I was trying to play around with Nikola’s code today and learnt about a documented weirdness of Python’s reload.

• Below are two versions of code – ORIGINAL and UPDATED respectively referring to the orginial code and the code after changes. The code only has a PLUGINS list, which is changed in each version of the code.

  # Work in a temporary directory
  import os
  import tempfile
  os.chdir(tempfile.mkdtemp())
  
  # Module content, original and updated
  ORIGINAL = "# PLUGINS = []"
  UPDATED = "PLUGINS = ['rss']"
  
  def create_conf_file(content):
      """Create a conf.py module with given content."""
      with open('conf.py', 'w') as f:
          f.write(content)
  

• conf doesn’t have a PLUGINS attribute in the ORIGINAL code. It’s been commented out!

  create_conf_file(ORIGINAL)
  import conf
  # PLUGINS is not defined in the module, originally.
  print(conf.PLUGINS)
  

• The code for conf has been updated, but the module doesn’t yet have a PLUGINS attribute, since the new module isn’t imported until we reload.

  create_conf_file(UPDATED)
  import conf
  print(conf.PLUGINS)
  

• PLUGINS has the expected value, after the reload

  import importlib
  importlib.reload(conf)
  print(conf.PLUGINS)
  

• What happens when we revert to the ORIGINAL code, and reload the module?

  # We write back the original file. PLUGINS should be empty!
  create_conf_file(ORIGINAL)
  import conf
  importlib.reload(conf)
  print(conf.PLUGINS, "<---Whaaaat!")
  

The behavior is well documented, along with a reasoning of why it is the way it is, but you can trip over it if you don’t know. I hit a bug and was wondering if there was a race condition somewhere, until I read the docs for reload. On reload, the module dict is updated, instead of creating a new dict. Any values not redefined in the new code for the module remain unchanged.