以前的字符串格式换方法
1. format 方法格式化
这种方法用的不多（对应 string.Formatter）。
replacement_field ::= "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name ::= arg_name ("." attribute_name | "[" element_index "]")*
arg_name ::= [identifier | digit+]
attribute_name ::= identifier
element_index ::= digit+ | index_string
index_string ::= <any source character except "]"> +
conversion ::= "r" | "s" | "a"
format_spec ::= [[fill]align][sign][#][0][width][grouping_option][.precision][type]
fill ::= <any character>
align ::= "<" | ">" | "=" | "^"
sign ::= "+" | "-" | " "
width ::= digit+
grouping_option ::= "_" | ","
precision ::= digit+
type ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
print('{} {}'.format('Hello', 'World'))
print('There are three people in my family: {0}, {1}, and I, and I love my {0} a litte more.'.format('father', 'mother'))
2. 模板字符串格式化
这种方法我只在文档中看到，从没真的用过。
PS: 被废弃的 PEP 215 曾建议采用 $'a = $a, b = $b' 这种语法。
from string import Template
Template('$who likes $what').substitute(who='tim', what='kung pao')
Template('$who likes $what').safe_substitute({'who': 'time'})
3. 百分号格式化
这应该是现在的最主流的字符串格式化方式。
printf-style String Formatting
print('Hello %s' % 'World')
print('action %s cost %.3f seconds' % ('download', 0.123456789))
print('%(language)s has %(number)03d quote types.' % {'language': "Python", "number": 2})
Python 3.6 新加入 f-string
f_string ::= (literal_char | "{{" | "}}" | replacement_field)*
replacement_field ::= "{" f_expression ["="] ["!" conversion] [":" format_spec] "}"
f_expression ::= (conditional_expression | "*" or_expr)
("," conditional_expression | "," "*" or_expr)* [","]
| yield_expression
conversion ::= "s" | "r" | "a"
format_spec ::= (literal_char | NULL | replacement_field)*
literal_char ::= <any code point except "{", "}" or NULL>
从 format 方法格式化语法中复用了很多 (格式化和 conversion 部分), 不过变得更强大了。
主要是里面支持条件语句，表达式 (包括 yield)。
a = 3.1415926
f'{a}'
f'{a:.2f}'
f"{1 + 1}", f"{{1 + 1}}", f"{{{1 + 1}}}"
# ('2', '{1 + 1}', '{2}')
注意：f-sting 里面不能使用反斜杠转义！
f'{John\'s}'
# SyntaxError: f-string expression part cannot include a backslash
r, s, a
!r->repr()!s->str()!a->ascii()
PS: ascii 方法是 Python 3 引入，和 repr 相似，但是 ascii 方法仅使用 ASCII 字符。
例如：a = '中国'; print(f'{a!a} {ascii(a)}') 输出 '\u4e2d\u56fd' '\u4e2d\u56fd'。
print(f'{a!r}')
=
有人提议加入 !d 表示输出表达式本身，然后加上等于号，加上计算值，例如 f'1 + 1!d' => 1 + 1=2。
后来实现成了这样：
a = 3.14
b = 1
print(f'a + b=') # a + b=4.140000000000001
print(f'a + b = ') # a + b = 4.140000000000001
很有趣！
参考资料与拓展阅读
- Bentley University, A Guide to f-string Formatting in Python
- RealPython, Python 3's f-Strings: An Improved String Formatting Syntax (Guide)
- Python，PEP 498 -- Literal String Interpolation
