reload Basics

Unlike import and from:

 

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  • reload is a function in Python, not a statement.
  • reload is passed an existing module object, not a name.
  • reload lives in a module in Python 3.0 and must be imported itself.

Because reload expects an object, a module must have been previously imported successfully before you can reload it (if the import was unsuccessful, due to a syntax or other error, you may need to repeat it before you can reload the module). Furthermore, the syntax of import statements and reload calls differs: reloads require parentheses, but imports do not. Reloading looks like this:

import module                     # Initial import
...use module.attributes...
...                               # Now, go change the module file
...
from imp import reload            # Get reload itself (in 3.0)
reload(module)                    # Get updated exports
...use module.attributes...

The typical usage pattern is that you import a module, then change its source code in a text editor, and then reload it. When you call reload, Python rereads the module file’s source code and reruns its top-level statements. Perhaps the most important thing to know about reload is that it changes a module object in-place; it does not delete and re-create the module object. Because of that, every reference to a module object anywhere in your program is automatically affected by a reload. Here are the details:

 

 
  • reload runs a module file’s new code in the module’s current namespace. Rerunning a module file’s code overwrites its existing namespace, rather than deleting and re-creating it.
  • Top-level assignments in the file replace names with new values. For instance, rerunning a def statement replaces the prior version of the function in the module’s namespace by reassigning the function name.
  • Reloads impact all clients that use import to fetch modules. Because clients that use import qualify to fetch attributes, they’ll find new values in the module object after a reload.
  • Reloads impact future from clients only. Clients that used from to fetch attributes in the past won’t be affected by a reload; they’ll still have references to the old objects fetched before the reload.