[FIXED] What is '_oh' in iPython locals()? And how to get locals back or create new one in standard python IDLE

Issue

I was playing around with namespaces in IPython (Spyder), and tried to see what happens if I dict.clear() locals(). So, without further ado:

Python 3.8.5 (default, Aug  5 2020, 09:44:06) [MSC v.1916 64 bit (AMD64)]
Type "copyright", "credits" or "license" for more information.

IPython 7.19.0 -- An enhanced Interactive Python.

In [1]: locals().clear()

In [2]: locals
Traceback (most recent call last):

  File "<ipython-input-2-f1c14746c80d>", line 1, in <module>
    locals

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 263, in __call__
    self.update_user_ns(result)

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 201, in update_user_ns
    if self.cache_size and result is not self.shell.user_ns['_oh']:

KeyError: '_oh'


In [3]: dict
Traceback (most recent call last):

  File "<ipython-input-3-0d8c7dca5f1a>", line 1, in <module>
    dict

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 263, in __call__
    self.update_user_ns(result)

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 201, in update_user_ns
    if self.cache_size and result is not self.shell.user_ns['_oh']:

KeyError: '_oh'




In [4]: globals
Traceback (most recent call last):

  File "<ipython-input-6-0d1754e6861d>", line 1, in <module>
    globals

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 263, in __call__
    self.update_user_ns(result)

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 201, in update_user_ns
    if self.cache_size and result is not self.shell.user_ns['_oh']:

KeyError: '_oh'

As builtins have separate namespaces, tried deleting local:

In [5]: del locals
Traceback (most recent call last):

  File "<ipython-input-5-61828b7e8872>", line 1, in <module>
    del locals

NameError: name 'locals' is not defined

Expected, as the namespace itself got deleted.

I tried importing builtins and assigning to __builtins__

In [6]: import builtins
In [7]: __builtins__ = builtins
In [8]: __builtins__.dict
Traceback (most recent call last):

  File "<ipython-input-11-8211c7f1d719>", line 1, in <module>
    __builtins__.dict

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 263, in __call__
    self.update_user_ns(result)

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 201, in update_user_ns
    if self.cache_size and result is not self.shell.user_ns['_oh']:

KeyError: '_oh'

At this point I noticed all the errors were attributed to _oh. So I restarted the kernel, and checked locals:

locals()
Out[1]: 
{'__name__': '__main__',
 '__doc__': 'Automatically created module for IPython interactive environment',
 '__package__': None,
 '__loader__': None,
 '__spec__': None,
 '__builtin__': <module 'builtins' (built-in)>,
 '__builtins__': <module 'builtins' (built-in)>,
 '_ih': ['', 'locals()'],
 '_oh': {},
 '_dh': ['D:\\Programs\\Python\\StackOv'],
 'In': ['', 'locals()'],
 'Out': {},
 'get_ipython': <bound method InteractiveShell.get_ipython of <ipykernel.zmqshell.ZMQInteractiveShell object at 0x000002C2953F37F0>>,
 'exit': <IPython.core.autocall.ZMQExitAutocall at 0x2c295420af0>,
 'quit': <IPython.core.autocall.ZMQExitAutocall at 0x2c295420af0>,
 '_': '',
 '__': '',
 '___': '',
 '_i': '',
 '_ii': '',
 '_iii': '',
 '_i1': 'locals()'}

So _oh seems to be an empty dictionary. Now:

In [2]: locals().clear()

In [3]: dict
Traceback (most recent call last):

  File "<ipython-input-3-0d8c7dca5f1a>", line 1, in <module>
    dict

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 263, in __call__
    self.update_user_ns(result)

  File "C:\Users\sayan\Anaconda3\envs\tfgpu_py38\lib\site-packages\IPython\core\displayhook.py", line 201, in update_user_ns
    if self.cache_size and result is not self.shell.user_ns['_oh']:

KeyError: '_oh'


In [4]: locals()['_oh'] = {}

In [5]: dict
Out[5]: dict

So I found how to get locals back, but did not understand why assigning an empty dict as _oh brought back locals.

But the same thing did not work in python IDLE:

Python 3.7.7 (default, May  7 2020, 21:25:33)
[GCC 7.3.0] :: Anaconda, Inc. on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> locals()
{'__name__': '__main__', '__doc__': None, '__package__': None, '__loader__': <class '_frozen_importlib.BuiltinImporter'>, '__spec__': None, '__annotations__': {}, '__builtins__': <module 'builtins' (built-in)>}
>>> locals().clear()
>>> locals()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'locals' is not defined
>>> dict
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'dict' is not defined
>>> import builtins
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: __import__ not found
>>> __builtin__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__builtin__' is not defined
>>>

---

There's no _oh and even the __import__ doesn't work.

• Question 1: If assignment (__builtins__ = builtins) worked, why couldn't I access builtins?
• Question 2: What is _oh in iPython and how does it help bring locals back, being an empty dict itself.
• Question 3: How to get locals() working in python IDLE after locals().clear() (Without restarting the IDLE)

If Question 3 is too broad within the scope of this answer I can ask a separate question. Thank you.

NOTE: I tried IPython on Windows 10, and standard python prompt on linux (Ubuntu). Don't think this makes a difference, added it for the sake of completeness.

Solution

In a running ipython session:

In [109]: id(Out)
Out[109]: 139634741914432
In [110]: id(_oh)
Out[110]: 139634741914432

_oh is another name for the Out dictionary, which we can use as:

In [112]: Out[109]
Out[112]: 139634741914432
In [113]: _oh[109]
Out[113]: 139634741914432
In [114]: _109
Out[114]: 139634741914432

So why they use another name, I don't know. In any case by locals().clear() you wiped out that whole dictionary of variables you see in Out[1], and have effectively disabled the ipython interactive session.

I don't know of any value to using locals.clear(). It might 'safer' inside a function.

Magic %who shows the interactive variables, the ones your own code has created. There may be a way of selectively deleting those, but I haven't needed it. If I need a clean ipython session I just start one in a new shell tab or window.

I haven't used IDLE in a long time (don't even have it installed), so can't help with its locals(). It has a much simpler history mechanism.

Looking at the magics, there's a %reset that can be used to remove user defined variables, and even clear the In/Out history. Read its docs.

See also %who_ls and %reset_selective.

Answered By - hpaulj