“return someString and partString in someString” in Python3

Question

test1 = "this is test line"  # non-empty string
test2 = ""                   # empty string

def test_line(line):
    return line and not "is" in line

test1_result = test_line(test1)
test2_result = test_line(test2)

print(test1_result)  # True
print(test2_result)  # empty line printed

I've seen several ppl using this kind of code for boolean output

return someString and not partString in someString # "not" is optional

Why test2 results in empty line? And by adding someString as an argument what additdional function we get compared to just using partString in someStirng ?

Answer 1

In your second function call with an empty string, when you essentially do

return "" and not "is" in ""

the and expression gets short-circuited due to the first condition evaluating as False essentially (you don't need both conditions to fail for an and to fail) and hence it gets returned as an empty string. The second boolean condition won't be evaluated.

>>> bool("")
False
>>> "" and 1 == 1/0
''
>>> False and 1 == 1/0
False

But when you do it with a non-empty string, both parts of the boolean expression would be evaluated and thus giving exception, as expected

>>> bool("b")
True
>>> "b" and 1 == 1/0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
>>> "b" and 1 == 1
True