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重新撸 Python 官方文档02--Tuples, Sets

重新撸 Python 官方文档02--Tuples, Sets

作者: 大飞哥 | 来源:发表于2017-12-02 17:33 被阅读12次

    1 Tuples and Sequences

    Sequence Types — list, tuple, range

    >>> t = 12345, 54321, 'hello!'
    >>> t[0]
    12345
    >>> t
    (12345, 54321, 'hello!')
    >>> # Tuples may be nested:
    ... u = t, (1, 2, 3, 4, 5)
    >>> u
    ((12345, 54321, 'hello!'), (1, 2, 3, 4, 5))
    >>> # Tuples are immutable:
    ... t[0] = 88888
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    TypeError: 'tuple' object does not support item assignment
    >>> # but they can contain mutable objects:
    ... v = ([1, 2, 3], [3, 2, 1])
    >>> v
    ([1, 2, 3], [3, 2, 1])
    

    tuple用逗号区分开元素
    输出时总是带括号,输入时可以不带括号,不过带括号比较好,不然用以引起误解.
    tuple 和 list 很像,不过,tuple是不可变的
    tuple 有零个元素还是一个元素

    >>> empty = ()
    >>> singleton = 'hello',    # <-- note trailing comma
    >>> len(empty)
    0
    >>> len(singleton)
    1
    >>> singleton
    ('hello',)
    

    构建tupple 与拆解tuple

    >>>t = x, y, z
    >>>x, y, z = t
    

    2 Sets

    set 是 没有重复元素的无序集合. 用花括号或者 set()来创建,但是创建空set要用set(),而不是{},{}用来创建空dictionary

    简略演示

    >>> 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 a or b or both
    {'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'}
    

    差不多就是与或并之类的逻辑

    set 也支持推导式

    >>> a = {x for x in 'abracadabra' if x not in 'abc'}
    >>> a
    {'r', 'd'}
    

    3 Dictionaries

    dictionary就是 key: value 对
    一些例子

    >>> tel = {'jack': 4098, 'sape': 4139}
    >>> tel['guido'] = 4127
    >>> tel
    {'sape': 4139, 'guido': 4127, 'jack': 4098}
    >>> tel['jack']
    4098
    >>> del tel['sape']
    >>> tel['irv'] = 4127
    >>> tel
    {'guido': 4127, 'irv': 4127, 'jack': 4098}
    >>> list(tel.keys())
    ['irv', 'guido', 'jack']
    >>> sorted(tel.keys())
    ['guido', 'irv', 'jack']
    >>> 'guido' in tel
    True
    >>> 'jack' not in tel
    False
    
    >>> dict([('sape', 4139), ('guido', 4127), ('jack', 4098)])
    {'sape': 4139, 'jack': 4098, 'guido': 4127}
    

    另外 字典也有 推导式

    >>> {x: x**2 for x in (2, 4, 6)}
    {2: 4, 4: 16, 6: 36}
    
    >>> dict(sape=4139, guido=4127, jack=4098)
    {'sape': 4139, 'jack': 4098, 'guido': 4127}
    

    4 循环技巧

    循环字典时,items()可以同时获取 key 和 value

    >>> knights = {'gallahad': 'the pure', 'robin': 'the brave'}
    >>> for k, v in knights.items():
    ...     print(k, v)
    ...
    gallahad the pure
    robin the brave
    

    循环序列时, enumerate()可以同时获得索引以及对应的值

    >>> for i, v in enumerate(['tic', 'tac', 'toe']):
    ...     print(i, v)
    ...
    0 tic
    1 tac
    2 toe
    

    循环两个或更多的序列时,可以用zip()来讲对应元素配对.

    >>> 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.
    

    逆循环时,可以用 reversed()

    >>> for i in reversed(range(1, 10, 2)):
    ...     print(i)
    ...
    9
    7
    5
    3
    1
    

    用排序后的顺序进行循环,可以用 sorted()返回一个排完序的列表同时不改变原来的序列

    >>> basket = ['apple', 'orange', 'apple', 'pear', 'orange', 'banana']
    >>> for f in sorted(set(basket)):
    ...     print(f)
    ...
    apple
    banana
    orange
    pear
    

    循环时改变一个列表总是很爽,但是更简单也更安全的是创建一个新的列表来替换.

    >>> import math
    >>> raw_data = [56.2, float('NaN'), 51.7, 55.3, 52.5, float('NaN'), 47.8]
    >>> filtered_data = []
    >>> for value in raw_data:
    ...     if not math.isnan(value):
    ...         filtered_data.append(value)
    ...
    >>> filtered_data
    [56.2, 51.7, 55.3, 52.5, 47.8]
    

    5 更多的条件语句

    while if 可以有任意运算符 而不仅仅是比较

    in not in 检查一个值是否在一个序列里面.
    is is not 检查两个对象是不是真的相同的对象.
    以上几个运算符有相同优先级,但是比数字操作符要低一些.

    逻辑运算符 andor有时会有 not 否定形式. not有最高优先级 而 or的优先级是最低的
    比如
    A and not B or C 等价于 (A and (not B)) or C

    andor又叫短路运算符,从左向右计算,如果值一旦确定,则计算停止.比如
    A``C是真的,B是假,那 A and B and C就不再去计算C的表达式

    把比较式 或者布尔表达式的结果赋值也是可以的,比如:

    >>> string1, string2, string3 = '', 'Trondheim', 'Hammer Dance'
    >>> non_null = string1 or string2 or string3
    >>> non_null
    'Trondheim'
    

    即,计算那个表达,就把该表达式赋给等号左边的变量.

    6 比较序列和其他类型

    序列对象和相同序列类型的其他对象可以比较.这种比较是逐字的.
    一个一个对比,而且是递归的
    一个序列若是另一个的从头开始的子集,则子集要更小.
    字符串的话,则是按照Unicod编码的顺序.例子如下

    (1, 2, 3)              < (1, 2, 4)
    [1, 2, 3]              < [1, 2, 4]
    'ABC' < 'C' < 'Pascal' < 'Python'
    (1, 2, 3, 4)           < (1, 2, 4)
    (1, 2)                 < (1, 2, -1)
    (1, 2, 3)             == (1.0, 2.0, 3.0)
    (1, 2, ('aa', 'ab'))   < (1, 2, ('abc', 'a'), 4)
    

    解释器不会给一个任意排序,而是给一个TypeError的异常.

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