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Recursive from Imports May Not Work

I saved the most bizarre (and, thankfully, obscure) gotcha for last. Because imports execute a file’s statements from top to bottom, you need to be careful when using modules that import each other (known as recursive imports). Because the statements in a module may not all have been run when it imports another module, some of its names may not yet exist.

If you use import to fetch the module as a whole, this may or may not matter; the module’s names won’t be accessed until you later use qualification to fetch their values. But if you use from to fetch specific names, you must bear in mind that you will only have access to names in that module that have already been assigned.

For instance, take the following modules, recur1 and recur2. recur1 assigns a name X, and then imports recur2 before assigning the name Y. At this point, recur2 can fetch recur1 as a whole with an import (it already exists in Python’s internal modules table), but if it uses from, it will be able to see only the name X; the name Y, which is assigned below the import in recur1, doesn’t yet exist, so you get an error:

# recur1.py
X = 1
import recur2                             # Run recur2 now if it doesn't exist
Y = 2

# recur2.py
from recur1 import X                      # OK: "X" already assigned
from recur1 import Y                      # Error: "Y" not yet assigned

C:\misc> C:\Python30\python
>>> import recur1
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "recur1.py", line 2, in <module>
    import recur2
  File "recur2.py", line 2, in <module>
    from recur1 import Y
ImportError: cannot import name Y

Python avoids rerunning recur1’s statements when they are imported recursively from recur2 (otherwise the imports would send the script into an infinite loop), but recur1’s namespace is incomplete when it’s imported by recur2.

The solution? Don’t use from in recursive imports (no, really!). Python won’t get stuck in a cycle if you do, but your programs will once again be dependent on the order of the statements in the modules.

There are two ways out of this gotcha:

 

You can usually eliminate import cycles like this by careful design—maximizing cohesion and minimizing coupling are good first steps.If you can’t break the cycles completely, postpone module name accesses by using import and qualification (instead of from), or by running your froms either inside functions (instead of at the top level of the module), or near the bottom of your file to defer their execution.