diff --git a/docs/source/trainer.rst b/docs/source/trainer.rst
index 19c394db48..393650cceb 100644
--- a/docs/source/trainer.rst
+++ b/docs/source/trainer.rst
@@ -9,7 +9,7 @@ Trainer
    :exclude-members:
         run_pretrain_routine,
         _abc_impl,
-        _Trainer__set_random_port,
+        _Trainer_set_random_port,
         _Trainer__set_root_gpu,
         _Trainer__init_optimizers,
         _Trainer__parse_gpu_ids,
diff --git a/pytorch_lightning/trainer/distrib_data_parallel.py b/pytorch_lightning/trainer/distrib_data_parallel.py
index 3b94c9ae3b..68629dc8c1 100644
--- a/pytorch_lightning/trainer/distrib_data_parallel.py
+++ b/pytorch_lightning/trainer/distrib_data_parallel.py
@@ -369,7 +369,7 @@ class TrainerDDPMixin(ABC):
         # don't make this debug... this is good UX
         rank_zero_info(f'CUDA_VISIBLE_DEVICES: [{os.environ["CUDA_VISIBLE_DEVICES"]}]')
 
-    def __set_random_port(self):
+    def set_random_port(self):
         """
         When running DDP NOT managed by SLURM, the ports might collide
         """
@@ -384,7 +384,6 @@ class TrainerDDPMixin(ABC):
         os.environ['MASTER_PORT'] = str(default_port)
 
     def spawn_ddp_children(self, model):
-        self.__set_random_port()
         port = os.environ['MASTER_PORT']
 
         master_address = '127.0.0.1' if 'MASTER_ADDR' not in os.environ else os.environ['MASTER_ADDR']
diff --git a/pytorch_lightning/trainer/trainer.py b/pytorch_lightning/trainer/trainer.py
index 35027cf3c1..cd77f1caa8 100644
--- a/pytorch_lightning/trainer/trainer.py
+++ b/pytorch_lightning/trainer/trainer.py
@@ -897,18 +897,19 @@ class Trainer(
                 self.ddp_train(task, model)
 
             elif self.distributed_backend == 'cpu_ddp':
-                self.__set_random_port()
+                self._set_random_port
                 self.model = model
                 mp.spawn(self.ddp_train, nprocs=self.num_processes, args=(model,))
 
             elif self.distributed_backend == 'ddp_spawn':
-                self.__set_random_port()
+                self._set_random_port
                 model.share_memory()
 
                 # spin up peers
                 mp.spawn(self.ddp_train, nprocs=self.num_processes, args=(model, ))
 
             elif self.distributed_backend == 'ddp':
+                self._set_random_port
                 self.spawn_ddp_children(model)
 
         # 1 gpu or dp option triggers training using DP module
@@ -1273,7 +1274,6 @@ class _PatchDataLoader(object):
         dataloader: Dataloader object to return when called.
 
     """
-
     def __init__(self, dataloader: Union[List[DataLoader], DataLoader]):
         self.dataloader = dataloader