Module-loading operations rely on
attributes of the built-in
sys module (covered in
Chapter 8). The module-loading process described
here is carried out by built-in function
__import__. Your code can call
directly, with the module name string as an argument.
__import__ returns the module object or raises
ImportError if the import
To import a module named
sys.modules, using string
M as the key. When key
M is in the dictionary,
__import__ returns the corresponding value as the requested
module object. Otherwise,
to a new empty module object with a
M, then looks for the right way to
initialize (load) the module, as covered in Section 7.2.2 later in this section.
Thanks to this mechanism, the loading operation takes place only the
first time a module is imported in a given run of the program. When a
module is imported again, the module is not reloaded, since
__import__ finds and returns the
module’s entry in
Thus, all imports of a module after the first one are extremely fast
because they’re just dictionary lookups.
module is loaded,
__import__ first checks
whether the module is built-in. Built-in modules are listed in tuple
sys.builtin_module_names, but rebinding that tuple does not affect module loading. A built-in module, like any other Python extension, is initialized by calling the module’s initialization function. The search ...