from Copies Names but Doesn’t Link

Although it’s commonly used, the from statement is the source of a variety of potential gotchas in Python. The from statement is really an assignment to names in the importer’s scope—a name-copy operation, not a name aliasing. The implications of this are the same as for all assignments in Python, but they’re subtle, especially given that the code that shares the objects lives in different files. For instance, suppose we define the following module, nested1.py:

# nested1.py
X = 99
def printer(): print(X)

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If we import its two names using from in another module, nested2.py, we get copies of those names, not links to them. Changing a name in the importer resets only the binding of the local version of that name, not the name in nested1.py:

# nested2.py
from nested1 import X, printer    # Copy names out
X = 88                            # Changes my "X" only!
printer()                         # nested1's X is still 99

% python nested2.py
99

If we use import to get the whole module and then assign to a qualified name, however, we change the name in nested1.py. Qualification directs Python to a name in the module object, rather than a name in the importer, nested3.py:

# nested3.py
import nested1                    # Get module as a whole
nested1.X = 88                    # OK: change nested1's X
nested1.printer()

% python nested3.py
88