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. The direct runtime semantics are the same as for any import statement: there is a standard module _future_, best 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; its 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 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.
Future statements A future statement is a directive to the compiler that a particular homework 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.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.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.
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. If you execute from. Subpkg2 import mod from within bpkg1 you will import. The specification for relative imports is contained within pep 328. Port_module is provided to support applications that determine which modules need to be loaded dynamically.
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The public names hero defined by a module are determined by checking the resume modules 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 modules 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.
_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 iltin_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.
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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 good dules (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 dules then it is to be added to that dict before initialization begins. If an make error occurs during loading of the module and it was added to dules it is to be removed from the dict. If an error occurs but the module was already in dules it is left in the dict. The loader must set several attributes on the module.
If the module is not contained in a package then th 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 th_importer_cache caches finders for paths and is checked for a finder. If the path does not have a finder cached then th_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 th_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 writing in th_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.
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. If a finder can find the module it returns a loader (discussed later) or returns None. If none of the finders on ta_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 th_hooks, th_importer_cache, and. 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.
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 essay 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 dules, the cache of all modules that have been imported previously.
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Simple statements are comprised within a single logical line. Several simple statements may occur on a single line separated by semicolons. The syntax for simple statements is:.12. The import statement import_stmt : "import" module "as" name ( dubai 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.