Files Generate Namespaces

So, how do files morph into namespaces? The short story is that every name that is assigned a value at the top level of a module file (i.e., not nested in a function or class body) becomes an attribute of that module.

For instance, given an assignment statement such as X = 1 at the top level of a module file M.py, the name X becomes an attribute of M, which we can refer to from outside the module as M.X. The name X also becomes a global variable to other code inside M.py, but we need to explain the notion of module loading and scopes a bit more formally to understand why:

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  • Module statements run on the first import. The first time a module is imported anywhere in a system, Python creates an empty module object and executes the statements in the module file one after another, from the top of the file to the bottom.
  • Top-level assignments create module attributes. During an import, statements at the top level of the file not nested in a def or class that assign names (e.g., =, def) create attributes of the module object; assigned names are stored in the module’s namespace.
  • Module namespaces can be accessed via the attribute__dict__ or dir(M). Module namespaces created by imports are dictionaries; they may be accessed through the built-in __dict__ attribute associated with module objects and may be inspected with the dir function. The dir function is roughly equivalent to the sorted keys list of an object’s __dict__ attribute, but it includes inherited names for classes, may not be complete, and is prone to changing from release to release.
  • Modules are a single scope (local is global). As we saw in Chapter 17, names at the top level of a module follow the same reference/assignment rules as names in a function, but the local and global scopes are the same (more formally, they follow the LEGB scope rule we met in Chapter 17, but without the L and E lookup layers). But, in modules, the module scope becomes an attribute dictionary of a module object after the module has been loaded. Unlike with functions (where the local namespace exists only while the function runs), a module file’s scope becomes a module object’s attribute namespace and lives on after the import.

Here’s a demonstration of these ideas. Suppose we create the following module file in a text editor and call it module2.py:

print('starting to load...')
import sys
name = 42

def func(): pass

class klass: pass

print('done loading.')

The first time this module is imported (or run as a program), Python executes its statements from top to bottom. Some statements create names in the module’s namespace as a side effect, but others do actual work while the import is going on. For instance, the two print statements in this file execute at import time:

>>> import module2
starting to load...
done loading.

Once the module is loaded, its scope becomes an attribute namespace in the module object we get back from import. We can then access attributes in this namespace by qualifying them with the name of the enclosing module:

>>> module2.sys
<module 'sys' (built-in)>

>>> module2.name
42

>>> module2.func
<function func at 0x026D3BB8>

>>> module2.klass
<class 'module2.klass'>

Here, sys, name, func, and klass were all assigned while the module’s statements were being run, so they are attributes after the import. We’ll talk about classes in Part VI, but notice the sys attribute—import statements really assign module objects to names, and any type of assignment to a name at the top level of a file generates a module attribute.

Internally, module namespaces are stored as dictionary objects. These are just normal dictionary objects with the usual methods. We can access a module’s namespace dictionary through the module’s __dict__ attribute (remember to wrap this in a list call in Python 3.0—it’s a view object):

>>> list(module2.__dict__.keys())
['name', '__builtins__', '__file__', '__package__', 'sys', 'klass', 'func',
'__name__', '__doc__']

The names we assigned in the module file become dictionary keys internally, so most of the names here reflect top-level assignments in our file. However, Python also adds some names in the module’s namespace for us; for instance, __file__ gives the name of the file the module was loaded from, and __name__ gives its name as known to importers (without the .py extension and directory path).