Dive into python中說道Tuple是不可變的List，一旦創建了一個Tuple，就不能以任何方式改變它。但是Tuple 比 list 操作速度快。如果您定義了一個值的常量集，並且唯一要用它做的是不斷地遍歷它，請使用 tuple 代替 list。
我寫了幾行代碼測試了一下：
example_list=list()
for i in range(0,500,1):
example_list.append(i)
example_tuple=tuple(example_list)
start=time.clock()
for item in example_list:
do something
print " iterate list:",time.clock()-start
start=time.clock()
for item in example_tuple:
do something
print " iterate tuple:",time.clock()-start
運行結果如下：
iterate list: 9.37050717647e-05
iterate tuple: 6.8270838e-05
我運行了幾次，雖然具體的時間會改變，但是遍歷tuple的速度確實比list快。為什么呢？stackoverflow上有人的解釋：
http://stackoverflow.com/questions/3340539/why-is-tuple-faster-than-list
大概有以下幾個要點：
1、tuple中是不可變的，在CPython中tuple被存儲在一塊固定連續的內存中，創建tuple的時候只需要一次性分配內存。但是List被的被存儲在兩塊內存中，一塊內存固定大小，記錄着Python Object（某個list對象）的信息，另一塊是不固定大小的內存，用來存儲數據。所以，查找時tuple可以快速定位（C中的數組）；list必須遍歷（C中的鏈表）。在編譯中，由於Tuple是不可變的，python編譯器將它存儲在它所在的函數或者模塊的“常量表”（constants table）中。運行時，只要找到這些預構建的常量元組。但是List是可變的，必須在運行中構建，分配內存。
3、當Tuple的元素是List的時候，它只存儲list的引用，（C中定長數組里一個元素是指向某個鏈表的指針），定位查找時它還是會比List快
4、CPython中已經做了相關優化以減少內存分配次數：釋放一個List對象的時候，它的內存會被保存在一個自由List中以重復使用。不過非空list的創建時，仍然需要給它分配內存存儲數據。
Tuple 源碼
class tuple(object):
"""
tuple() -> empty tuple
tuple(iterable) -> tuple initialized from iterable's items
If the argument is a tuple, the return value is the same object.
"""
def count(self, value): # real signature unknown; restored from __doc__
""" T.count(value) -> integer -- return number of occurrences of value """
return 0
def index(self, value, start=None, stop=None): # real signature unknown; restored from __doc__
"""
T.index(value, [start, [stop]]) -> integer -- return first index of value.
Raises ValueError if the value is not present.
"""
return 0
def __add__(self, y): # real signature unknown; restored from __doc__
""" x.__add__(y) <==> x+y """
pass
def __contains__(self, y): # real signature unknown; restored from __doc__
""" x.__contains__(y) <==> y in x """
pass
def __eq__(self, y): # real signature unknown; restored from __doc__
""" x.__eq__(y) <==> x==y """
pass
def __getattribute__(self, name): # real signature unknown; restored from __doc__
""" x.__getattribute__('name') <==> x.name """
pass
def __getitem__(self, y): # real signature unknown; restored from __doc__
""" x.__getitem__(y) <==> x[y] """
pass
def __getnewargs__(self, *args, **kwargs): # real signature unknown
pass
def __getslice__(self, i, j): # real signature unknown; restored from __doc__
"""
x.__getslice__(i, j) <==> x[i:j]
Use of negative indices is not supported.
"""
pass
def __ge__(self, y): # real signature unknown; restored from __doc__
""" x.__ge__(y) <==> x>=y """
pass
def __gt__(self, y): # real signature unknown; restored from __doc__
""" x.__gt__(y) <==> x>y """
pass
def __hash__(self): # real signature unknown; restored from __doc__
""" x.__hash__() <==> hash(x) """
pass
def __init__(self, seq=()): # known special case of tuple.__init__
"""
tuple() -> empty tuple
tuple(iterable) -> tuple initialized from iterable's items
If the argument is a tuple, the return value is the same object.
