How do I make a decorator conditional?

Question

On a Windows PC using python3 I'm using a Raspberry Pi simulator to develop a system whilst building the hardware. I'm now at the point where I would like to test on a real Raspberry Pi. It would be nice to have the same code and switch between the two modes (simulator & real hardware) by just changing a switch.

The code using the simulator is at the start (after some import json code) is:

# load the Rasp Pi tkgpio simulator
from tkgpio import TkCircuit
# get simulator configuration settings
with open("EMSSimV8.json", "r") as file:
    configuration = load(file)
circuit = TkCircuit(configuration)
@circuit.run

def main():

I would like to do:

simulation = True     # or False
if simulation:
    # load the Rasp Pi tkgpio simulator
    from tkgpio import TkCircuit
    # get simulator configuration settings
    with open("EMSSimV8.json", "r") as file:
        configuration = load(file)
    circuit = TkCircuit(configuration)
    @circuit.run
else:               # production on a real Pi
    main().run

def main():

but python gives an "unexpected unindent" at the else: statement

Is there some way to achieve this?

Answer 1

The @ in Python means you're dealing with a decorator, which is a (usually) more convenient way of wrapping or modifying a function, method, or class to do something slightly different. This code:

@some_decorator
def my_function():
    ...

is essentially the same as this:

def my_function():
    ...

my_function = some_decorator(my_function)

After either one of these, anything later in the code that calls my_function() will actually be calling whatever some_decorator(my_function) returned; this will usually be a wrapped or modified version of the original my_function that has some additional behavior.

I'm not familiar with Raspberry Pi or the packages you're using, so I can't exactly promise this will work, but you should be able to do something very much like this:

from tkgpio import TkCircuit

simulation = True     # or False

def main():
    ...

if simulation:
    with open("EMSSimV8.json", "r") as file:
        configuration = load(file)
    circuit = TkCircuit(configuration)
    main = circuit.run(main)

if __name__ == '__main__':
    main()

That way you only create the TkCircuit and use it to wrap main if needed.

Answer 2

If you are short on time skip to the bottom. The example I provided is not guaranteed as I do not have enough information from you.

To clarify I am answering the underlying question, "how can I selectively decorate a function". I don't have your underlying code or a definition of the decorator function to work with.

EDIT An "avoid repeating yourself" concern was raised about the main function. You can conditionally include modules through the same mechanism in python. Just add your main routine to two different files and include the routine you want to use. I have demonstrated how to do this at the bottom.

There are many ways to skin this cat, this is just the method I prefer since everything stays nice and encapsulated. The underlying principle is the same though : you can include decorators and other symbols that don't exist without generating "compile time" errors in python provided the section including them is "dead code". Python is a lot more lenient than other languages are where this is concerned.

I will include an example that might work at the bottom, but again, I don't have your underlying code.

First the principle.

To selectively decorate a function, you can just decorate a method

def external_func(func):
  def dostuff():
    print("hi there!")
  return dostuff

class selectively_decorate :
  def __init__(self):
    global dodecorate
    if dodecorate:
      """
      Try replacing this with @doesntexist
      if dodecorate is falsy you can still create an instance
      """
      @external_func
      def doTheThing():
        return 1
    else:
      def doTheThing():
        print("well hello yourself!")
        return 1
    doTheThing()

Example

dodecorate=0
a=selectively_decorate()

Outputs

well hello yourself!

Example

dodecorate=1
a=selectively_decorate()

Outputs

hi there!

In your case this might work :

# load the Rasp Pi tkgpio simulator
from tkgpio import TkCircuit

is_simulation = 0

class config_loader:
  def __init__(self, jsonFile):
    global is_simulation

    # get simulator configuration settings
    with open(jsonFile, "r") as file:
      self.configuration = load(file)
    self.circuit = TkCircuit(self.configuration)

    """"
    You do not need to repeat the decorator or definition of main
    It is included here only to show that it can be selectively
    included
    """
    if is_simulation :
      from module1 import main_routine
      """
      In other words, you can delete the next three lines...
      """
      @circuit.run
      def main() :
        main_routine()
    else :
      from module2 import main.routine
      """
      ... and decrease the indent for the next three one level
      """
      @circuit.run
      def main() :
        main_routine()
    main()

load = config_loader("EMSSimV8.json")

Obviously this example can be simplified further, but hopefully the idea has been adequately demonstrated.