我的python列表理解由于语法错误而失败

Question

Python noob using version 3.2.3...

class Card:
    def __init__(self, suitID, rank):
            self.suits = ("Spades", "Hearts", "Diamonds","Clubs")
            self.rank = rank
            self.suit = self.suits[suitID]

    def __str__(self):
            return (str("%s of %s" % (self.rank, self.suit)))


class Deck:
    def __init__(self):
            self.cards = [Card(i//13, i%13) for i in range(52)]

The above is how I have things set up, seems fine to me. The confusion is in the following (please ignore the return ("") statement as this is only for testing)

Case 1:

def __str__(self):
    print ( self.cards [i for i in range (52) ] )
    return("")

Output:

File "cardgame.py", line 21
print (self.cards[i for i in range(52)])
                      ^
SyntaxError: invalid syntax

Why the error? I thought that was how LC's were used...

---

Case 2:

def __str__(self):
    print (myCards for myCards in self.cards)
return ("")

Output:

<generator object <genexpr> at 0x2256fa0>

Again, what does that mean, and why does it not output the data?

---

Case 3:

def __str__(self):
    for myCard in self.cards:
          print (myCard)
    return ("")

Output:

0 of Spades
1 of Spades
2 of Spades
3 of Spades
4 of Spades
...
..
.

Now this outputs exactly what I want, but that confuses me ever more. I was under the impression that all three cases are equivalent and they are clearly not. What are the differences?

Thanks in advance.

Answer 1

Case 1:

You should pass an integer rather than i for i in range (52) as the index of a list. Try print [self.cards[i] for i in range(52)] . And you can just write print self.cards if there are only 52 cards in self.cards .

Case 2 & Case 3:

(myCards for myCards in self.cards) is a generator in Python. You can iterate it like a list in Case 3, but it will not return all its elements at first, which is an important feature of generators.

Answer 2

The is is List Comprehensions, you'd better to learn it's gramar.

li = [1,2,3]

la = [i for i in li] # the la is a list [1,2,3]

lb = (i for i in li) # lb is a generator object, and list(lb) is [1,2,3]

in case 1, print [self.cars[i] for i in range(52)] will work.

in your code (self.cars[i for i in range(52)]) , the [i for i in range(52)] is a list,so self.cars[list objeject] is invalid.

Answer 3

print (self.cards[i for i in range(52)])

You are trying to use a list to index into a list. This is like doing the following:

>>> l = [4,5,6]
>>> l[i for i in range (3)]
  File "<stdin>", line 1
    l[i for i in range (3)]

Answer 4

For your first case, that is not how you arrange Python syntax.

print(self.cards[i for i in range (52)])

You are indexing a list with another list . It doesn't make any sense. What you probably meant to do was this:

print([self.cards[i] for i in range(52)])

Note that this just creates a new list . The expression [self.cards[i] for i in range(52)] , operating on a list with 52 elements, has the same result as self.cards . You are adding unnecessary stuff and going in circles.

For your second case, this:

print(myCards for myCards in self.cards)

is printing a generator expression, which is represented as:

<generator object <genexpr> at 0x2256fa0>

For your third case, which prints each item in that object rather than printing the object itself, you can unpack your generator expression above with * and specify a custom separator:

print(*(myCards for myCards in self.cards), sep='\n')

Of course, note that you are once again copying that list for no reason. Just use the original one instead of spending time and effort (both yours and the computer's) making a new one.

print(*myCards, sep='\n')