Python中，我们在预定义某类具有相似格式的变量或者输出一句含有多个变量的提示语句时，往往用到占位符，而占位符有两种表达方式：
%方式：
下面这段代码摘自matplotlib的_init_.py文件中class RcParams的定义：
1 msg_depr = "%s is deprecated and replaced with %s; please use the latter." 2 msg_depr_set = ("%s is deprecated. Please remove it from your " 3 "matplotlibrc and/or style files.") 4 msg_depr_ignore = "%s is deprecated and ignored. Use %s instead." 5 msg_obsolete = ("%s is obsolete. Please remove it from your matplotlibrc " 6 "and/or style files.") 7 msg_backend_obsolete = ("The {} rcParam was deprecated in version 2.2. In" 8 " order to force the use of a specific Qt binding," 9 " either import that binding first, or set the " 10 "QT_API environment variable.")
其中，%s表示此展位符可用相关字符串代替，这是一个比较简单的实例，%方式的占位符具体格式为：
%[(name)][flags][width].[precision]typecode
注：Python中百分号格式化是不存在自动将整数转换成二进制表示的方式
tpl = "i am %s" % "alex" print(tpl)
tpl = "i am %s age %d" % ("alex", 18) print(tpl)
tpl = "i am %(name)s age %(age)d" % {"name": "alex", "age": 18} print(tpl)
tpl = "percent %.2f" % 99.97623 print(tpl)
tpl = "i am %(pp).2f" % {"pp": 123.425556, } print(tpl)
tpl = "i am %(pp).2f %%" % {"pp": 123.425556, } print(tpl)
i am alex
i am alex age 18
i am alex age 18
percent 99.98
i am 123.43
i am 123.43 %
format方式：
以下是同时输出字典roll_dict的key-value对的format表达方式：
total_times = 1000
roll_dict={"1":56, "2":65, "3":458, "4":25}
for i, result in roll_dict.items():
print(‘点数是{}的次数是：{}，频率是：{}‘ .format(i, result, result/total_times))
点数是1的次数是：56，频率是：0.056
点数是2的次数是：65，频率是：0.065
点数是3的次数是：458，频率是：0.458
点数是4的次数是：25，频率是：0.025
如上述例子所示：forma占位需要结合{}使用，{}中可以带入相关变量的相关参数，上述例子中{}是空白，未指定类型，则默认是None，表示实际变量i，result，result/total_times依次取代前面三个{}。
format的{}格式具体如下：
[[fill]align][sign][#][0][width][,][.precision][type]
1 tpl = "i am {}, age {}, {}".format("seven", 18, ‘alex‘) 2 print(tpl) 3 4 tpl = "i am {}, age {}, {}".format(*["seven", 18, ‘alex‘]) 5 print(tpl) 6 7 tpl = "i am {0}, age {1}, really {0}".format("seven", 18) 8 print(tpl) 9 10 tpl = "i am {0}, age {1}, really {0}".format(*["seven", 18]) 11 print(tpl) 12 13 tpl = "i am {name}, age {age}, really {name}".format(name="seven", age=18) 14 print(tpl) 15 16 tpl = "i am {name}, age {age}, really {name}".format(**{"name": "seven", "age": 18}) 17 print(tpl) 18 19 tpl = "i am {0[0]}, age {0[1]}, really {0[2]}".format([1, 2, 3], [11, 22, 33]) 20 print(tpl) 21 22 tpl = "i am {:s}, age {:d}, money {:f}".format("seven", 18, 88888.1) 23 print(tpl) 24 25 tpl = "i am {:s}, age {:d}".format(*["seven", 18]) 26 print(tpl) 27 28 tpl = "i am {name:s}, age {age:d}".format(name="seven", age=18) 29 print(tpl) 30 31 tpl = "i am {name:s}, age {age:d}".format(**{"name": "seven", "age": 18}) 32 print(tpl) 33 34 tpl = "numbers: {:b},{:o},{:d},{:x},{:X}, {:%}".format(15, 15, 15, 15, 15, 15.87623, 2) 35 print(tpl) 36 37 tpl = "numbers: {:b},{:o},{:d},{:x},{:X}, {:%}".format(15, 15, 15, 15, 15, 15.87623, 2) 38 print(tpl) 39 40 tpl = "numbers: {0:b},{0:o},{0:d},{0:x},{0:X}, {0:%}".format(15) 41 print(tpl) 42 43 tpl = "numbers: {num:b},{num:o},{num:d},{num:x},{num:X}, {num:%}".format(num=15) 44 print(tpl)
i am seven, age 18, alex
i am seven, age 18, alex
i am seven, age 18, really seven
i am seven, age 18, really seven
i am seven, age 18, really seven
i am seven, age 18, really seven
i am 1, age 2, really 3
i am seven, age 18, money 88888.100000
i am seven, age 18
i am seven, age 18
i am seven, age 18
numbers: 1111,17,15,f,F, 1587.623000%
numbers: 1111,17,15,f,F, 1587.623000%
numbers: 1111,17,15,f,F, 1500.000000%
numbers: 1111,17,15,f,F, 1500.000000%
从上述例子中 可以看出{}在语句中出现的位置依次与format中变量出现的位置从左到右依次对应，但如果{}中有整型数据，则整型数据值表示format中元素的索引位置：
1 tpl = "i am {0[0]}, age {0[1]}, really {0[2]}".format([1, 2, 3], [11, 22, 33]) 2 print(tpl)
输出为：
i am 1, age 2, really 3
如果改成：
1 tpl = "i am {0}, age {1[1]}, really {1[2]}".format([1, 2, 3], [11, 22, 33]) 2 print(tpl)
则输出为：
i am [1, 2, 3], age 22, really 33
此种方式更显灵活。
原文：https://www.cnblogs.com/SpringFull/p/10169858.html