Module Reloading and Unloading

Python provides no real support for reloading or unloading of previously imported modules. Although you can remove a module from sys.modules, this does not generally unload a module from memory.This is because references to the module object may still exist in other program components that used import to load that module. Moreover, if there are instances of classes defined in the module, those instances contain references back to their class object, which in turn holds references to the module in which it was defined.

The fact that module references exist in many places makes it generally impractical to reload a module after making changes to its implementation. For example, if you remove a module from sys.modules and use import to reload it, this will not retroactively change all of the previous references to the module used in a program. Instead, you'll have one reference to the new module created by the most recent import statement and a set of references to the old module created by imports in other parts of the code. This is rarely what you want and never safe to use in any kind of sane production code unless you are able to carefully control the entire execution environment.

Older versions of Python provided a reload() function for reloading a module. However, use of this function was never really safe (for all of the aforementioned reasons), and its use was actively discouraged except as a possible debugging aid. Python 3 removes this feature entirely. So, it's best not to rely upon it.

Finally, it should be noted that C/C++ extensions to Python cannot be safely unloaded or reloaded in any way. No support is provided for this, and the underlying operating system may prohibit it anyways. Thus, your only recourse is to restart the Python interpreter process.

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