On Tue, 16 Apr 2013 19:57:25 -0700, Bruce McGoveran 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 and no. According to the Python grammar as documented, no, identifiers are just another name for, er, name. But according to common practice, yes, we call many things identifiers: x # a bare name, or "identifier" according to the grammar x.attr # name with an attribute, or "attributeref" x[key] # name with a subscript, or "subscription" x[5] # name with an indexed subscript, or "slicing" x[start:stop:step] # name with a slice subscript > 2. Section 5.3 defines primaries as the most tightly bound operations > of Python. What does this mean? It means that primaries are evaluated at the highest priority. For example, given: x.a+b that is evaluated as: (x.a) + b rather than: x . (a+b) In particular, if you have a variable: name = "Fred" then x.name will look for an attribute "name", *not* x.Fred. > In particular, if an atom is a > primary, what operation is the atom performing that leads to the label > "most tightly bound"? To put it a different way, I think of atoms as > things (i.e. identifiers). Correct. > The documentation makes me think atoms > actually do something, as opposed to being things (I think I have in my > mind the difference between a noun and a verb as I write this). The only thing they do is be evaluated. > 3. Section 5.3.1 offers this definition of an attributeref: > attributeref ::= primary "." identifier > > Now, I was at first a little concerned to see the non-terminal primary > on the right hand side of the definition, since primary is defined to > include attributeref in section 5.3 (so this struck me as circular). Am > I correct in thinking attributeref is defined this way to allow for > situations in which the primary, whether an atom, attributeref (example: > an object on which a method is called that returns another object), > subscription, slicing, or call, returns an object with property > identifier? Yes. It means you can write things like this: module.Class.method()[0](arg).attr.name[2]['spam'].aardvark() and have it evaluated from left to right. With respect, I think you may be confusing yourself unnecessarily with an excessive concern for the formal grammar of Python. You may find it makes a lot more sense in practice than it makes in theory. I strongly recommend you open up an interactive interpreter and just play around with the syntax and see what happens. -- Steven -- http://mail.python.org/mailman/listinfo/python-list
