How do I undo True = False in python interactive mode?

Question

So I tried the "evil" thing Ned Deily mentioned in his answer here. Now I have that the type True is now always False. How would I reverse this within the interactive window?

Thing to not do:

True = False

Since True has now been completely overridden with False, there doesn't seem to be an obvious way to back-track. Is there a module that True comes from that I can do something like:

True = <'module'>.True

All this does is create a local variable named `True`, which hides the builtin constant. The "real" `True` is unaffected, and can still be created e.g. with `(1 == 1)` or the `bool()` function. It also lives in `__builtins__.True`. — Kevin, May 31 '15 at 23:27
Note that this is not possible in Python 3, which treats `True` and `False` as keywords. — Lambda Fairy, May 31 '15 at 23:53
I think you can still use `True = not False` or in this case also `True = not True` to recover. Otherwise, as stated in an answer below, use `del True` to recover the old value. — Simon Hessner, Aug 08 '18 at 09:07

score 137 · Accepted Answer · answered May 31 '15 at 23:23

137

You can simply del your custom name to set it back to the default:

>>> True = False
>>> True
False
>>> del True
>>> True
True
>>>

answered May 31 '15 at 23:23

4

So the `True = False` command actually creates a new, local name that shadows the keyword, rather than actually replacing a variable? And this just cleans it up? – jpmc26 Jun 01 '15 at 19:18
10

@jpmc26 - Yes, that is correct. Note however that `True` and `False` are not keywords in Python 2.x; they are builtins. That is why this is possible. In Python 3.x, where `True` and `False` *are* keywords, `True = False` would raise a syntax error because you cannot assign to a keyword. – Jun 01 '15 at 19:21
5

Interesting to note that this fails if `__builtins__.True` has been changed, as hinted at in the answer from @simonwo. – 101 Jun 01 '15 at 21:51
7

@figs - Yea, `del` will not work if you've modified `__builtins__`. In that case, it would be best to do `True = 1 == 1` like Roberto Bonvallet proposed. Of course, if you are already going so far as to modify `__builtins__`, then you could also do `del __builtins__` and really ruin the interpreter. At some point, it might be best to just restart it instead of trying to repair it piecemeal. :) – Jun 01 '15 at 22:13
@iCodez del assignment – PyRulez Jun 02 '15 at 01:34
`del` works to recover other builtins too, `list` being a common one to accidentally shadow when mucking around. – Nick T Jun 02 '15 at 03:46

101 · Answer 2 · 2015-06-02T03:54:02.207

47

This works:

>>> True = False
>>> True
False
>>> True = not False
>>> True
True

but fails if False has been fiddled with as well. Therefore this is better:

>>> True = not None

as None cannot be reassigned.

These also evaluate to True regardless of whether True has been reassigned to False, 5, 'foo', None, etc:

>>> True = True == True   # fails if True = float('nan')
>>> True = True is True
>>> True = not True or not not True
>>> True = not not True if True else not True
>>> True = not 0

edited Jun 02 '15 at 03:54

answered May 31 '15 at 23:25

101

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As much as I like this, I think iCodez's is the more general solution to these override type issues. – horta May 31 '15 at 23:28
Especially since this falls apart if you do `True, False = False, True` instead. – Kevin May 31 '15 at 23:28
2

@Kevin `True, False = not True, True` fixes that. As long as you have a reference to either `True` or `False` you can deduce the other it seems. – 101 May 31 '15 at 23:35
5

@figs in fact, if you've done `True, False = False, True`, you can just do it again, and you'll be back to the same place. – porglezomp Jun 01 '15 at 02:13
1

`True == True` will not work if `True` has been redefined to be `float('nan')`: `True == True → False` – sleblanc Jun 01 '15 at 14:35
1

@sebleblanc However, `True is True` does return `True` even then (though `float('nan') is float('nan')` is still `False`). – Reinstate Monica -- notmaynard Jun 01 '15 at 19:58
Right. `1 == 1` works too, and `False = 2 + 2 == 5`. – sleblanc Jun 01 '15 at 20:32

score 37 · Answer 3 · answered Jun 01 '15 at 15:13

37

Another way:

>>> True = 1 == 1
>>> False = 1 == 2

answered Jun 01 '15 at 15:13

Roberto Bonvallet

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That's a pretty creative, yet simple solution. I'm impressed. – Jun 01 '15 at 19:16

score 17 · Answer 4 · answered Jun 01 '15 at 11:54

For completeness: Kevin mentions that you could also fetch the real True from __builtins__:

>>> True = False
>>> True
False
>>> True = __builtins__.True
>>> True
True

But that True can also be overriden:

>>> __builtins__.True = False
>>> __builtins__.True
False

So better to go with one of the other options.

score 11 · Answer 5 · answered Jun 01 '15 at 06:52

11

Just do this:

True = bool(1)

Or, because booleans are essentially integers:

True = 1

answered Jun 01 '15 at 06:52

Loovjo

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`bool = int`. Oops! `bool(1) → 1`. – sleblanc Jun 01 '15 at 14:25
1

@sebleblanc I meant that they work in the same way – Loovjo Jun 01 '15 at 14:26
And I mean that your answer does not work in all cases, in the same way that figs and simonwo's answers have pitfalls. If bool was also redefined, you cannot get the value `True` from that function. – sleblanc Jun 01 '15 at 14:33
On the other hand, if you don't explicitly need the True or False values, `True = 1` is a foolproof way of getting boolean-like behaviour. – sleblanc Jun 01 '15 at 14:37
`True = 1` violates some essential properties, e.g. check `type()`. – Karoly Horvath Jun 02 '15 at 08:53

PythonNut · Answer 6 · 2015-06-02T03:01:15.393

2

Solutions that use no object literals but are as durable as 1 == 1. Of course, you can define False once True is defined, so I'll supply solutions as half pairs.

def f(): pass
class A(): pass
True = not f()
False = A != A

False = not (lambda:_).__gt__(_)
True = not (lambda:_).__doc__

edited Jun 02 '15 at 03:01

answered Jun 02 '15 at 02:53

PythonNut

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How do I undo True = False in python interactive mode?

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