python: str-format

在 python 中，有三种 string format: %-format, {}-format, $-format

{}-format

Format String Syntax

Format strings contain replacement fields surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

The grammar for a replacement field:

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 ::=  + 
conversion ::= "r" | "s" | "a" 
format_spec ::=

field_name
The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument.
! conversion
Three conversion flags are currently supported: !s which calls str() on the value, !r which calls repr() and !a which calls ascii().
format_spec
The general form of a standard format specifier is:

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

关于解释, 详见官方文档: Format Specification Mini-Language

str.format(*args, **kwargs)

Perform a string formatting operation. The string on which this method is called can contain literal text or replacement fields delimited by braces {}. Each replacement field contains either the numeric index of a positional argument, or the name of a keyword argument. Returns a copy of the string where each replacement field is replaced with the string value of the corresponding argument.

>>> "The sum of 1 + 2 is {0}".format(1+2)
'The sum of 1 + 2 is 3'

更多栗子详见 Format examples

format(value[, format_spec])

Convert a value to a “formatted” representation, as controlled by format_spec.

>>> x = 12.345
>>> format(x, '>10.2f')
'     12.35'
>>>

f-string

A formatted string literal or f-string is a string literal that is prefixed with f or F. These strings may contain replacement fields, which are expressions delimited by curly braces {}. While other string literals always have a constant value, formatted strings are really expressions evaluated at run time.

>>> 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'."

$-format

Template strings

$$ is an escape; it is replaced with a single $.
$identifier names a substitution placeholder matching a mapping key of identifier.

Examples

>>> from string import Template
>>> 
>>> s = Template('$who likes $what')
>>> s.template
'$who likes $what'
>>> 
>>> s.substitute(who='tim', what='kung pao')
'tim likes kung pao'
>>> d = dict(who='tim')
>>> Template('$who likes $what').substitute(d)
Traceback (most recent call last):
...
KeyError: 'what'
>>> Template('$who likes $what').safe_substitute(d)
'tim likes $what'

References

pyDoc: string
pyDoc: Built-in Type: str.format
pyDoc: Built-in Functions: format
pyDoc: Lexical analysis: Formatted string literals

Custom String Formatting
PEP 3101 -- Advanced String Formatting
PEP 292 -- Simpler String Substitutions
PEP 498 -- Literal String Interpolation