so I'm doing some basic computations that I would normally do in Python.

I want to get a value from a dictionary using a key. In Python I would simply :

sequences = ["ATG","AAA","TAG"]
D_codon_aa = {"ATG": "M", "AAA": "R", "TAG": "*"}
for seq in sequences:
    print D_codon_aa[seq]
>>>
M
R
*

When I try this in Swift.

let sequences = ["ATG","AAA","TAG"]
let D_codon_aa = ["ATG": "M", "AAA": "R", "TAG": "*"]
for seq in sequences
    {
    var codon = D_codon_aa[seq]
    println(codon)
    }
>>>
Optional("M")
Optional("R")
Optional("*")

1) What is Optional() and why is it around the dictionary value?

2) Why can't I make a dictionary with multiple types of objects inside?

In Python I can do this:

sequence= {'A':0,'C':1, 'G':'2', 'T':3.0}

In Swift I can't do this:

let sequences = ["A":0,"C":1, "G":"2", "T":3.0]

Answer 1

1: Look at the declaration of the dictionarys subscript:

subscript(key: Key) -> Value?

It returns an optional, since you can use any key you want in subscripts, but they might not associated with values, so in that case it returns nil, otherwise the value wrapped in an optional.

2: Actually, you can, if you define your dictionary as for eg. ["String": AnyObject], and now you can associate keys with values, thats conforms to the AnyObject protocol.

Updated And your example let sequences = ["A":0,"C":1, "G":"2", "T":3.0] compiles fine in Xcode 6.1.1.