Python：集合与字符串格式化

1. 集合

　　定义：集合是由不同元素组合而成，集合中是一组无序排列的可hash值，可做为字典的key。

　　特征：集合的目的是将不同的值放在一起，不同的集合可以用来做关系运算。集合中的元素必须是不可变类型。可以帮助简单的去重。

1.1集合的创建

>>> {1,2,3,1}
{1, 2, 3}

或者

>>> set_test = set("hello")
>>> set_test
{'o', 'h', 'l', 'e'}

集合中的元素是不会重复的

frozenset()函数将集合改变为不可变集合

>>> f_set_test=frozenset(set_test)
>>> f_set_test
frozenset({'o', 'h', 'l', 'e'})

1.2集合的函数

class set(object):
    """
    set() -> new empty set object
    set(iterable) -> new set object

    Build an unordered collection of unique elements.
    """
    def add(self, *args, **kwargs): # real signature unknown
        """增加元素，传一个参数"""
        """
        Add an element to a set.

        This has no effect if the element is already present.
        """
        pass

    def clear(self, *args, **kwargs): # real signature unknown
        """清除集合内的所有元素"""
        """ Remove all elements from this set. """
        pass

    def copy(self, *args, **kwargs): # real signature unknown
        """浅拷贝集合内的元素"""
        """ Return a shallow copy of a set. """
        pass

    def difference(self, *args, **kwargs): # real signature unknown
        """求差集 相当于集合return a-b"""
        """
        Return the difference of two or more sets as a new set.

        (i.e. all elements that are in this set but not the others.)
        """
        pass

    def difference_update(self, *args, **kwargs): # real signature unknown
        """a.difference_update(b) 相当于 a = a - b"""
        """ Remove all elements of another set from this set. """
        pass

    def discard(self, *args, **kwargs): # real signature unknown
        """删除一个元素，元素不存在时不会报错"""
        """
        Remove an element from a set if it is a member.

        If the element is not a member, do nothing.
        """
        pass

    def intersection(self, *args, **kwargs): # real signature unknown
        """求两个集合的交集；相当于 return a & b"""
        """
        Return the intersection of two sets as a new set.

        (i.e. all elements that are in both sets.)
        """
        pass

    def intersection_update(self, *args, **kwargs): # real signature unknown
        """求两个集合的交集 更新；相当于 a = a & b"""
        """ Update a set with the intersection of itself and another. """
        pass

    def isdisjoint(self, *args, **kwargs): # real signature unknown
        """如果两个集合的交集为空，返回true"""
        """ Return True if two sets have a null intersection. """
        pass

    def issubset(self, *args, **kwargs): # real signature unknown
        """a.issubset(b) a是否是b的子集 相当于 a <= b"""
        """ Report whether another set contains this set. """
        pass

    def issuperset(self, *args, **kwargs): # real signature unknown
        """a.issubset(b) a是否是b的父集 相当于 a > b"""
        """ Report whether this set contains another set. """
        pass

    def pop(self, *args, **kwargs): # real signature unknown
        """随机删除一个元素"""
        """
        Remove and return an arbitrary set element.
        Raises KeyError if the set is empty.
        """
        pass

    def remove(self, *args, **kwargs): # real signature unknown
        """指定一个元素删除，只能有一个元素;删除元素不存在会报错"""
        """
        Remove an element from a set; it must be a member.

        If the element is not a member, raise a KeyError.
        """
        pass

    def symmetric_difference(self, *args, **kwargs): # real signature unknown
        """交叉差集; 两个集合;相当于先并集再去除他们相同的元素； a ^ b"""
        """
        Return the symmetric difference of two sets as a new set.

        (i.e. all elements that are in exactly one of the sets.)
        """
        pass

    def symmetric_difference_update(self, *args, **kwargs): # real signature unknown
        """交叉差集更新; 两个集合;相当于先并集再去除他们相同的元素；a = a ^ b"""
        """ Update a set with the symmetric difference of itself and another. """
        pass

    def union(self, *args, **kwargs): # real signature unknown
        """求并集 相当于 return a | b"""
        """
        Return the union of sets as a new set.

        (i.e. all elements that are in either set.)
        """
        pass

    def update(self, *args, **kwargs): # real signature unknown
        """更新 元素，可以更新多个值，但是'int' object is not iterable;相当于 a = a | b"""
        """ Update a set with the union of itself and others. """
        pass

1.3集合的关系运算

in , not in

==, != , <=, <, >=, >

|, |= : 并集

&, &= : 交集

-, -= : 差集

^, ^= : 交叉差集

2.字符串格式化

Python中字符串格式有两种：百分号方式，format方式。

2.2百分号字符串的拼接

%[(name)][flags][width].[precision]typecode

• name　　　　可选，用于指定的key
• flags　　　　 可选，
• 　　　　+（右对齐前面加符号）
• 　　　　 -（左对齐，正数无符号，负数有）
• 　　　　 空格（右对齐，正数前加空格，负数前负号）
• 　　　　 0（左对齐，正数无符号，负数有；用0填充空白处）
• width　　　　可选，占有的宽度
• precision         小数点后保留的位数
• typecode         必选，格式类型

常用格式

tp = "i am a %s"%"boy"

tp = "i am a %s,age %d"%("boy",18)

tp = "i am a %(sex)s,age %(age)d"%{"sex":"boy","age":18}

tp = "percent %.2f"%99.888

tp = "percent %(p).2f"%{"p":123.456}

tp = "percent %(p).2f%%"%{"p":123.456} #在字符串中连续两个%是表示百分号

2.3Format方式

[[fill]align][sign][#][0][width][,][.precision][type]

• fill                        可选，空白处填充的字符
• aligh                    可选，对齐方式，需要与width配合使用
  • <               内容左对齐
  • >               内容右对齐（默认）
  • =               内容左对齐，将符号放在填充字符的左侧
  • ^                内容居中
• sign                     可选 ，有无符号数字
• +               正数加+，负数加-
• -                正数不变，负数加-
• 空格          正数加空格，负数加-
• #                          可选，对于二，八，十六进制，如果加上#，会显示0b/0o/0x/否则不显示
• ,                           可选，数字添加分隔符  如 100，000
• width                   可选，位宽
• .precision            可选，保留小数点后位数
• type                     可选，格式化类型

常用格式：

tp = "i am a {}, age {}, {}".format("boy",18,"abc")

tp = "i am a {0}, age {0}, {2}".format("boy",18,"abc")

tp = "i am a {1}, age {0}, {2}".format("boy",18,"abc")

tp = "i am a {sex}, age {age}, {abc}".format(sex="boy",age=18,abc="abc")

tp = "i am a {sex}, age {age}, {abc}".format(**{"sex":"boy","age":18,"abc":"abc"})

tp = "i am a {0}, age {0[0]}, {0[0]}".format([1,2,3],[4,5,6])

tp = "i am a {:s}, age {:d}, {:f}".format("boy",18,18.123)

tp = "i am a {:s}, age {:d}, {:f}".format(*["boy",18,18.123])

tp = "i am a {sex:s}, age {age:d}, {abc:f}".format(**{"sex":"boy","age":18,"abc":18.123})