Python：集合與字符串格式化

1. 集合

　　定義：集合是由不同元素組合而成，集合中是一組無序排列的可hash值，可做為字典的key。

　　特征：集合的目的是將不同的值放在一起，不同的集合可以用來做關系運算。集合中的元素必須是不可變類型。可以幫助簡單的去重。

1.1集合的創建

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

或者

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

集合中的元素是不會重復的

frozenset()函數將集合改變為不可變集合

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

1.2集合的函數

 

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，傳一個參數"""
        """
        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
        """清除集合內的所有元素"""
        """ Remove all elements from this set. """
        pass

    def copy(self, *args, **kwargs): # real signature unknown
        """淺拷貝集合內的元素"""
        """ Return a shallow copy of a set. """
        pass

    def difference(self, *args, **kwargs): # real signature unknown
        """求差集 相當於集合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) 相當於 a = a - b"""
        """ Remove all elements of another set from this set. """
        pass

    def discard(self, *args, **kwargs): # real signature unknown
        """刪除一個元素，元素不存在時不會報錯"""
        """
        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
        """求兩個集合的交集；相當於 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
        """求兩個集合的交集 更新；相當於 a = a & b"""
        """ Update a set with the intersection of itself and another. """
        pass

    def isdisjoint(self, *args, **kwargs): # real signature unknown
        """如果兩個集合的交集為空，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 相當於 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 相當於 a > b"""
        """ Report whether this set contains another set. """
        pass

    def pop(self, *args, **kwargs): # real signature unknown
        """隨機刪除一個元素"""
        """
        Remove and return an arbitrary set element.
        Raises KeyError if the set is empty.
        """
        pass

    def remove(self, *args, **kwargs): # real signature unknown
        """指定一個元素刪除，只能有一個元素;刪除元素不存在會報錯"""
        """
        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; 兩個集合;相當於先並集再去除他們相同的元素； 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; 兩個集合;相當於先並集再去除他們相同的元素；a = a ^ b"""
        """ Update a set with the symmetric difference of itself and another. """
        pass

    def union(self, *args, **kwargs): # real signature unknown
        """求並集 相當於 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，可以更新多個值，但是'int' object is not iterable;相當於 a = a | b"""
        """ Update a set with the union of itself and others. """
        pass

 

1.3集合的關系運算

in , not in

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

|, |= : 並集

&, &= : 交集

-, -= : 差集

^, ^= : 交叉差集

2.字符串格式化

 

Python中字符串格式有兩種：百分號方式，format方式。

2.2百分號字符串的拼接

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

• name　　　　可選，用於指定的key
• flags　　　　 可選，
• 　　　　+（右對齊前面加符號）
• 　　　　 -（左對齊，正數無符號，負數有）
• 　　　　 空格（右對齊，正數前加空格，負數前負號）
• 　　　　 0（左對齊，正數無符號，負數有；用0填充空白處）
• width　　　　可選，占有的寬度
• precision         小數點后保留的位數
• typecode         必選，格式類型

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} #在字符串中連續兩個%是表示百分號

 

2.3Format方式

 

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

• fill                        可選，空白處填充的字符
• aligh                    可選，對齊方式，需要與width配合使用
  • <               內容左對齊
  • >               內容右對齊（默認）
  • =               內容左對齊，將符號放在填充字符的左側
  • ^                內容居中
• sign                     可選 ，有無符號數字
• +               正數加+，負數加-
• -                正數不變，負數加-
• 空格          正數加空格，負數加-
• #                          可選，對於二，八，十六進制，如果加上#，會顯示0b/0o/0x/否則不顯示
• ,                           可選，數字添加分隔符  如 100，000
• width                   可選，位寬
• .precision            可選，保留小數點后位數
• type                     可選，格式化類型

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})