第508页 | Learning Python | 阅读 ‧ 电子书库

Imports Versus Scopes

As we’ve learned, it is never possible to access names defined in another module file without first importing that file. That is, you never automatically get to see names in another file, regardless of the structure of imports or function calls in your program. A variable’s meaning is always determined by the locations of assignments in your source code, and attributes are always requested of an object explicitly.

For example, consider the following two simple modules. The first, moda.py, defines a variable X global to code in its file only, along with a function that changes the global X in this file:

X = 88                        # My X: global to this file only
def f():
    global X                  # Change this file's X
    X = 99                    # Cannot see names in other modules

The second module, modb.py, defines its own global variable X and imports and calls the function in the first module:

X = 11                        # My X: global to this file only

import moda                   # Gain access to names in moda
moda.f()                      # Sets moda.X, not this file's X
print(X, moda.X)

When run, moda.f changes the X in moda, not the X in modb. The global scope for moda.f is always the file enclosing it, regardless of which module it is ultimately called from:

% python modb.py
11 99

In other words, import operations never give upward visibility to code in imported files—an imported file cannot see names in the importing file. More formally:

 

Functions can never see names in other functions, unless they are physically enclosing. Module code can never see names in other modules, unless they are explicitly imported.

Such behavior is part of the lexical scoping notion—in Python, the scopes surrounding a piece of code are completely determined by the code’s physical position in your file. Scopes are never influenced by function calls or module imports.[51]