import_stmt: "import" identifier ("," identifier)*
| "from" identifier "import" identifier ("," identifier)*
| "from" identifier "import" "*"
Import statements are executed in two steps: (1) find a module, and
initialize it if necessary; (2) define a name or names in the local
name space (of the scope where the import statement occurs).
The first form (without from) repeats these steps for each
identifier in the list, the from form performs them once, with
the first identifier specifying the module name.
The system maintains a table of modules that have been initialized,
indexed by module name. (The current implementation makes this table
accessible as sys.modules.) When a module name is found in
this table, step (1) is finished. If not, a search for a module
definition is started. This first looks for a built-in module
definition, and if no built-in module if the given name is found, it
searches a user-specified list of directories for a file whose name is
the module name with extension ".py". (The current
implementation uses the list of strings sys.path as the search
path; it is initialized from the shell environment variable
$PYTHONPATH, with an installation-dependent default.)
If a built-in module is found, its built-in initialization code is
executed and step (1) is finished. If no matching file is found,
ImportError is raised. If a file is found, it is parsed,
yielding an executable code block. If a syntax error occurs,
SyntaxError is raised. Otherwise, an empty module of the given
name is created and inserted in the module table, and then the code
block is executed in the context of this module. Exceptions during
this execution terminate step (1).
When step (1) finishes without raising an exception, step (2) can begin.
The first form of import statement binds the module name in the
local name space to the module object, and then goes on to import the
next identifier, if any. The from from does not bind the
module name: it goes through the list of identifiers, looks each one
of them up in the module found in step (1), and binds the name in the
local name space to the object thus found. If a name is not found,
ImportError is raised. If the list of identifiers is replaced
by a star (*), all names defined in the module are bound,
except those beginning with an underscore(_).
Names bound by import statements may not occur in global
statements in the same scope.
The from form with * may only occur in a module scope.
(The current implementation does not enforce the latter two restrictions, but programs should not abuse this freedom, as future implementations may enforce them or silently change the meaning of the program.)