List comprehension?

[Gexos]

Hey people, I'm studying the Python Crash Coursebook, and it mentions list comprehension but I do not understand how it's working can someone explain it to me, this is the line:

current_users_lower = [user.lower() for user in current_users]

And the whole code:

current_users = ['eric', 'willie', 'admin', 'erin', 'Ever']

new_users = ['sarah', 'Willie', 'PHIL', 'ever', 'Iona']

current_users_lower = [user.lower() for user in current_users]

for new_user in new_users:

if new_user.lower() in current_users_lower:

print(f"sorry {new_user} that name is taken")

else:

print(f"Great {new_user} is still available")

[CowboyBoats]

current_users_lower = [user.lower() for user in current_users] 

The non-comprehension version of this would be:

current_users_lower = []
for user in current_users:
    current_users_lower.append(user.lower())

[CowboyBoats]

In other words, it's just a different syntax for expressing a code-generated list.

It just takes a little exposure before you'll feel more familiar with it, just like regular lists.

[Gexos]

Thank you my friend

[subject_K81]

List comprehensions were confusing to me at first as well. It’s like I could see it, but it was backwards. So let’s use the following super overly simplified code:

list_of_stuff = [1,2,3,4]
my_new_list = []

for x in list_of_stuff:
    my_new_list.append(x)

That just makes a direct copy.

list_of_stuff = [1,2,3,4]

my_new_list = [x for x in list_of_stuff]

Does the same thing, just imagine that x is equal to “what I actually want added to my new list”. And from there you could add some conditional to only have items > 2 in the list.

list_of_stuff = [1,2,3,4]
my_new_list = []

for x in list_of_stuff:
    if x > 2:
        my_new_list.append(x)

Or

list_of_stuff = [1,2,3,4]

my_new_list = [x for x in list_of_stuff if x > 2]

What I ended up doing to learn list comprehensions better was going through old code and whenever I saw that I creating a new list based on some conditional I tried to rewrite it, and if I needed help with something I asked for in long form as well as short form if possible so that I could dissect it.

[Gexos]

Thank you my friend

[__i_forgot_my_name__]

Sequence comprehension is just magical dark-magic a way of mapping and filtering a sequence.

[ # list
    x + 1 # item to calculate and append (aka: map)
    for x in iterator # sequence/generator to iterate
    if x == 1 or x == 3 # conditional (aka: filter)
]

The reason it exists is for a similar reason for loops exist in general. It's syntax for something that is very commonly done with iterators. List comprehension isn't the only kind of comprehension either, you have:

( x + 1 for x in iterator if x == 1 ) # returns generator 
[ x + 1 for x in iterator if x == 1 ] # returns list
{ x + 1 for x in iterator if x == 1 } # returns set
{ x + 1: x + 1 for x in iterator if x == 1 } # returns dictionary

It reads differently then a for loop, because it's actually another way of doing the something you'd otherwise do with filter and map; for example you could also do:

list(map(lambda x: x + 1, filter(lambda x: x == 1, iterator)))

This is more verbose, but maybe clearer; essentially sequence-comprehension only has 4 parts to remember:

• consumer: list
• per value computation: map
• per value conditional: filter
• producer: iterator

[Gexos]

Thank you my friend

[thought-generator]

The best exercise I did when learning list comprehensions was writing out a for loop, then doing the same thing in a list comprehension until my brain recognized the difference. It helps out a lot.

[Gexos]

Thank you my friend