Atoms, Identifiers, and Primaries

[Ian Kelly]

On Tue, Apr 16, 2013 at 8:57 PM, Bruce McGoveran
<bruce.mcgoveran at gmail.com> wrote:
> These are terms that appear in section 5 (Expressions) of the Python online documentation.  I'm having some trouble understanding what, precisely, these terms mean.  I'd appreciate the forum's thoughts on these questions:
>
> 1.  Section 5.2.1 indicates that an identifier occurring as an atom is a name.  However, Section 2.3 indicates that identifiers are names.  My question:  can an identifier be anything other than a name?

Yes.  For example:

from a import b

Here "a" is an identifier but not a name, as it does not carry
object-binding semantics.

> 2.  Section 5.3 defines primaries as the most tightly bound operations of Python.  What does this mean?

"Tightly bound" here refers to operator precedence.  For example, we
say that the multiplication operator binds more tightly [to the
surrounding operands] than the arithmetic operator, because the
multiplication takes precedence.  This section defines that the most
tightly bound operations in Python are attribute references,
subscriptions, slices and calls; these always take precedence over
other neighboring operations.

> In particular, if an atom is a primary, what operation is the atom performing that leads to the label "most tightly bound"?

An atom doesn't perform an operation.  The grammar defines that a
primary can be just an atom, so that anywhere in the grammar that
expects a primary, a simple atom with no primary operation performed
on it can equally be used.