0x01 Format string
替换规则
replacement_field ::= "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name ::= arg_name ("." attribute_name | "[" element_index "]")*
arg_name ::= [identifier | integer]
attribute_name ::= identifier
element_index ::= integer | index_string
index_string ::= <any source character except "]"> +
conversion ::= "r" | "s"
format_spec ::= Format Specification
replacement_field Example
"First, thou shalt count to {0}" # References first positional argument
"Bring me a {}" # Implicitly references the first positional - argument
"From {} to {}" # Same as "From {0} to {1}"
"My quest is {name}" # References keyword argument 'name'
"Weight in tons {0.weight}" # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}" # First element of keyword argument 'players'.
个人理解：可以按照顺序进行格式化，其中需要进行替代的参数只要能够获取到就行。
例如：
>>> c = 3-5j
>>> ('The complex number {0} is formed from the real part {0.real} '
... 'and the imaginary part {0.imag}.').format(c)
'The complex number (3-5j) is formed from the real part 3.0 and the imaginary part -5.0.'
>>> class Point(object):
... def __init__(self, x, y):
... self.x, self.y = x, y
... def __str__(self):
... return 'Point({self.x}, {self.y})'.format(self=self)
...
>>> str(Point(4, 2))
'Point(4, 2)'
conversion
在某些情况下，需要把格式化的参数先替换成字符串，然后再进行格式化，这样，conversion参数就有用武之地了。
Two conversion flags are currently supported: '!s' which calls str() on the value, and '!r' which calls repr().
现仅支持两类conversion标识："!s"和"!r", "!s"会调用替换值得str()，而'!r'则会调用repr()
"Harold's a clever {0!s}" # Calls str() on the argument first
"Bring out the holy {name!r}" # Calls repr() on the argument first
0x02 Format Specification Mini-Language
format_spec ::= [[fill]align][sign][#][0][width][,][.precision][type]
fill ::= <any character>
align ::= "<" | ">" | "=" | "^"
sign ::= "+" | "-" | " "
width ::= integer
precision ::= integer
type ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
fill option
在有对齐的情况下，对空格进行替换的字符，可以是任何字符。
align options
| Option | Meaning |
|---|---|
| '<' | 左对齐 |
| '>' | 右对齐 |
| '^' | 居中对齐 |
| '=' | 仅对数字有效 |
>>> '{:<30}'.format('left aligned')
'left aligned '
>>> '{:>30}'.format('right aligned')
' right aligned'
>>> '{:^30}'.format('centered')
' centered '
>>> '{:*^30}'.format('centered') # use '*' as a fill char
'***********centered***********'
>>> b="{:0=20}"
>>> b.format(11)
'00000000000000000011'
>>> b.format(-1)
'-0000000000000000001'
>>> b.format(+1)
'00000000000000000001'
>>> a="{:<20}"
>>> a.format(1)
'1 '
>>> b="{:<20}"
>>> b="{:=20}"
>>> b.format(1)
' 1'
sign options
sign option只对数字类型的数字有效，有以下几种:
- '+': indicates that a sign should be used for both positive as well as negative numbers.无论正负数，都会加上正负号。
- '-': indicates that a sign should be used only for negative numbers (this is the default behavior).只有在数字为负数时会添加上负号
- space: indicates that a leading space should be used on positive numbers, and a minus sign on negative numbers.在正数前面有个空格，在负数前面是个负号
>>> '{:+f}; {:+f}'.format(3.14, -3.14) # show it always
'+3.140000; -3.140000'
>>> '{: f}; {: f}'.format(3.14, -3.14) # show a space for positive numbers
' 3.140000; -3.140000'
>>> '{:-f}; {:-f}'.format(3.14, -3.14) # show only the minus -- same as '{:f}; {:f}'
'3.140000; -3.140000'
'#' option
仅对整型数据有效，并且只能输出二进制、八进制或十六进制的转换数据，获得的输出数据前缀依次是'0b', '0o', 或'0x'。
>>> # format also supports binary numbers
>>> "int: {0:d}; hex: {0:x}; oct: {0:o}; bin: {0:b}".format(42)
'int: 42; hex: 2a; oct: 52; bin: 101010'
>>> # with 0x, 0o, or 0b as prefix:
>>> "int: {0:d}; hex: {0:#x}; oct: {0:#o}; bin: {0:#b}".format(42)
'int: 42; hex: 0x2a; oct: 0o52; bin: 0b101010'
',' option
此可选参数表示格式化需要千位分隔符','。在本地化的分隔符中，用'n'整型表示类型来代替。
>>> '{:,}'.format(1234567890)
'1,234,567,890'
width option
width is a decimal integer defining the minimum field width. If not specified, then the field width will be determined by the content.
