Python——import与reload模块的区别

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1. import

The "import" statement

**********************

   import_stmt     ::= "import" module ["as" name] ( "," module ["as" name] )*
                   | "from" relative_module "import" identifier ["as" name]
                   ( "," identifier ["as" name] )*
                   | "from" relative_module "import" "(" identifier ["as" name]
                   ( "," identifier ["as" name] )* [","] ")"
                   | "from" module "import" "*"
   module          ::= (identifier ".")* identifier
   relative_module ::= "."* module | "."+
   name            ::= identifier

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
namespace (of the scope where the "import" statement occurs). The
statement comes in two forms differing on whether it uses the "from"
keyword. The first form (without "from") repeats these steps for each
identifier in the list. The form with "from" performs step (1) once,
and then performs step (2) repeatedly.

To understand how step (1) occurs, one must first understand how
Python handles hierarchical naming of modules. To help organize modules 
and provide a hierarchy in naming, Python has a concept of
packages. A package can contain other packages and modules while
modules cannot contain other modules or packages. From a file system
perspective, packages are directories and modules are files.

Once the name of the module is known (unless otherwise specified, the
term "module" will refer to both packages and modules), searching for
the module or package can begin. The first place checked is
"sys.modules", the cache of all modules that have been imported
previously. If the module is found there then it is used in step (2)
of import.

If the module is not found in the cache, then "sys.meta_path" is
searched (the specification for "sys.meta_path" can be found in **PEP
302**). The object is a list of *finder* objects which are queried in
order as to whether they know how to load the module by calling their
"find_module()" method with the name of the module. If the module
happens to be contained within a package (as denoted by the existence
of a dot in the name), then a second argument to "find_module()" is
given as the value of the "__path__" attribute from the parent package
(everything up to the last dot in the name of the module being imported). 
If a finder can find the module it returns a *loader*
(discussed later) or returns "None".

If none of the finders on "sys.meta_path" are able to find the module
then some implicitly defined finders are queried. Implementations of
Python vary in what implicit meta path finders are defined. The one
they all do define, though, is one that handles "sys.path_hooks",
"sys.path_importer_cache", and "sys.path".

The implicit finder searches for the requested module in the "paths"
specified in one of two places ("paths" do not have to be file system
paths). If the module being imported is supposed to be contained
within a package then the second argument passed to "find_module()",
"__path__" on the parent package, is used as the source of paths. If
the module is not contained in a package then "sys.path" is used as
the source of paths.

Once the source of paths is chosen it is iterated over to find a
finder that can handle that path. The dict at
"sys.path_importer_cache" caches finders for paths and is checked for a finder. If the path does not have a finder cached then
"sys.path_hooks" is searched by calling each object in the list with a
single argument of the path, returning a finder or raises
"ImportError". If a finder is returned then it is cached in
"sys.path_importer_cache" and then used for that path entry. If no
finder can be found but the path exists then a value of "None" is
stored in "sys.path_importer_cache" to signify that an implicit, file-
based finder that handles modules stored as individual files should be
used for that path. If the path does not exist then a finder which
always returns "None" is placed in the cache for the path.

If no finder can find the module then "ImportError" is raised.
Otherwise some finder returned a loader whose "load_module()" method
is called with the name of the module to load (see **PEP 302** for the
original definition of loaders). A loader has several responsibilities
to perform on a module it loads. First, if the module already exists
in "sys.modules" (a possibility if the loader is called outside of the
import machinery) then it is to use that module for initialization and
not a new module. But if the module does not exist in "sys.modules", then it is to be added to that dict before initialization begins. If
an error occurs during loading of the module and it was added to
"sys.modules" it is to be removed from the dict. If an error occurs
but the module was already in "sys.modules" it is left in the dict.

The loader must set several attributes on the module. "__name__" is to
be set to the name of the module. "__file__" is to be the "path" to
the file unless the module is built-in (and thus listed in
"sys.builtin_module_names") in which case the attribute is not set. If
what is being imported is a package then "__path__" is to be set to a
list of paths to be searched when looking for modules and packages
contained within the package being imported. "__package__" is optional
but should be set to the name of package that contains the module or
package (the empty string is used for module not contained in a
package). "__loader__" is also optional but should be set to the
loader object that is loading the module.

If an error occurs during loading then the loader raises "ImportError"
if some other exception is not already being propagated. Otherwise the
loader returns the module that was loaded and initialized.

