A doubt with 'in' and 'not in' operators with strings

[newbieAuggie2019]

Hi!

I was just reading that the 'in' and 'not in' operators can be used with strings just like with list values and that an expression with two strings joined using 'in' or 'not in' will evaluate to a Boolean True or False.

That's fine with me, but I cannot understand the logic of the first case of these inputs and outputs:

Output:
>>> '' in 'spam'
True
>>> '' in ' spam'
True
>>> '' in 'spam '
True
>>> '' in ' spam '
True
>>> 

I can understand why "" is True in the last 3 cases ...

Wait! I think my light bulb has just turned on ... I thought it meant this:

Output:
>>> ' ' in 'spam'
False
>>> ' ' in ' spam'
True
>>> ' ' in 'spam '
True
>>> ' ' in ' spam '
True
>>> 

but probably the concept of "" is similar to 'the empty set' in set theory, so:

For any set A: 
The empty set is a subset of A: 
∀ A : ∅ ⊆ A 

https://en.m.wikipedia.org/wiki/Empty_set

So dealing with strings, an empty string (represented by "" or '') is included in any string, like an empty set is included in any set.

Is that it?

All the best,

[ichabod801]

I don't know if that was Guido's reasoning, but that is certainly the behavior. The empty string is in every string, to the point of being in the empty string.

[newbieAuggie2019]

The empty string is in every string, to the point of being in the empty string.

Thank you!

One doubt less!!!

All the best,

[perfringo]

This behaviour pointed out in documentation: Membership test operations

Empty strings are always considered to be a substring of any other string, so "" in "abc" will return True.

One way of mentally parsing it will be remember that slicing string will return empty string even if index is out of range (and not None or whatever) and you can verify that there is empty string in every string and every character in string is considered empty string as well:

>>> a = ''
>>> a[0:0]
''
>>> a[0:]
''
>>> a[1:]
''
>>> a[7:42]
''
>>> ''.count('')
1
>>> 'a'.count('')
2
>>> 'ab'.count('')
3
>>> 'abc'.count('')
4

[newbieAuggie2019]

Empty strings are always considered to be a substring of any other string [ ... ].

Thank you for your answer!

I can now understand that, comparing it to 'set theory'.

[ ... ] there is empty string in every string [ ... ]

I can now understand that, for the same reason as before.

[ ... ] and every character in string is considered empty string as well

Wow! That's a bit more difficult for me to comprehend. I can understand that 'every character is a string' and that from the axiom that 'there is empty string in every string' or expressed in other terms, that 'Empty strings are always considered to be a substring of any other string', I can see as a corollary that 'therefore any string of any character HAS an empty string in it', but I cannot manage to see 'any string of any character BEING an empty string'. I'm not sure if I have clearly explained myself.

All the best,

[ichabod801]

I think this may help:

For the string and bytes types, x in y is True if and only if x is a substring of y. An equivalent test is y.find(x) != -1. Empty strings are always considered to be a substring of any other string, so "" in "abc" will return True.

The thing is, y.find('') is always 0. So every string sort of starts with an empty string. And so does every substring, as 'abc'.find('', 1) is 1 and 'abc'.find('', 2) is 2 and 'abc'.find('', 3) is 3.

[newbieAuggie2019]

I think this may help:

For the string and bytes types, x in y is True if and only if x is a substring of y. An equivalent test is y.find(x) != -1. Empty strings are always considered to be a substring of any other string, so "" in "abc" will return True.

Thank you!

[perfringo]

I agree that ichabod801 explanation is much better.