width选项，是一个十进制整型数据，用来定义字段的最短宽度。如果没有指定，那么字段的宽度将由字段本身的内容决定。
precision option
precision是一个十进制数据
- 当一个浮点型数据需要被格式化'f'、'F'时，小数点后面需要保留的位数。
- 当一个浮点型数据需要被格式化'g'、'G'时，小数点前后总共的位数。
- 当一个非数字的字段需要被格式化时，表示最大保留的字段长度，即字段中多少个字符会被使用
不能对整型数据使用
type option:
type参数需要分情况使用
string类型数据
| Type | Meaning |
|---|---|
| 's' | 字符串格式化。字符串的默认类型。可能被取代。 |
| None | 同's'. |
整型数据格式化
| Type | Meaning |
|---|---|
| 'b' | 转化为2进制。 |
| 'c' | 转化为unicode字符. |
| 'd' | 转化为10进制。 |
| 'o' | 转化为8进制 |
| 'x' | 转化为16进制，小写 |
| 'X' | 转化为16进制，大写 |
| 'n' | 数字. 同'd', except that it uses the current locale setting to insert the appropriate number separator characters. |
| None | 同'd'. |
浮点型数据格式化
| Type | Meaning |
|---|---|
| 'e' | 指数标识。使用科学计数法，默认精度为6. |
| 'E' | 同'e'，只是把指数标识换成了'E'. |
| 'f' | 固定精度的浮点型. 默认精度是6. |
| 'F' | 同'f'. |
| 'g' | 见'g'解释 |
| 'G' | 同'g'，例外情况：except switches to 'E' if the number gets too large. The representations of infinity and NaN are uppercased, too. |
| 'n' | Number. 同'g', except that it uses the current locale setting to insert the appropriate number separator characters. |
| '%' | 百分比转换 |
| None | 同'g'. |
'g'解释
General format. For a given precision p >= 1, this rounds the number to p significant digits and then formats the result in either fixed-point format or in scientific notation, depending on its magnitude.
一般的格式化。如果给定的精度p>=1,这将对数字p进行四舍五入，然后根据大小将结果格式化为固定精度的数据或是科学计数的数据。
精度的规则为：将结果格式化为科学计数e和精度为p-1的表达式exp。如果-4<=exp<p,那么数据将会转化为'f'类型，并且精度为p-1。如果-4>exp,那么数据将会被格式化为'e'类型，精度为p-1.在这两种情况中，末尾的0将会移除，如果没有小数，小数点也会移除
>>> '{:.10g}'.format(0.00097867008822282723426)
'0.0009786700882'
>>> '{:.10g}'.format(0.000097867008822282723426)
'9.786700882e-05'
>>> '{:.10g}'.format(0.0000900)
'9e-05'
>>> '{:.10g}'.format(0.000900)
'0.0009'
>>> '{:.10g}'.format(0.0000900)
'9e-05'
>>> '{:.10g}'.format(0.0000000097867008822282723426)
'9.786700882e-09'
正负无穷，正0负0和 nans将会依次被格式化为inf,-inf,0, -0，不管精度为何值。
精度0将被看做和精度1相同。默认的精度为6
日期格式化
>>>
>>> import datetime
>>> d = datetime.datetime(2010, 7, 4, 12, 15, 58)
>>> '{:%Y-%m-%d %H:%M:%S}'.format(d)
'2010-07-04 12:15:58'