# (copied from class doc)
"""
pass
def __iter__(self): # real signature unknown; restored from __doc__
""" x.__iter__() <==> iter(x) """
pass
def __len__(self): # real signature unknown; restored from __doc__
""" x.__len__() <==> len(x) """
pass
def __le__(self, y): # real signature unknown; restored from __doc__
""" x.__le__(y) <==> x<=y """
pass
def __lt__(self, y): # real signature unknown; restored from __doc__
""" x.__lt__(y) <==> x<y """
pass
def __mul__(self, n): # real signature unknown; restored from __doc__
""" x.__mul__(n) <==> x*n """
pass
@staticmethod # known case of __new__
def __new__(S, *more): # real signature unknown; restored from __doc__
""" T.__new__(S, ...) -> a new object with type S, a subtype of T """
pass
def __ne__(self, y): # real signature unknown; restored from __doc__
""" x.__ne__(y) <==> x!=y """
pass
def __repr__(self): # real signature unknown; restored from __doc__
""" x.__repr__() <==> repr(x) """
pass
def __rmul__(self, n): # real signature unknown; restored from __doc__
""" x.__rmul__(n) <==> n*x """
pass
def __sizeof__(self): # real signature unknown; restored from __doc__
""" T.__sizeof__() -- size of T in memory, in bytes """
pass
List源碼
class list(object):
"""
list() -> new empty list
list(iterable) -> new list initialized from iterable's items
"""
def append(self, p_object): # real signature unknown; restored from __doc__
""" L.append(object) -- append object to end """
pass
def count(self, value): # real signature unknown; restored from __doc__
""" L.count(value) -> integer -- return number of occurrences of value """
return 0
def extend(self, iterable): # real signature unknown; restored from __doc__
""" L.extend(iterable) -- extend list by appending elements from the iterable """
pass
def index(self, value, start=None, stop=None): # real signature unknown; restored from __doc__
"""
L.index(value, [start, [stop]]) -> integer -- return first index of value.
Raises ValueError if the value is not present.
"""
return 0
def insert(self, index, p_object): # real signature unknown; restored from __doc__
""" L.insert(index, object) -- insert object before index """
pass
def pop(self, index=None): # real signature unknown; restored from __doc__
"""
L.pop([index]) -> item -- remove and return item at index (default last).
Raises IndexError if list is empty or index is out of range.
"""
pass
def remove(self, value): # real signature unknown; restored from __doc__
"""
L.remove(value) -- remove first occurrence of value.
Raises ValueError if the value is not present.
"""
pass
def reverse(self): # real signature unknown; restored from __doc__
""" L.reverse() -- reverse *IN PLACE* """
pass
def sort(self, cmp=None, key=None, reverse=False): # real signature unknown; restored from __doc__
"""
L.sort(cmp=None, key=None, reverse=False) -- stable sort *IN PLACE*;
cmp(x, y) -> -1, 0, 1
"""
pass
def __add__(self, y): # real signature unknown; restored from __doc__
""" x.__add__(y) <==> x+y """
pass
def __contains__(self, y): # real signature unknown; restored from __doc__
""" x.__contains__(y) <==> y in x """
pass
def __delitem__(self, y): # real signature unknown; restored from __doc__
""" x.__delitem__(y) <==> del x[y] """
pass
def __delslice__(self, i, j): # real signature unknown; restored from __doc__
"""
x.__delslice__(i, j) <==> del x[i:j]
Use of negative indices is not supported.
"""
pass
def __eq__(self, y): # real signature unknown; restored from __doc__
""" x.__eq__(y) <==> x==y """
pass
def __getattribute__(self, name): # real signature unknown; restored from __doc__
""" x.__getattribute__('name') <==> x.name """
pass
def __getitem__(self, y): # real signature unknown; restored from __doc__
""" x.__getitem__(y) <==> x[y] """
pass
def __getslice__(self, i, j): # real signature unknown; restored from __doc__
"""
x.__getslice__(i, j) <==> x[i:j]
Use of negative indices is not supported.
