Overriding True and False ?
python at deborahswanson.net
Mon Jan 30 02:50:21 EST 2017
Irv Kalb wrote, on Sunday, January 29, 2017 9:04 PM
> I teach intro to programming using Python. In my first
> assignment, students are asked to assign variables of
> different types and print out the values.
> One student (who really did not understand Booleans) turned
> in the following for his/her interpretation of Booleans (Python 2.7):
> True = 'shadow'
> False = 'light'
> print "If the sun is behind a cloud, there is", True
> print "If it is a clear day, there is", False
> And it printed:
> If the sun is behind a cloud, there is shadow
> If it is a clear day, there is light
> It seems very odd that Python allows you to override the
> values of True and False. In the code, True and False were
> clearly recognized as keywords as they were colored purple.
> But there was no error message.
> You cannot assign new values to other keywords. Simple tests
> of things like:
> for = 5
> while = 2
> not = 3
> As expected, all result in SyntaxError: invalid syntax. Why
> would Python allow you to override the values of True and
> False? I wonder if this is some sort of historical thing as
> these are the only keywords besides None that are uppercased.
> This line:
> None = 5
> Even gives a special SyntaxError: cannot assign to None
> Just curious,
Just guessing, but in the examples you give in Python 2.7, substitute
strings are syntactically correct in print statements, but:
5 in list('abc'):
if a 3 b:
would all be syntactical errors.
As is 'None = 5'.
Looks like the moral of the story is that in Python 2.7 you can redefine
keywords, so long as you don't get any syntax errors after (or during)
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