I'm trying to build a Boolean expression, based on (unpredictable) user input. I find that I'm building a string that looks proper, but doesn't work. I've looked at python.org, Google and Stackoverflow and couldn't find what goes wrong here.
Example of the code:
print stringDing
newVariable = stringDing.replace('False','0')
print newVariable
print bool(newVariable)
Output from this:
False or False or False
0 or 0 or 0
True
Yet when the string is pasted into python, python responds as expected:
>>> False or False or False
False
I think I need to build this as a string because the user can add 'OR', 'AND' and brackets, which I would need to fit in at the proper place.
How to proceed?
Interpreting a non-empty string as bool will always evaluate to True. I.e.:
print bool("False") # True
print bool("0") # True
This is, because a str is an iterable object (such as list, set or dict). All iterable objects are considered to be True, if they are non-empty. A str is an iterable, that iterates over its characters. This is useful, e.g. if you want to test if a string s is empty. In such a case you can just write:
if s:
# do something with non-empty string s.
However, if you want to evaluate the expression that is represented by the string, then call eval:
print eval("False") # False
print eval("0") # 0
print bool(eval("0")) # False