Why does this list comprehension only "sometimes" work?

>>> l = [] 
>>> for x in range(10):
...     l.append(x**2)
>>> l
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> list(map(lambda x: x**2, range(10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> [x**2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> [x.strip() for x in ('foo\n', 'bar\n', 'baz\n')]
['foo', 'bar', 'baz']

>>> [int(x) for x in ('1', '2', '3')]
[1, 2, 3]

>>> d = {'foo': '10', 'bar': '20', 'baz': '30'}
>>> [d[x] for x in ['foo', 'baz']]
['10', '30']

>>> d = {'foo': '10', 'bar': '20', 'baz': '30'}
>>> [int(d[x].rstrip('0')) for x in ['foo', 'baz']]
[1, 3]

>>> l = []
>>> for x in range(10):
...     if x%2:
...         l.append(x)
>>> l
[1, 3, 5, 7, 9]

>>> [x for x in range(10) if x%2]
[1, 3, 5, 7, 9]

>>> [i if i%2 != 0 else None for i in range(10)]
[None, 1, None, 3, None, 5, None, 7, None, 9]

>>> [i for x in [[1, 2, 3], [4, 5, 6]] for i in x]
[1, 2, 3, 4, 5, 6]

>>> [j for x in [[[1, 2], [3]], [[4, 5], [6]]] for i in x for j in i]
[1, 2, 3, 4, 5, 6]

>>> [j for j in x for x in [[1, 2, 3], [4, 5, 6]]]
Traceback (most recent call last):
  File "<input>", line 1, in <module>
NameError: name 'x' is not defined

>>> {x for x in [1, 1, 2, 3, 3, 1]}
{1, 2, 3}

>>> set([i for i in [1, 1, 2, 3, 3, 1]])
{1, 2, 3}

>>> {i: i**2 for i in range(5)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

>>> d = {}
>>> for i in range(5):
...     d[i] = i**2
>>> d
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

>>> (i for i in range(5))
<generator object <genexpr> at 0x7f52703fbca8>

>>> def gen():
...     for i in range(5):
...         yield i
>>> gen()
<generator object gen at 0x7f5270380db0>

>>> gen = (i for i in range(5))
>>> next(gen)
0
>>> next(gen)
1
>>> list(gen)
[2, 3, 4]
>>> next(gen)
Traceback (most recent call last):
  File "<input>", line 1, in <module>
StopIteration

>>> sum(i**2 for i in range(5))
30

filter(None, (x.strip() for x in str_list))

[y for y in (x.strip() for x in str_list) if y]

for element in g:
    print(element)

for x in q:
    for y in p:
        yield [x, y]

friends = ["Wolf", "Frootie", "charlean", "Jenny"]
guests = ["xavier", "Bobbie", "wolf", "Charlean", "ashley"]

#friends_lcase = [f.lower() for f in friends]
friends_lcase = []
for f in friends:
    friends_lcase.append(f.lower()) <--- Change

# present_friends = [
#   name.title() for name in guests if name.lower() in friends_lcase
# ]

present_friends = []
for name in guests:
    if name.lower() in friends_lcase:
        present_friends.append(name.title())


print(present_friends)