Overriding True and False ?

Deborah Swanson 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,
> 
> Irv

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'):

2 True:

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)
redefinition.



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