python 格式化字符串（官方文档）

A conversion specifier contains two or more characters and has the following components, which must occur in this order:

1. The "%" character, which marks the start of the specifier.
2. Mapping key (optional), consisting of a parenthesised sequence of characters (for example, (somename)).
3. Conversion flags (optional), which affect the result of some conversion types.
4. Minimum field width (optional). If specified as an "*" (asterisk), the actual width is read from the next element of the tuple in values, and the object to convert comes after the minimum field width and optional precision.
5. Precision (optional), given as a "." (dot) followed by the precision. If specified as "*" (an asterisk), the actual width is read from the next element of the tuple in values, and the value to convert comes after the precision.
6. Length modifier (optional).
7. Conversion type.

When the right argument is a dictionary (or other mapping type), then the formats in the string must include a parenthesised mapping key into that dictionary inserted immediately after the "%" character. The mapping key selects the value to be formatted from the mapping. For example:

>>> print '%(language)s has %(#)03d quote types.' % \          {'language': "Python", "#": 2}Python has 002 quote types.

In this case no * specifiers may occur in a format (since they require a sequential parameter list).

The conversion flag characters are:

FlagMeaning#The value conversion will use the ``alternate form'' (where defined below).0The conversion will be zero padded for numeric values.-The converted value is left adjusted (overrides the "0" conversion if both are given). (a space) A blank should be left before a positive number (or empty string) produced by a signed conversion.+A sign character ("+" or "-") will precede the conversion (overrides a "space" flag).

A length modifier (h, l, or L) may be present, but is ignored as it is not necessary for Python.

The conversion types are:

ConversionMeaningNotesd有符号整数 i有符号整数 o无符号八进制(1)u无符号十进制 x无符号十六进制(小写).(2)X无符号十六进制(大写).(2)e浮点数指数格式(小写). E浮点数指数格式(大写). f浮点数小数格式 F浮点数小数格式 g指数大于-4或者小于精度，则和e相同,反之则和 "f" 相同. G指数大于-4或者小于精度，则和e相同,反之则和 "f" 相同. c单字符 (接受整数或者单字符串). r字符串 （将任意值转化为字符串，使用 repr()).(3)s字符串 （将任意值转化为字符串，使用 str()).(4)%不转换参数，结果为“ ％”字符。 

Notes:

(1)

The alternate form causes a leading zero ("0") to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero.

(2)

The alternate form causes a leading '0x' or '0X' (depending on whether the "x" or "X" format was used) to be inserted between left-hand padding and the formatting of the number if the leading character of the result is not already a zero.

(3)

The %r conversion was added in Python 2.0.

(4)

If the object or format provided is a unicode string, the resulting string will also be unicode.

Since Python strings have an explicit length, %s conversions do not assume that '\0' is the end of the string.

For safety reasons, floating point precisions are clipped to 50; %f conversions for numbers whose absolute value is over 1e25 are replaced by %g conversions.^2.9 All other errors raise exceptions.