#!/usr/bin/env python
# coding: utf-8

# # Trying to clear object references in the 'last traceback' in IPython
# 
# IPython holds a reference (in a few places) on the last error and its traceback.
# This can cause problems, because any object references in any frame in the traceback will not get garbage collected naturally.
# 
# If we can find the references, we can clean them up (and maybe suggest some improvements to IPython)
# 
# Prompted by https://mastodon.social/@hannorein/113697884922216639
# 
# Setup: an object that raises an exception in a method

# In[1]:


import sys

class Test:
    def exception(self):
        raise ValueError("test")

    def __del__(self):
        print("freed!")

t = Test()
print(sys.getrefcount(t))


# In[2]:


t.exception()


# In[3]:


print(sys.getrefcount(t))


# In[4]:


del t


# That should have printed 'freed', but it didn't.
# This is because the last error's traceback still holds a reference on `t`
# and IPython has references to the traceback in 3 places:
# 
# 1. `exception.__traceback__` on the exception raised (easier than clearing every reference to the Exception itself)
# 2. `sys.last_traceback` for debugger support
# 3. `get_ipython().InteractiveTB.tb` which is a cache value, and probably a bug that the value is still held
# 
# If we clear these, the refcount goes back to normal and `t` is freed

# In[5]:


# clear held references to the latest traceback
import gc

sys.last_value.__traceback__ = None
sys.last_traceback = None
get_ipython().InteractiveTB.tb = None
# gc.collect seems to be needed to resolve some cycles somewhere
gc.collect();


# We can use IPython's `post_run_cell` hook to do this automatically on every cell execution,
# so you can put this in your `ipython_config.py`:

# In[6]:


# do the above automatically when a cell fails
import gc
import sys
import traceback

def clear_last_tb(result):
    if result.error_in_exec:
        result.error_in_exec.__traceback__ = None
        sys.last_traceback = None
        get_ipython().InteractiveTB.tb = None
        gc.collect()
    
get_ipython().events.register("post_run_cell", clear_last_tb)


# In[7]:


t = Test()
t.exception()


# In[8]:


print(sys.getrefcount(t))
del t


# Now we have enough information to propose an enhancement to IPython itself, perhaps.