[英]what is the meaning of colon in python's string format?
In the reading of Python's Format Specification Mini-Language , 在阅读Python的格式规范迷你语言时 ,
format_spec ::= [[fill]align][sign][#][0][width][,][.precision][type]
fill ::= <any character>
align ::= "<" | ">" | "=" | "^"
sign ::= "+" | "-" | " "
width ::= integer
precision ::= integer
type ::= "b" | "c" | "d" | "e" | "E" | "f" | "F" | "g" | "G" | "n" | "o" | "s" | "x" | "X" | "%"
the grammar really confused me. 语法真让我困惑。
For example, if I want to convert int to binary representation I can do this 例如,如果我想将int转换为二进制表示,我可以这样做
"{0:b}".format(100)
"{:b}".format(100) # but this is fine too, so what dose the 0 do?
I know that b
represent the type
part in the specification, but I can't figure out the role for 0
and :
, what are they doing? 我知道
b
代表规范中的type
部分,但我无法弄清0
和0
的作用:
它们在做什么?
You are only looking at the grammar for the format_spec
, the full grammar is specified higher up on the same page : 您只查看
format_spec
的语法,在同一页面上指定完整语法:
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"
format_spec ::= <described in the next section>
In the replacement_field
syntax notice the :
preceding the format_spec
. 在
replacement_field
语法中,请注意:
format_spec
之前的:
。
The
field_name
is optionally followed by a conversion field, which is preceded by an exclamation point'!'
所述
field_name
是任选地随后通过转换场,这是由前面带有感叹号'!'
, and aformat_spec
, which is preceded by a colon':'
和
format_spec
,前面有冒号':'
When the field_name
and/or conversion
are specified, :
marks the end of former and the start of the format_spec
. 指定
field_name
和/或conversion
, :
标记前者的结束和format_spec
的开始。
In your example, 在你的例子中,
>>> "{0:b}".format(100)
'1100100'
zero specifies the optional field_name
which in this case corresponds to the index of the item to be formatted in the passed parameter tuple; zero指定可选的
field_name
,在这种情况下,对应于要在传递的参数元组中格式化的项的索引; it is optional so it can be dropped. 它是可选的,因此可以删除。
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