Python substring in string

Question

Suppose this code

x = "boo"
if "a" or "z" in x:
    print(True)
else:
    print(False)

returns True.

But why? I expected it to return False, becuause neither a nor z is in x. Do I misunderstand in?

I frequently use in to see if a string contains a single substring. E.g.

x = "eggs and spam"
if "spam" in x:
    return "add sausage"

voted to reopen as the duplicate offers no explanation why this does not work. — hiro protagonist
– hiro protagonist, Commented Mar 11, 2019 at 13:43

hiro protagonist · Accepted Answer · 2019-03-11 13:36:22Z

4

"a" or "z" in x is evaluated as "a" or ("z" in x) (see operator precedence to understand why)
"z" in x is False for x = "boo"
"a" or False is True (as "a" is a non-empty string; bool("a") = True).

what you mean to do is this:

if "a" in x or "z" in x:
    ...

if you are comfortable working with sets you could also try this:

if set("az") & set(x):
    ...

answered Mar 11, 2019 at 13:13

hiro protagonist

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