I dont think there is an actual Python debugger available in FME. I usually use log messages at key points in my scripts to debug. You can log strings using logMessageString() https://docs.safe.com/fme/html/fmepython/api/fmeobjects/_log/fmeobjects.FMELogFile.logMessageString.html

I`ll try 

Thanks a lot

If possible, I try to avoid using Python and use baked-in FME functionality as much as possible
I don't know if it's an option here, but maybe you could consider an HTTPCaller to perform your downloadAPI calls?

I`ll try using HTTPCaller to perform my download API calls?

With some fiddeling it is possible. After working on a project which grew very fast in complexity FMEs transformers could not handle it and the python code grew to much so i spent some time banging my head against the wall.

After some time i found this post about developing FME packages that mentioned debugpy, but the same principle applies to running pythoncallers.

https://docs.safe.com/fme/html/fpkg-sdk/hello-world-package/#__tabbed_1_2

TLDR ..

• Install debugpy package into an existing python installation (Esri ArcGIS Python 3.9 for me, and i used PIP). This MAY cause instability with your existing python installation. We use an existing python installation because FME refused to accept a custom one, plus we make sure its compatible with FME Server.

•  In the same folder as your FME-file, create a python file (.py). In that python module, paste the default python-code 

  

   

• Create another python module, debug.py and add the following code

•  

• # Debug -> Attach to process -> tcp://127.0.0.1:5678
  
  
  def start_debugging():
      import fme
      if fme.macroValues.get('FME_SERVER_HOST') == '':
          print('Running locally')
          import psutil
          targets = �'devenv.exe', 'code.exe'] # Visual Studio and VSCode
          processes = sorted(�proc.name() for proc in psutil.process_iter(c'name'])])
          
          if any(target in processes for target in targets):
              print('Visual Studio or VSCode running, enabling debugging')
              import debugpy
  
              socket = ("127.0.0.1", 5678)
              print('Starting debugging session...')
              print('Setting path')
              debugpy.configure(python=r'C:\Program Files\ArcGIS\Pro\bin\Python\envs\arcgispro-py3\python.exe')
              print('Trying to listen')
              debugpy.listen(socket)
              print(f'Listening ... waiting for debugger to connect to socket {socket}. \nIn Visual Studio -> Debugging -> Attach to process, connect to tcp://127.0.0.1:5678')
              debugpy.wait_for_client()
              print('Client connected')
              # debugpy.breakpoint()  # Optional: Set a breakpoint right after
          else:
              print('Visual studio or VSCode not running, no debugging enabled')
  
  
      else:
          print('Running on server, no debugging')
              
  # Use this method to find the python path to use in the debugpy.configure line
  def print_python_paths():
      import sys
      print(f'System version  {sys.version_info}')
  
      for path in sys.path:
          print(f'System path  {path}')

  
   

• Create a PythonCaller add the following code”. Make sure that the “Class to Process Features has the same name as the class you are importing.

• 

  In Visual Studio (VS Code works as well) create a new Python project. Under Python Environments add the arcgispro-py3 one and set it to active. It will now fild all installed packages.

• To not get errors from fme and fmeobjects, copy the .py and .pyd files from the python installation you are using into the same folder that your fme-file and python modules are

  • fme.py

  • fmebootstrap.py

  • fmeobjects.cp39-win_amd64.pyd

• When you start your script you should see this text 

  

• So go ahead and connect your debugger and enjoy breakpoints, code completion, copilot, linting and everything else that comes with a good IDE