"""
pass
def __ge__(self, y): # real signature unknown; restored from __doc__
""" x.__ge__(y) <==> x>=y """
pass
def __gt__(self, y): # real signature unknown; restored from __doc__
""" x.__gt__(y) <==> x>y """
pass
def __iadd__(self, y): # real signature unknown; restored from __doc__
""" x.__iadd__(y) <==> x+=y """
pass
def __imul__(self, y): # real signature unknown; restored from __doc__
""" x.__imul__(y) <==> x*=y """
pass
def __init__(self, seq=()): # known special case of list.__init__
"""
list() -> new empty list
list(iterable) -> new list initialized from iterable's items
# (copied from class doc)
"""
pass
def __iter__(self): # real signature unknown; restored from __doc__
""" x.__iter__() <==> iter(x) """
pass
def __len__(self): # real signature unknown; restored from __doc__
""" x.__len__() <==> len(x) """
pass
def __le__(self, y): # real signature unknown; restored from __doc__
""" x.__le__(y) <==> x<=y """
pass
def __lt__(self, y): # real signature unknown; restored from __doc__
""" x.__lt__(y) <==> x<y """
pass
def __mul__(self, n): # real signature unknown; restored from __doc__
""" x.__mul__(n) <==> x*n """
pass
@staticmethod # known case of __new__
def __new__(S, *more): # real signature unknown; restored from __doc__
""" T.__new__(S, ...) -> a new object with type S, a subtype of T """
pass
def __ne__(self, y): # real signature unknown; restored from __doc__
""" x.__ne__(y) <==> x!=y """
pass
def __repr__(self): # real signature unknown; restored from __doc__
""" x.__repr__() <==> repr(x) """
pass
def __reversed__(self): # real signature unknown; restored from __doc__
""" L.__reversed__() -- return a reverse iterator over the list """
pass
def __rmul__(self, n): # real signature unknown; restored from __doc__
""" x.__rmul__(n) <==> n*x """
pass
def __setitem__(self, i, y): # real signature unknown; restored from __doc__
""" x.__setitem__(i, y) <==> x[i]=y """
pass
def __setslice__(self, i, j, y): # real signature unknown; restored from __doc__
"""
x.__setslice__(i, j, y) <==> x[i:j]=y
Use of negative indices is not supported.
"""
pass
def __sizeof__(self): # real signature unknown; restored from __doc__
""" L.__sizeof__() -- size of L in memory, in bytes """
pass
__hash__ = None
List和Tuple相同的用法：
>>> t = ("a", "b", "mpilgrim", "z", "example")
>>> t
('a', 'b', 'mpilgrim', 'z', 'example')
>>> t[0]
'a'
>>> t[-1]
'example'
>>> t[1:3]
('b', 'mpilgrim')
list和tuple定義、索引、分片（slice）方式一樣
List和Tuple不同用法：
（1）
>>> t
('a', 'b', 'mpilgrim', 'z', 'example')
>>> t.append("new")
Traceback (innermost last):
File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'append'
>>> t.remove("z")
Traceback (innermost last):
File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'remove'
>>> t.index("example")
Traceback (innermost last):
File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'index'
>>> "z" in t
True
從源碼也可以看出，tuple中沒有append\extend\remove\pop\index等用法。但是可以使用 in 來查看一個元素是否存在於 tuple 中。
（2）
d=dict() d[example_tuple]="a" d[example_list]="b"
Traceback (most recent call last):
File "N:/WorkSpace/python-workspace/tuple_test.py", line 15, in <module>
d[example_list]="b"
TypeError: unhashable type: 'list'
tuple2=([2,3,4,5],[1,2,4,5])
d=dict()
d[tuple2]="a"
Traceback (most recent call last):
File "N:/WorkSpace/python-workspace/tuple_test.py", line 14, in <module>
d[tuple2]="a"
TypeError: unhashable type: 'list'
Tuples 可以在 dictionary 中被用做 key，但是 list 不行。實際上，事情要比這更復雜。Dictionary key 必須是不可變的。Tuple 本身是不可改變的，但是如果您有一個 list 的 tuple，那就認為是可變的了，用做 dictionary key 就是不安全的。只有字符串、整數或其它對 dictionary 安全的 tuple 才可以用作 dictionary key。
Tuple和List相互轉換：
example_list=list()
for i in range(0,10,1):
example_list.append(i)
example_tuple=tuple(example_list)
example_list2=list(example_tuple)
print "tuple",example_tuple
print "list",example_list2
運行結果：
tuple (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
list [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
內置的 tuple 函數接收一個 list，並返回一個有着相同元素的 tuple。而 list 函數接收一個 tuple 返回一個 list。