>>> from collections import deque
>>> queue = deque(["Eric", "John", "Michael"])
>>> queue.append("Terry") # Terry arrives
>>> queue.append("Graham") # Graham arrives
>>> queue.popleft() # The first to arrive now leaves
'Eric'
>>> queue.popleft() # The second to arrive now leaves
'John'
>>> queue # Remaining queue in order of arrival
deque(['Michael', 'Terry', 'Graham'])
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5.1.3. 列表推导式
squares = [x**2 for x in range(10)]
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上面这个方法更加简明且易读.
列表推导式由包含一个表达式的括号组成，表达式后面跟随一个 for 子句，之后可以有零或多个 for 或 if 子句。结果是一个列表，由表达式依据其后面的 for 和 if 子句上下文计算而来的结果构成。
例如，如下的列表推导式结合两个列表的元素，如果元素之间不相等的话:
>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
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等同于:
>>> combs = []
>>> for x in [1,2,3]:
... for y in [3,1,4]:
... if x != y:
... combs.append((x, y))
...
>>> combs
[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
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值得注意的是在上面两个方法中的 for 和 if 语句的顺序。
如果想要得到一个元组（例如，上面例子中的 (x, y)），必须要加上括号:
>>> vec = [-4, -2, 0, 2, 4]
>>> # create a new list with the values doubled
>>> [x*2 for x in vec]
[-8, -4, 0, 4, 8]
>>> # filter the list to exclude negative numbers
>>> [x for x in vec if x >= 0]
[0, 2, 4]
>>> # apply a function to all the elements
>>> [abs(x) for x in vec]
[4, 2, 0, 2, 4]
>>> # call a method on each element
>>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
>>> [weapon.strip() for weapon in freshfruit]
['banana', 'loganberry', 'passion fruit']
>>> # create a list of 2-tuples like (number, square)
>>> [(x, x**2) for x in range(6)]
[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
>>> # the tuple must be parenthesized, otherwise an error is raised
>>> [x, x**2 for x in range(6)]
File "<stdin>", line 1, in ?
[x, x**2 for x in range(6)]
^
SyntaxError: invalid syntax
>>> # flatten a list using a listcomp with two 'for'
>>> vec = [[1,2,3], [4,5,6], [7,8,9]]
>>> [num for elem in vec for num in elem]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
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列表推导式可使用复杂的表达式和嵌套函数:
>>> from math import pi
>>> [str(round(pi, i)) for i in range(1, 6)]
['3.1', '3.14', '3.142', '3.1416', '3.14159']
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5.1.4. 嵌套的列表推导式
>>> basket = {'apple', 'orange', 'apple', 'pear', 'orange', 'banana'}
>>> print(basket) # show that duplicates have been removed
{'orange', 'banana', 'pear', 'apple'}
>>> 'orange' in basket # fast membership testing
True
>>> 'crabgrass' in basket
False
>>> # Demonstrate set operations on unique letters from two words
...
>>> a = set('abracadabra')
>>> b = set('alacazam')
>>> a # unique letters in a
{'a', 'r', 'b', 'c', 'd'}
>>> a - b # letters in a but not in b
{'r', 'd', 'b'}
>>> a | b # letters in either a or b
{'a', 'c', 'r', 'd', 'b', 'm', 'z', 'l'}
>>> a & b # letters in both a and b
{'a', 'c'}
>>> a ^ b # letters in a or b but not both
{'r', 'd', 'b', 'm', 'z', 'l'}
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类似 列表推导式，这里有一种集合推导式语法:
>>> a = {x for x in 'abracadabra' if x not in 'abc'}
>>> a
{'r', 'd'}
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5.5. 字典
>>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
>>> for k, v in knights.items():
... print(k, v)
...
gallahad the pure
robin the brave
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在序列中循环时，索引位置和对应值可以使用 enumerate() 函数同时得到:
>>> for i, v in enumerate(['tic', 'tac', 'toe']):
... print(i, v)
...
0 tic
1 tac
2 toe
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同时循环两个或更多的序列，可以使用 zip() 整体打包:
>>> questions = ['name', 'quest', 'favorite color']
>>> answers = ['lancelot', 'the holy grail', 'blue']
>>> for q, a in zip(questions, answers):
... print('What is your {0}? It is {1}.'.format(q, a))
...
What is your name? It is lancelot.
What is your quest? It is the holy grail.
What is your favorite color? It is blue.
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需要逆向循环序列的话，先正向定位序列，然后调用 reversed() 函数:
>>> for i in reversed(range(1, 10, 2)):
... print(i)
...
9
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basket = [‘apple’, ‘orange’, ‘apple’, ‘pear’, ‘orange’, ‘banana’]
for f in sorted(set(basket)):
… print(f)
…
apple
banana
orange
pear
若要在循环内部修改正在遍历的序列（例如复制某些元素），建议您首先制作副本。在序列上循环不会隐式地创建副本。切片表示法使这尤其方便:
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words = [‘cat’, ‘window’, ‘defenestrate’]
for w in words[:]: # Loop over a slice copy of the entire list.
… if len(w) > 6:
… words.insert(0, w)
…
words
[‘defenestrate’, ‘cat’, ‘window’, ‘defenestrate’]
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#5.7. 深入条件控制
while 和 if 语句中使用的条件不仅可以使用比较，而且可以包含任意的操作。
比较操作符 in 和 not in 审核值是否在一个区间之内。操作符 is 和 is not 比较两个对象是否相同；这只和诸如列表这样的可变对象有关。所有的比较操作符具有相同的优先级，低于所有的数值操作。
比较操作可以传递。例如 a < b == c 审核是否 a 小于 b 并且 b 等于 c。
比较操作可以通过逻辑操作符 and 和 or 组合，比较的结果可以用 not 来取反义。这些操作符的优先级又低于比较操作符，在它们之中，not 具有最高的优先级， or 优先级最低，所以 A and not B or C 等于 (A and (notB)) or C。当然，括号也可以用于比较表达式。
逻辑操作符 and 和 or 也称作短路操作符：它们的参数从左向右解析，一旦结果可以确定就停止。例如，如果 A 和 C 为真而 B 为假， A and B and C 不会解析 C。作用于一个普通的非逻辑值时，短路操作符的返回值通常是最后一个变量。
