what does {} and format do in this command python?

Question

Hi i am reading a source and it has a line:

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

I think the {} mean a dict but i do not understand what dict here ? Is is a dict took from format ?

Answer 1

The {} is a replacement field, not a dict. This is mentioned in the Python Documentation:

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 }}.

One simple example is:

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

Here are a few more examples from the Python Documentation:

"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'.

And this is the grammar of the replacement field:

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>