Cross-list comprehension in Python

Question

Let's say I have two lists of strings:

a = ['####/boo', '####/baa', '####/bee', '####/bii', '####/buu']

where #### represents 4-digit random number. And

b = ['boo', 'aaa', 'bii']

I need to know which string entry in list a contains any given entry in b. I was able to accomplish this by couple of nested loops and then using the in operator for checking the string contains the current entry in b. But, being relatively new to py, I'm almost positive this was not the most pythonic or elegant way to write it. So, is there such idiom to reduce my solution?

Answer 1

The following code gives you an array with the indexes of a where the part after the slash is an element from b.

a_sep = [x.split('/')[1] for x in a]
idxs = [i for i, x in enumerate(a_sep) if x in b]

To improve performance, make b a set instead of a list.

Demo:

>>> a = ['####/boo', '####/baa', '####/bee', '####/bii', '####/buu']
>>> b = ['boo', 'aaa', 'bii']
>>> a_sep = [x.split('/')[1] for x in a]
>>> idxs = [i for i, x in enumerate(a_sep) if x in b]
>>> idxs
[0, 3]
>>> [a[i] for i in idxs]
['####/boo', '####/bii']

---

If you prefer to get the elements directly instead of the indexes:

>>> a = ['####/boo', '####/baa', '####/bee', '####/bii', '####/buu']
>>> b = ['boo', 'aaa', 'bii']
>>> [x for x in a if x.split('/')[1] in b]
['####/boo', '####/bii']