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Live module editing and development enhancement supporting hot reloading for Python 3.4+
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limeade limeade: Release version 0.1.1 Sep 28, 2019
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README.rst

Limeade

https://github.com/CFSworks/limeade/blob/master/.github/readme/logo.png?raw=true

pip install limeade

Live module editing and development enhancement supporting hot reloading for Python 3.4+.

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Usage

Equip your app with the ability to call limeade.refresh() on command (e.g. via hotkey) and invoke after making edits to your source files!

https://github.com/CFSworks/limeade/blob/master/.github/readme/demo.gif?raw=true

Features

  • Speeds up development: Don't restart your app, refresh your Python code!
  • Automatic: Just call to limeade.refresh(); Limeade does the rest!
  • Thorough: Updates your classes/functions even if instantiated/referenced!
  • Object reuse: Mutable objects are modified in-place; doesn't break is!
  • Good source of vitamin A

Caveats

Limeade is still experimental. I would be deeply appreciative if you could use it and find ways in which it breaks!

The API is not yet stable. I will keep limeade.refresh() working, but that is the extent of the public API for the time being.

Some things which it cannot (even theoretically) handle are:

  • Closures: These aren't defined at module reload time; even so, it's unsafe to mutate code within instantiated closures. Closures will be updated the next time they are instantiated.
  • Changes in metaclass: A class can be mutated if its type (metaclass) isn't changing, but Python does not permit changing the type of an existing class.
  • Threads: This is not incompatible with threading, but note that threads may observe the program in an inconsistent state if they're running during a refresh operation. Make sure your threads are out of harm's way!
  • Changes to __init__ functions: While the __init__ change is handled normally just like any other function, a common pitfall to watch for is that the __init__ function only runs when an instance is first created - as so, if any code added in the refresh depends on extra code added to __init__, it may fail for classes which are already instantiated. An automatic update facility may be added in the future.

Some things which it may handle in the future:

  • Renaming functions/classes: Currently, old and new definitions are matched via the qualified name. Heuristics can be used to match definitions when they are under different names, but this is not yet done.
  • Changes in __slots__ attribute: New slot descriptors cannot be created; however, Limeade could invent its own descriptors for new slots and insert those. They wouldn't be as efficient but they would get the job done. Descriptors for deleted slots can be cached in case the slot is brought back in a future mutation.
  • Changes in base classes: Initial experiments show that Python is much more picky about this one would expect.
  • Automatic rollback: In case of refresh/mutate failure, it would be great to rollback everything to the state it was in before, so that the running app isn't left in a half-updated state.

Comparison

The idea of merely reloading modules in Python is not a new one. What sets Limeade apart is that it attempts to update extant classes (and their instances) and functions. Here are some other approaches and how Limeade differs:

  • importlib.reload(): This simply re-executes the module in the same module namespace. The definitions of classes and functions are overwritten, but the classes and functions themselves are not updated. Because the module namespace is reused, a reload can be detected by first checking if certain names already exist. Note that Limeade uses the importlib.reload() call itself, so the same namespace reuse occurs at the module level.
  • IPython's autoreload extension: This is a much more similar approach to Limeade from a theoretical standpoint. There are practical differences, however: The autoreload extension contains imports that pull in IPython, while Limeade is intended to function standalone. IPython is intended for interactive use, while Limeade may be useful in hotpatching daemons and other long-running standalone Python programs. Limeade also aspires to integrate well with custom loaders and user logic.

License

All code licensed under 3-clause BSD.

Logo licensed under CC-BY-SA 4.0 with attribution to Elizabeth Reedy.

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