DragonautX DragonautX - 1 month ago 9
Python Question

What does the star in the field_name input mean in the Python str.format() docs?

In "6.1.3. Format String Syntax" in the Python 3.5.2 docs, the

field_name
for the replacement field grammar for
str.format()
is written like this:

field_name ::= arg_name ("." attribute_name | "[" element_index "]")*


What does the star at the very right mean? I can guess that I can get an object's attribute with
arg_name.attribute
or element with
arg_name[element_index]
, but I don't know if that extra star means I can do more with the object. I'm guessing it means that
.attribute_name
and
[element_index]
are optional, but I thought the parentheses already implied that.

Answer

Python Language Reference, section 1.2. says:

The descriptions of lexical analysis and syntax use a modified BNF grammar notation. This uses the following style of definition:

name      ::=  lc_letter (lc_letter | "_")*
lc_letter ::=  "a"..."z"

The first line says that a name is an lc_letter followed by a sequence of zero or more lc_letters and underscores. An lc_letter in turn is any of the single characters 'a' through 'z'. (This rule is actually adhered to for the names defined in lexical and grammar rules in this document.)

Each rule begins with a name (which is the name defined by the rule) and ::=. A vertical bar (|) is used to separate alternatives; it is the least binding operator in this notation. A star (*) means zero or more repetitions of the preceding item; likewise, a plus (+) means one or more repetitions, and a phrase enclosed in square brackets ([ ]) means zero or one occurrences (in other words, the enclosed phrase is optional). The * and + operators bind as tightly as possible; parentheses are used for grouping. Literal strings are enclosed in quotes. White space is only meaningful to separate tokens. Rules are normally contained on a single line; rules with many alternatives may be formatted alternatively with each line after the first beginning with a vertical bar.

So, asterisk * means, as expected, zero or more repetitions of the preceding group.