Document that IDLE -python difference for `del __builtins__`

[pppery]

BPO	25564
Nosy	@terryjreedy, @stevendaprano, @pppery

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

Show more details

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = <Date 2016-08-25.05:29:29.052>
created_at = <Date 2015-11-05.23:26:04.761>
labels = ['interpreter-core', 'expert-IDLE', 'type-bug', 'docs']
title = 'Document that IDLE -python difference for `del __builtins__`'
updated_at = <Date 2016-08-25.06:18:19.663>
user = 'https://github.com/pppery'

bugs.python.org fields:

activity = <Date 2016-08-25.06:18:19.663>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = True
closed_date = <Date 2016-08-25.05:29:29.052>
closer = 'terry.reedy'
components = ['Documentation', 'IDLE', 'Interpreter Core']
creation = <Date 2015-11-05.23:26:04.761>
creator = 'ppperry'
dependencies = []
files = []
hgrepos = []
issue_num = 25564
keywords = []
message_count = 10.0
messages = ['254149', '254150', '254151', '254153', '254173', '273594', '273617', '273625', '273627', '273629']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'steven.daprano', 'python-dev', 'ppperry']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25564'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']

[pppery]

In IDLE the following code silently works:
>>> del __builtins__
>>> min
<built-in function min>

In the standard interpreter, it produces an error:
>>> del __builtins__
>>> min
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'min' is not defined

Note that saying __builtins__ = 7 fails in idle as well.

[pppery]

If you type del __builtins__;min an error is raise in both IDLE and the standard interpreter.

[pppery]

`del __builtins__;min` only fails in IDLE if someone has previously set `__builtins__ to something else.
>>>__builtins__ = 7
>>> min
Traceback (most recent call last):
  File "<pyshell#352>", line 1, in <module>
    min
NameError: name 'min' is not defined
>>>del __builtins__;min
Traceback (most recent call last):
  File "<pyshell#353>", line 1, in <module>
    del __builtins__;min
NameError: name 'min' is not defined
>>del __builtins__;min
<built-in function min>

[stevendaprano]

__builtins__ is a private implementation detail in CPython. There is no guarantees made about whether it exists or not. E.g. it doesn't exist in Jython.

steve@orac:~/python$ jython
Jython 2.5.1+ (Release_2_5_1, Aug 4 2010, 07:18:19)
[OpenJDK Server VM (Sun Microsystems Inc.)] on java1.6.0_31
Type "help", "copyright", "credits" or "license" for more information.
>>> __builtins__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name '__builtins__' is not defined

You should use __builtin__ in Python 2 and builtins in Python 3. Anything you do to __builtins__ with an S is implementation-dependent.

I don't think it is a bug that CPython behaves differently regarding __builtins__ depending on whether IDLE is running or not.

[terryjreedy]

We try to have IDLE work the same as Python (and CPython, where relevant), except where differences are intentional or unavoidable. I am trying to eliminate some unintentional avoidable differences in other issues. However, this one is unavoidable given IDLE's basic design. Also, Steven is correct; see
https://docs.python.org/3/library/builtins.html#module-builtins

IDLE executes user code with "exec(code, self.locals)" (run.py, l.351 in 3.5).
https://docs.python.org/3/library/functions.html#exec
says "If the globals dictionary does not contain a value for the key __builtins__, a reference to the dictionary of the built-in module builtins is inserted under that key."

(Doc issus: From the builtins doc and the Jython example, this seems implementation dependent. Steven, does
  >>> d = {}; exec('', d); d.keys()
  dict_keys(['__builtins__'])
have this result in Jython?)

In the IDLE shell, each statement is exec'ed separately. With two statements, __builtins__ is added back before 'min' , while with one statement, it is not.

Editor: Run Module execs the entire file at once. I expected print(min) to fail either way, but it works either way. I verified that globals().keys() lost '__builtins__', so I don't know how __builtins__.min is found.

I left this open to consider adding a line to
https://docs.python.org/3/library/idle.html#idle-console-differences

[terryjreedy]

Steven: "You should use __builtin__ in Python 2 and builtins in Python 3." I presume this is for import statements.

ppperry: Titles should fit in the box, so they fit in search listing results.

I am thinking of something like "Since Python inserts '__builtins__' into the exec global namespace when not present and IDLE uses exec, '__builtins__' is defined at the start of each statement or file even when it otherwise would not be."