Boolean expression in Python or a string? Always returns 'True'

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?

Solution

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