PEP 805: Safe Parallel Python

markshannon · August 25, 2026, 4:05pm

Most aren’t safe with the GIL, like if self.attr is None: self.attr = initial_value() in this case, the race is present with the GIL, but it is much more likely in FT.
If the race manifests as a leak, or some other small effect, then the difference between the GIL build and FT could be running to completion vs not.

However, there are some cases where the GIL does prevent a race.

The “best” example I have found is this:

github.com/python/cpython

Lib/collections/init.py

main


      
              self.data = {}
              if dict is not None:
                  self.update(dict)
              if kwargs:
                  self.update(kwargs)
          
          def __len__(self):
              return len(self.data)
          
          def __getitem__(self, key):
              if key in self.data:
                  return self.data[key]
              if hasattr(self.__class__, "__missing__"):
                  return self.__class__.__missing__(self, key)
              raise KeyError(key)
          
          def __setitem__(self, key, item):
              self.data[key] = item
          
          def __delitem__(self, key):
              del self.data[key]

where (provided the keys are strings or other simple objects) the GIL prevents the race with __setitem__