This file provides a Promise class that can be used to simulate asynchronous behavior in Python using threads and callbacks.

There's no installation required for this code. You can simply copy the promise.py file to your project directory and import it into your Python scripts.

1. Import the Promise class:

   from promise import Promise

2. Create a Promise instance:

   promise = Promise()

3. Define asynchronous operations:

   • You'll need to implement your own asynchronous operations (functions that take some time to complete). These functions should return the result value when successful.

4. Resolve the promise:

   • Once your asynchronous operation completes, call the resolve method of the promise object, passing the result value as an argument. This will trigger any registered then callback.

   def my_async_operation():
       # Simulate some asynchronous work (e.g., network request, file I/O)
       import time
       time.sleep(2)  # Replace with your actual asynchronous operation
       return "Asynchronous result"
   
   # Call the asynchronous operation and resolve the promise
   promise.resolve(lambda: my_async_operation)

5. Handle successful completion (optional):

   • Use the then method to register a callback function that will be executed when the promise is resolved. This callback function will receive the result value from the asynchronous operation.

   promise.then(lambda result: print("Success:", result))

6. Handle errors (optional):

   • Use the catch method to register a callback function that will be executed if the promise is rejected due to an error during the asynchronous operation. This callback function will receive the error object.

   def handle_error(error):
       print("Error:", error)
   
   promise.catch(handle_error)

7. Reject promise (Optional)

   • Sometimes an error might occasionally occur an error after calling the promise in the main thread. So the promise isn’t nessesary anymore. Then you can reject the promise with reject. After rejecting the promise and passing the reason (error), the catch will be called.

   def function():
       promise = Promise()
   
       def my_async_operation():
          import time
          time.sleep(2)  # Simulate asynchronous work
          return "Asynchronous result"
   
       promise.resolve(lambda: my_async_operation)
   
       try:
           #error occur here so the promise isnt nessesary.
       except Exception as e:
           promise.reject(e)
       
       return promise
    
    promise = function()
    promise.then(lambda result: print("Success:", result))
    promise.catch(lambda error: print("Error:", error)) # Will be called

from promise import Promise

def main():
    promise = Promise()

    def my_async_operation():
        import time
        time.sleep(2)  # Simulate asynchronous work
        return "Asynchronous result"

    promise.resolve(lambda: my_async_operation)

    promise.then(lambda result: print("Success:", result))
    promise.catch(lambda error: print("Error:", error))

if __name__ == "__main__":
    main()

• The threading.Lock object is used to ensure thread-safe access to the promise's internal state. This is crucial when multiple threads might interact with the same promise object.

• Using threads can introduce complexity to your code. Consider using higher-level abstractions for asynchronous programming in Python whenever possible.