Why does Python throw an error when a substring is not found?

Question

What's the design thinking behind this?

To me it's easier to do something like

if string.index(substring) > -1:
    # do stuff

than trying to catch an exception. If the substring is not found, at least your program doesn't break.

Someone told me 'returning -1 is a bad pattern'. Why is that?

What is the Pythonic way for checking substring?

Answer 1

str.index() throws an exception; you were perhaps thinking of the str.find() method instead:

if string.find(substring) > -1:

The str.index() documentation explicitly states this:

Like find(), but raise ValueError when the substring is not found.

However, the correct way to test for substring membership is to use in:

if substring in string:

Answer 2

Because this way at least the return type of the function is fixed. Also your example is not pythonic, it should read:

if str in string:
    # do stuff