String Format Syntax

 2018/01/08

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” | “a”
format_spec       ::=  <described in the next section>

field_name that specifies the object whose value is to be formatted and inserted into the output instead of the replacement field. The field_name is optionally followed by a conversion field, which is preceded by an exclamation point '!', and a format_spec, which is preceded by a colon ':'. They are all optional.

Three conversion flags are currently supported: '!s' which calls str() on the value, '!r' which calls repr() and '!a' which calls ascii().目的是进行类型转换。

format_spec     ::=  [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill            ::=  <any character>
align           ::=  “<” | “>” | “=” | “^”
sign            ::=  “+” | “-” | ” “
width           ::=  integer
grouping_option ::=  “_” | “,”
precision       ::=  integer
type            ::=  “b” | “c” | “d” | “e” | “E” | “f” | “F” | “g” | “G” | “n” | “o” | “s” | “x” |

The meaning of the various align options is as follows:

Option	Meaning
`'<'`	左对齐，除数字外，对大多数对象默认
`'>'`	右对齐，对数字默认
`'='`	用等号之后的符号（不指定的话默认为‘0’）来填补数字正负号之后和数字之前的空格（此符号只适用于数字，比如 +000000120）
`'^'`	居中

The sign option is only valid for number types, and can be one of the following:

Option	Meaning
`'+'`	正负数均需要有正负号表示
`'-'`	如果不是负数则无需添加前置符号（默认）
space	表示正数的前面加空格，负数的前面加负号

The '#' option is only valid for integer, float, complex and Decimal types. For integers, when binary, octal, or hexadecimal output is used, this option adds the prefix respective ‘0b’, ‘0o’, or ‘0x’ to the output value.也就是说，对于二进制、八进制、十六进制的数字打印出来时会在前面加上对应的进制类型 。For floats, complex and Decimal the alternate form causes the result of the conversion to always contain a decimal-point character, even if no digits follow it.换句话说就是强制添加小数点，无论是否有小数。

The ',' option signals the use of a comma for a thousands separator.也就是千位分隔符。

The '_' option signals the use of an underscore for a thousands separator.类似于千位分隔符，不过将逗号换为了连字符。有其他功能，但不常用。

width is a decimal integer defining the minimum field width. If not specified, then the field width will be determined by the content.

The precision is a decimal number indicating how many digits should be displayed after the decimal point for a floating point value.用小数点后的数字表示小数点后的精度。

The type determines how the data should be presented.

The available string presentation types are:

Type	Meaning
`'s'`	String format. This is the default type for strings and may be omitted.
None	The same as `'s'`.

The available integer presentation types are:

Type Meaning

'b' Binary format. Outputs the number in base 2.

'c' Character. Converts the integer to the corresponding unicode character before printing.

'd' Decimal Integer. Outputs the number in base 10.

'o' Octal format. Outputs the number in base 8.

'x' Hex format. Outputs the number in base 16, using lower- case letters for the digits above 9.

'X' Hex format. Outputs the number in base 16, using upper- case letters for the digits above 9.

'n' Number. This is the same as 'd', except that it uses the current locale setting to insert the appropriate number separator characters.

None The same as 'd'.

Type	Meaning
`'b'`	Binary format. Outputs the number in base 2.
`'c'`	Character. Converts the integer to the corresponding unicode character before printing.
`'d'`	Decimal Integer. Outputs the number in base 10.
`'o'`	Octal format. Outputs the number in base 8.
`'x'`	Hex format. Outputs the number in base 16, using lower- case letters for the digits above 9.
`'X'`	Hex format. Outputs the number in base 16, using upper- case letters for the digits above 9.
`'n'`	Number. This is the same as `'d'`, except that it uses the current locale setting to insert the appropriate number separator characters.
None	The same as `'d'`.