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 namespace to the module object, and then goes on to import the
next identifier, if any.  If the module name is followed by "as", the
name following "as" is used as the local name for the module.

The "from" form 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 namespace to the object thus
found.  As with the first form of "import", an alternate local name
can be supplied by specifying ""as" localname".  If a name is not
found, "ImportError" is raised.  If the list of identifiers is
replaced by a star ("'*'"), all public names defined in the module are
bound in the local namespace of the "import" statement..

The *public names* defined by a module are determined by checking the
module's namespace for a variable named "__all__"; if defined, it must
be a sequence of strings which are names defined or imported by that
module.  The names given in "__all__" are all considered public and
are required to exist.  If "__all__" is not defined, the set of public
names includes all names found in the module's namespace which do not
begin with an underscore character ("'_'"). "__all__" should contain
the entire public API. It is intended to avoid accidentally exporting
items that are not part of the API (such as library modules which were
imported and used within the module).

The "from" form with "*" may only occur in a module scope.  If the
wild card form of import --- "import *" --- is used in a function and
the function contains or is a nested block with free variables, the
compiler will raise a "SyntaxError".

When specifying what module to import you do not have to specify the
absolute name of the module. When a module or package is contained
within another package it is possible to make a relative import within
the same top package without having to mention the package name. By
using leading dots in the specified module or package after "from" you
can specify how high to traverse up the current package hierarchy
without specifying exact names. One leading dot means the current
package where the module making the import exists. Two dots means up
one package level. Three dots is up two levels, etc. So if you execute
"from . import mod" from a module in the "pkg" package then you will
end up importing "pkg.mod". If you execute "from ..subpkg2 import mod"
from within "pkg.subpkg1" you will import "pkg.subpkg2.mod". The
specification for relative imports is contained within **PEP 328**.

"importlib.import_module()" is provided to support applications that
determine which modules need to be loaded dynamically.


Future statements
=================

A *future statement* is a directive to the compiler that a particular
module should be compiled using syntax or semantics that will be
available in a specified future release of Python.  The future
statement is intended to ease migration to future versions of Python
that introduce incompatible changes to the language.  It allows use of
the new features on a per-module basis before the release in which the
feature becomes standard.

   future_statement ::= "from" "__future__" "import" feature ["as" name]
                        ("," feature ["as" name])*
                        | "from" "__future__" "import" "(" feature ["as" name]
                        ("," feature ["as" name])* [","] ")"
   feature          ::= identifier
   name             ::= identifier

A future statement must appear near the top of the module.  The only
lines that can appear before a future statement are:

* the module docstring (if any),

* comments,

* blank lines, and

* other future statements.

The features recognized by Python 2.6 are "unicode_literals", "print_function", 
"absolute_import", "division", "generators",
"nested_scopes" and "with_statement".  "generators", "with_statement",
"nested_scopes" are redundant in Python version 2.6 and above because
they are always enabled.

A future statement is recognized and treated specially at compile
time: Changes to the semantics of core constructs are often
implemented by generating different code.  It may even be the case
that a new feature introduces new incompatible syntax (such as a new
reserved word), in which case the compiler may need to parse the
module differently.  Such decisions cannot be pushed off until
runtime.

For any given release, the compiler knows which feature names have
been defined, and raises a compile-time error if a future statement
contains a feature not known to it.

The direct runtime semantics are the same as for any import statement:
there is a standard module "__future__", described later, and it will
be imported in the usual way at the time the future statement is
executed.

The interesting runtime semantics depend on the specific feature
enabled by the future statement.

Note that there is nothing special about the statement:

   import __future__ [as name]

That is not a future statement; it's an ordinary import statement with
no special semantics or syntax restrictions.

Code compiled by an "exec" statement or calls to the built-in
functions "compile()" and "execfile()" that occur in a module "M"
containing a future statement will, by default, use the new  syntax or 
semantics associated with the future statement.  This can, starting
with Python 2.2 be controlled by optional arguments to "compile()" ---
see the documentation of that function for details.

A future statement typed at an interactive interpreter prompt will
take effect for the rest of the interpreter session.  If an
interpreter is started with the "-i" option, is passed a script name
to execute, and the script includes a future statement, it will be in
effect in the interactive session started after the script is
executed.

See also:

  **PEP 236** - Back to the __future__
     The original proposal for the __future__ mechanism.

2. reload

reload(...)
    reload(module) -> module

    Reload the module.  The module must have been successfully imported before.

3. 

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