What is the meaning of a colon in string formatting?

python, string-formatting

Solution

You are only looking at the grammar for the `format_spec`, the full grammar is specified higher up on the same page:

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`.

The `field_name` is optionally followed by a conversion field, which is preceded by an exclamation point `'!'`, and a `format_spec`, which is preceded by a colon `':'`

When the `field_name` and/or `conversion` are specified, `:` marks the end of former and the start of the `format_spec`.

In your example,

>>> "{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; it is optional so it can be dropped.

Problem

In the reading of Python's Format Specification Mini-Language, ``` 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. For example, if I want to convert int to binary representation I can do this ``` "{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?

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