Python – How Does a Symbol Table Relate to a Namespace

python

The official tutorial uses the term symbol table in a few places where I would expect the term namespace.

The execution of a function introduces a new symbol table used for the
local variables of the function. More precisely, all variable
assignments in a function store the value in the local symbol table;
whereas variable references first look in the local symbol table, then
in the local symbol tables of enclosing functions, then in the global
symbol table, and finally in the table of built-in names.

_{2. More on modules}

Each module has its own private symbol table, which is used as the
global symbol table by all functions defined in the module. Thus, the
author of a module can use global variables in the module without
worrying about accidental clashes with a user’s global variables.

I found Eli Bendersky's blog where he quotes the symtable module:

Symbol tables are generated by the compiler from AST just before
bytecode is generated. The symbol table is responsible for calculating
the scope of every identifier in the code.

So it seems like a symbol table precedes a namespace. Yet another quote, from the first source, leads me to believe they also exist at the same time.

The actual parameters (arguments) to a function call are introduced in
the local symbol table of the called function when it is called; thus,
arguments are passed using call by value (where the value is always an
object reference, not the value of the object).¹

Is a symbol table involved with the creation of a namespace? Does a symbol table "contain" a namespace or simply information that a namespace contains?
In short,
how does a symbol table relate to a namespace?

Best Answer

A symbol table is an implementation detail. Namespaces are implemented using symbol tables, but symbol tables are used for more than just namespaces. For example, functions have their own symbol table for local variables, but those variables do not exist in any namespace (that is, it is impossible to somehow access the local variables of a function using a fully-qualified name).

Related Solutions

Python Variables – Using Prime Symbol in Variable Names

You cannot just use any Unicode character in Python identifiers. There are rules governing what can be used:

identifier   ::=  xid_start xid_continue*
id_start     ::=  <all characters in general categories Lu, Ll, Lt, Lm, Lo, Nl, the underscore, and characters with the Other_ID_Start property>
id_continue  ::=  <all characters in id_start, plus characters in the categories Mn, Mc, Nd, Pc and others with the Other_ID_Continue property>
xid_start    ::=  <all characters in id_start whose NFKC normalization is in "id_start xid_continue*">
xid_continue ::=  <all characters in id_continue whose NFKC normalization is in "id_continue*">
The Unicode category codes mentioned above stand for:

Lu - uppercase letters
Ll - lowercase letters
Lt - titlecase letters
Lm - modifier letters
Lo - other letters
Nl - letter numbers
Mn - nonspacing marks
Mc - spacing combining marks
Nd - decimal numbers
Pc - connector punctuations
Other_ID_Start - explicit list of characters in PropList.txt to support backwards compatibility
Other_ID_Continue - likewise

The above is the Unicode equivalent to any number of letters, digits and underscores, as long as you don't start with a digit.

The U+2032 PRIME codepoint doesn't fall in any of those classes; it is considered Po - other punctuation instead.

I've linked the Unicode categories to codepoint.net queries for you so you can see what kind of characters are permitted. Your other identifier choices are all Ll lowercase letter characters, by the way.

You do want to be careful and use some common sense in what you use as identifiers. Keep them readable and recognisable. Preferably they should be easy to type too, so personally I'd steer away from using greek lowercase letters myself. I'd rather use alpha, gamma, epsilon and s_prime instead here.

Python – Relationship Between Scope and Namespaces

A namespace is a dictionary, mapping names (as strings) to values. When you do an assignment, like a = 1, you're mutating a namespace. When you make a reference, like print(a), Python looks through a list of namespaces to try and find one with the name as a key.

A scope defines which namespaces will be looked in and in what order. The scope of any reference always starts in the local namespace, and moves outwards until it reaches the module's global namespace, before moving on to the builtins (the namespace that references Python's predefined functions and constants, like range and getattr), which is the end of the line.

Imagine you have a function named inner, nested within a global function named outer, and inner contains a reference to a name. Python first looks in the inner namespace. If the name's not there, Python then looks in the outer namespace. If that fails, Python tries the module's global namespace, then the builtin namespace, eventually throwing a NameError if the name isn't found.

When we say x is in a function's namespace, we mean it is defined there, locally within the function. When we say x is in the function's scope, we mean x is either in the function's namespace or in any of the outer namespaces that the function's namespace is currently nested within.

Whenever you define a function, you create a new namespace and a new scope. The namespace is the new, local hash of names. The scope is the implied chain of namespaces that starts at the new namespace, then works its way through any outer namespaces (outer scopes), up to the global namespace (the global scope), and on to the builtins.

The terms can be used almost interchangeably, but that's not because they mean the same thing; it's because they overlap a lot in what they imply.

Best Answer

Related Solutions

Python Variables – Using Prime Symbol in Variable Names

Python – Relationship Between Scope and Namespaces

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