Learning Python

Import Model

As we’ve seen, qualification is needed only when you use import to fetch a module as a whole. When you use the from statement, you copy names from the module to the importer, so the imported names are used without qualifying. Here are a few more details on the import process.

Imports Happen Only Once

One of the most common questions beginners seem to ask when using modules is: why won’t my imports keep working? The first import works fine, but later imports during an interactive session (or in a program) seem to have no effect. They’re not supposed to, and here’s why:

• Modules are loaded and run on the first import or from.

• Running a module’s code creates its top-level names.

• Later import and from operations fetch an already loaded module.

Python loads, compiles, and runs code in a module file only on the first import, on purpose; since this is an expensive operation, Python does it just once per process by default. Moreover, since code in a module is usually executed once, you can use it to initialize variables. For example:

% cat simple.py
print 'hello'
spam = 1                   # initialize variable

% python
>>> import simple          # first import: loads and runs file's code
hello
>>> simple.spam            # assignment makes an attribute
1
>>> simple.spam = 2        # change attribute in module
>>>
>>> import simple          # just fetches already-loaded module
>>> simple.spam            # code wasn't rerun: attribute unchanged
2

In this example, the print and = statements run only the first time the module is imported. The second ...