I'm using ternary operator for short conditional variable definition. I was wondering when the expression returned True instead given in the expression value.
>>> digits = '123456'
>>> conv_d = digits != None if int(digits) else None
>>> conv_d
>>> True
>>> int(digits)
>>> 123456
Explain to me please, how this happens? What is the logical difference between a ternary operator and regular conditional expression in Python?
int(digits) == 123456 which is a true-ish value. So conv_d = digits != None. Since digits is not None, conv_d is set to true.
You probably wanted this:
conv_d = int(digits) if digits is not None else None
Remember that a string containing something not a number will raise an exception though! If you prefer 0 or None for those values, write a small function:
def toint(s):
try:
return int(s)
except (ValueError, TypeError):
return None # or 0
The Python conditional operator is not in the same order as in other languages. And you should never compare equality with None unless you're certain that you need to.
conv_d = int(digits) if digits is not None else None
Alternatives to ternary-operators:
conv_d = digits != None and int(digits) or None #short-circuiting; just wrong in this context as it doesn't work for digits = "0" - please see comment(s) below
or
conv_d = int(digits) if digits is not None else None
The second expression is clearer, hence preferred.
int(digits) is 0. Both will fail if digits doesn't compare equality fairly with None.
Notice how the keywords are placed. "X if Y else Z". "Y" is the part that follows "if", so it is the condition. "X if Y" is a perfectly valid (though less common) English construction meaning the same thing as "if Y, X", thus X is the expression evaluated when Y is satisfied. Z is the expression evaluated when Y is not satisfied, similarly.
