Can you use AND in List comprehension conditional statements?

Question

I am trying to use List Comprehension to perform the following. I want to make a new list (unique) that only has the common numbers from both lists.

unique = []
for listcomp in range(len(list1)):
    if list1[listcomp] in list2 and list1[listcomp] not in unique:
        unique.append(list1[listcomp])
    else:
        continue

Above works fine but when I create the List comprehension below I get duplicates if list1 has duplicate numbers. ie list1 = [1, 1, 2], list2 = [1, 5]. I created my list comprehension as

unique = [list1[listcomp] for listcomp in range(len(list1)) if list1[listcomp] in list2 and list1[listcomp] not in unique]

If I'm getting duplicates I assume the "and" statement isn't being applied? I have read other queries about moving the if statement further up the comprehension statement but this didn't work. Can you use AND to extend your conditions?

Many thanks

My full code is:-

import random as rnd
# Randomly generate the size of your list
list1size = rnd.randint(1,20)
list2size = rnd.randint(1,20)
# Declare your list variables
list1 = []
list2 = []
# Fill your lists with randomly generated numbers upto the listsize generated above
for x in range(list1size):
    list1.append(rnd.randint(1,15))
for y in range(list2size):
    list2.append(rnd.randint(1,15))
# Not required but easier to read lists once sorted
list1.sort()
list2.sort()
print(list1)
print(list2)
# Now to compare old school

unique = []
# for listcomp in range(len(list1)):
#     if list1[listcomp] in list2 and list1[listcomp] not in unique:
#         unique.append(list1[listcomp])
#     else:
#         continue

# Now to compare with list comprehension
unique = [list1[listcomp] for listcomp in range(len(list1)) if list1[listcomp] in list2 and list1[listcomp] not in unique]
# Above doesn't stop duplicates if they are in List1 so I assume you can't use AND        

print(f"The common numbers in both lists are {unique}")

Answer 1

You can't access elements produced by a list comprehension as you go along. Your condition list1[listcomp] not in unique will always return True since at that moment in time unique is defined as the empty list intialised in unique = [] .

So the and statement is being applied, but not the in way you want.

Instead, you can create a "seen" set holding items you have already found and omit them. The standard implementation is found in the itertools unique_everseen recipe .

If you have the 3rd party toolz library, you can use the identical toolz.unique and feed a generator expression. More Pythonic, you can iterate elements directly rather than using indices:

from toolz import unique

unique = list(unique(i for i in list1 if i in list2))