格式字符串字面值

格式字符串字面值#

格式字符串字面值 或称 f-string 是标注了 'f' 或 'F' 前缀的字符串字面值。这种字符串可包含替换字段，即以 {} 标注的表达式。其他字符串字面值只是常量，格式字符串字面值则是可在运行时求值的表达式。

句法格式：

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>

除非字面值标记为原始字符串，否则，与在普通字符串字面值中一样，转义序列也会被解码。

  • 双花括号 '{{' 或 '}}' 被替换为单花括号，花括号外的字符串仍按字面值处理。

  • 单左花括号 '{' 标记以 Python 表达式开头的替换字段。替换字段以右花括号 '}' 为结尾。

  • 在表达式后加等于号 '='，可在求值后，同时显示表达式文本及其结果（用于调试）。

  • 用叹号 '!' 标记的转换字段。

  • 还可以在冒号 ':' 后附加格式说明符。

  • 指定了转换符时，表达式求值的结果会先转换，再格式化。转换符 '!s' 调用 str() 转换求值结果，'!r' 调用 repr()，'!a' 调用 ascii()。

name = "Fred"
f"He said his name is {name!r}."

"He said his name is 'Fred'."

f"He said his name is {repr(name)}."  # repr() is equivalent to !r

"He said his name is 'Fred'."

import decimal
width = 10
precision = 4
value = decimal.Decimal("12.34567")
f"result: {value:{width}.{precision}}"  # nested fields

'result:      12.35'

from datetime import datetime

today = datetime(year=2017, month=1, day=27)
f"{today:%B %d, %Y}"  # using date format specifier

'January 27, 2017'

f"{today=:%B %d, %Y}" # using date format specifier and debugging

'today=January 27, 2017'

number = 1024
f"{number:#0x}"  # using integer format specifier

'0x400'

foo = "bar"
f"{ foo = }" # preserves whitespace

" foo = 'bar'"

line = "The mill's closed"
f"{line = }"

'line = "The mill\'s closed"'

f"{line = :20}"

"line = The mill's closed   "

f"{line = !r:20}"

'line = "The mill\'s closed" '