String vs list membership check

Question

So i'm wondering why this:

'alpha' in 'alphanumeric'

is True, but

list('alpha') in list('alphanumeric')

is False.

Why does x in s succeed when x is a substring of s, but x in l doesn't when x is a sublist of l?

Answer 1

When you use list function with any iterable, a new list object will be created with all the elements from the iterable as individual elements in the list.

In your case, strings are valid Python iterables, so

>>> list('alpha')
['a', 'l', 'p', 'h', 'a']
>>> list('alphanumeric')
['a', 'l', 'p', 'h', 'a', 'n', 'u', 'm', 'e', 'r', 'i', 'c']

So, you are effectively checking if one list is a sublist of another list.

In Python only Strings have the in operator to check if one string is part of another string. For all other collections, you can only use individual members. Quoting the documentation,

The operators in and not in test for collection membership. x in s evaluates to true if x is a member of the collection s, and false otherwise. x not in s returns the negation of x in s. The collection membership test has traditionally been bound to sequences; an object is a member of a collection if the collection is a sequence and contains an element equal to that object. However, it make sense for many other object types to support membership tests without being a sequence. In particular, dictionaries (for keys) and sets support membership testing.

For the list and tuple types, x in y is true if and only if there exists an index i such that x == y[i] is true.

For the Unicode and string types, x in y is true if and only if x is a substring of y. An equivalent test is y.find(x) != -1. Note, x and y need not be the same type; consequently, u'ab' in 'abc' will return True. Empty strings are always considered to be a substring of any other string, so "" in "abc" will return True.

Answer 2

For the second one you are asking if

['a', 'l', 'p', 'h', 'a'] in ['a', 'l', 'p', 'h', 'a', 'n', 'u', 'm', 'e', 'r', 'i', 'c']

and there is no sub-list in the second list only characters.

['a', 'l', 'p', 'h', 'a'] in [['a', 'l', 'p', 'h', 'a'], ['b', 'e', 't', 'a']]

would be true