｛｝和format在这个命令python中做什么

Question

嗨，我正在读取一个源，它有一行：

fn = 'total-listmix-{}-{}.xml'.format(opt.os, opt.compiler)
exept Exception as e:
raise Exception('error merging coverage: {}'.format(e))

我认为{}是指dict，但我不明白这里的dict是什么？dict是否取自format？

Answer 1

{}是一个替换字段，而不是dict。Python文档：中提到了这一点

格式化字符串包含由大括号{}包围的"替换字段"。任何不包含在大括号中的内容都被视为文字文本，它被原封不动地复制到输出中。如果需要在文本中包含大括号字符，可以通过加倍{{和}}来转义。

一个简单的例子是：

name = "Tom"
print("Hello, {}".format(name)) # prints "Hello, Tom"

以下是Python文档中的更多示例：

"First, thou shalt count to {0}"  # References first positional argument
"Bring me a {}"                   # Implicitly references the first positional argument
"From {} to {}"                   # Same as "From {0} to {1}"
"My quest is {name}"              # References keyword argument 'name'
"Weight in tons {0.weight}"       # 'weight' attribute of first positional arg
"Units destroyed: {players[0]}"   # First element of keyword argument 'players'.

这就是替换字段的语法：

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       ::=  <described in the next section>