How can I remove/unimport symbols from __future__ after importing them?

Question

In python 2.x, dividing two integers returns an integer. However, if you use

from ___future___ import division

you can get a float value:

>>> 3/2
1
>>> from __future__ import division
>>> 3/2
1.5
>>> 
>>> 
>>> 3//2
1
>>> 4/3
1.3333333333333333
>>> 

After the import, you have to use // instead of / to do integer division. How can I revert the import so that / does integer division again?

Answer 1

__future__ imports are special, and cannot be undone. You can read up on their behavior here.

Here are a few relevant portions:

A future statement is recognized and treated specially at compile time: Changes to the semantics of core constructs are often implemented by generating different code. It may even be the case that a new feature introduces new incompatible syntax (such as a new reserved word), in which case the compiler may need to parse the module differently. Such decisions cannot be pushed off until runtime.
...
A future statement typed at an interactive interpreter prompt will take effect for the rest of the interpreter session.

Since __future__ statements are handled at compile time as opposed to runtime, there is no runtime method for reverting the changed behavior.

With normal modules you can remove or unimport the module by deleting whatever you imported from the namespace, and deleting the entry for that import in sys.modules (this second part may not be necessary depending on the use case, all it does is force the reloading of the module if it is imported again).

Answer 2

import statements are local to the file you import in, so for example, if you have this file as example.py:

from __future__ import division
print(1/2)

Then you load it in another file:

import example # prints 0.5 because `division` is imported in example.py
print(1/2) # prints 0 because `division` is not imported in this file

So if you want an import that's only used in some of your code, put that code in a separate file.

In the case you gave, I'm not sure it's helpful though. Why not just use // when you need integer division?