How do i extract a sub string which is composed of certain characters from a string in python?

Question

For example, suppose I have a string "beabeefeab". I want to extract a substring which is composed of only 'b' and 'a' that is "babab".

I applied brute force by implementing a nested loop and deleting all characters but 'b' and 'a'

Answer 1

You can do it using a simple list comprehension

a = "beabeefeab"
print("".join([i for i in a if (i == 'a' or i =='b')]))

Output:

babab

Answer 2

Not very elegant, but it works.

a = "beabeefeab"
answer = ""
for char in a:
    if char == "a" or char == "b":
        answer += char

print(answer)

Output

babab

Answer 3

Using a set to keep the allowed chars, making this a bit more extensible:

s = "beabeefeab"
allowed = set('ab')
print("".join(x for x in s if x in allowed))