The available presentation types for floating point and decimal values are:

`'e'`	Exponent notation. Prints the number in scientific notation using the letter ‘e’ to indicate the exponent. The default precision is `6`.
`'E'`	Exponent notation. Same as `'e'` except it uses an upper case ‘E’ as the separator character.
`'f'`	Fixed point. Displays the number as a fixed-point number. The default precision is `6`.
`'F'`	Fixed point. Same as `'f'`, but converts `nan` to `NAN` and `inf` to `INF`.
`'g'`	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.The precise rules are as follows: suppose that the result formatted with presentation type `'e'` and precision `p-1` would have exponent `exp`. Then if `-4 <= exp < p`, the number is formatted with presentation type `'f'` and precision `p-1-exp`. Otherwise, the number is formatted with presentation type `'e'` and precision `p-1`. In both cases insignificant trailing zeros are removed from the significand, and the decimal point is also removed if there are no remaining digits following it.Positive and negative infinity, positive and negative zero, and nans, are formatted as `inf`, `-inf`, `0`, `-0` and `nan` respectively, regardless of the precision.A precision of `0` is treated as equivalent to a precision of `1`. The default precision is `6`.
`'G'`	General format. Same as `'g'` except switches to `'E'` if the number gets too large. The representations of infinity and NaN are uppercased, too.
`'n'`	Number. This is the same as `'g'`, except that it uses the current locale setting to insert the appropriate number separator characters.
`'%'`	Percentage. Multiplies the number by 100 and displays in fixed (`'f'`) format, followed by a percent sign.
None	Similar to `'g'`, except that fixed-point notation, when used, has at least one digit past the decimal point. The default precision is as high as needed to represent the particular value. The overall effect is to match the output of `str()` as altered by the other format modifiers.

Format examples

This section contains examples of the str.format() syntax and comparison with the old %-formatting.

In most of the cases the syntax is similar to the old %-formatting, with the addition of the {} and with : used instead of %. For example, '%03.2f' can be translated to '{:03.2f}'.

The new format syntax also supports new and different options, shown in the follow examples.

Accessing arguments by position:

>>>

>>> '{0}, {1}, {2}'.format('a', 'b', 'c')
'a, b, c'
>>> '{}, {}, {}'.format('a', 'b', 'c')  # 3.1+ only
'a, b, c'
>>> '{2}, {1}, {0}'.format('a', 'b', 'c')
'c, b, a'
>>> '{2}, {1}, {0}'.format(*'abc')      # unpacking argument sequence（*号相当于list）
'c, b, a'
>>> '{0}{1}{0}'.format('abra', 'cad')   # arguments' indices can be repeated
'abracadabra'

Accessing arguments by name:

>>>

>>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
'Coordinates: 37.24N, -115.81W'
>>> coord = {'latitude': '37.24N', 'longitude': '-115.81W'}
>>> 'Coordinates: {latitude}, {longitude}'.format(**coord)
'Coordinates: 37.24N, -115.81W'

Accessing arguments’ attributes:

>>>

>>> 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:
...     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)'

Accessing arguments’ items:

>>>

1
2
3

>>> coord = (3, 5)
>>> 'X: {0[0]};  Y: {0[1]}'.format(coord)
'X: 3;  Y: 5'

Replacing %s and %r:

>>>

1 2	>>> "repr() shows quotes: {!r}; str() doesn't: {!s}".format('test1', 'test2') "repr() shows quotes: 'test1'; str() doesn't: test2"

Aligning the text and specifying a width:

>>>

>>> '{:<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***********'

Replacing %+f, %-f, and % f and specifying a sign:

>>>

>>> '{:+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'

Replacing %x and %o and converting the value to different bases:

>>>

>>> # 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'

Using the comma as a thousands separator:

>>>

1 2	>>> '{:,}'.format(1234567890) '1,234,567,890'

Expressing a percentage:

>>>

>>> points = 19
>>> total = 22
>>> 'Correct answers: {:.2%}'.format(points/total)
'Correct answers: 86.36%'

Using type-specific formatting:

>>>

>>> 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'

Nesting arguments and more complex examples:

>>>

>>> for align, text in zip('<^>', ['left', 'center', 'right']):
...     '{0:{fill}{align}16}'.format(text, fill=align, align=align)
...
'left<<<<<<<<<<<<'
'^^^^^center^^^^^'
'>>>>>>>>>>>right'
>>>
>>> octets = [192, 168, 0, 1]
>>> '{:02X}{:02X}{:02X}{:02X}'.format(*octets)
'C0A80001'
>>> int(_, 16)
3232235521
>>>
>>> width = 5
>>> for num in range(5,12): 
...     for base in 'dXob':
...         print('{0:{width}{base}}'.format(num, base=base, width=width), end=' ')
...     print()
...
    5     5     5   101
    6     6     6   110
    7     7     7   111
    8     8    10  1000
    9     9    11  1001
   10     A    12  1010
   11     B    13  1011

CATALOG

1. Format examples

