Tukukkk

I am about two months into programming with Python (no previous experience) and I tried to write a program which would generate a key and assign it to a student.

import random

import string





code_list = 

students = ["Aljoša", "Blažka", "Nana", "Kaja", "Alja", "Tjaša", "Ana",

           "Gal", "Danijela", "Alma", "Neja", "Žiga K.", "Patricija", "Aja",

           "Kristjan",  "Urban",  "Janja", "Lea", "Žana", "Aljaž", "Tilen",

           "Matic", "Marija", "Žiga T."]



#It generates the code and puts it in a list

size = 6

code = string.ascii_uppercase + string.digits

for i in range(24):

    code_list.append(str(''.join(random.choice(code) for _ in range(size))))



#Combines the two lists (students and codes) into one

students_plus_codes = [x + str(" - " + y) for y in students for x in 

                      code_list]



#It prints only every 25th element from the combined list

new = [x for i, x in enumerate(students_plus_codes) if i % 25 == 0]



for i in new:

    print(i)`

My problem is that this code is not very elegant and just want to ask how I could write it more efficiently (especially the last part where I have to manually print out every 25th element of a list because every key gets assigned to every student).

The reason that you are getting so many more entries is because this list comprehension:

 students_plus_codes = [x + str(" - " + y) for y in students for x in code_list]

It's a nested for loop. If we expand it we can see what is happening:

students_plus_codes = 

for y in students:  # students has 24 items

    for x in code_list:  # code_list has 24 items

        students_plus_codes(x + str(" - " + y))

So for every iteration of students you do 24 iterations of code_list.

Fortunately, python has a builtin function called zip that will merge lists for us. So we can replace the nested list comprehension with this line:

students_plus_codes = list(zip(students, code_list))

Which results in a list of tuples:

[('Aljoša', '256D2B'), ('Blažka', 'OEGJL9'), ('Nana', 'GB1PJL'), ('Kaja', 'F0P0F2'), 

 ('Alja', '62KU94'), ... ('Matic', 'E7CJIP'), ('Marija', '1D2UCL'), ('Žiga T.', '6X1DD5')]

Generating the codes:

First, I would replace for i in range(24): with for i in range(len(students)): as this allows you to change the number of students without having change other aspects of your code. In this instance 24 is a magic number that could cause issues down the line.

You could even create a function that generates the the code, then you can use an easy to read list comprehension.

# <=python3.5

def generate_code(n, characters):

    return ''.join([random.choice(characters) for _ in range(n)]) 



# >=python3.6

def create_code(n, characters):

    return ''.join(random.choices(characters, k=n))

Printing the codes:

It isn't ideal to join the names and codes a single string as this makes it hard to use them separately later. For this reason, I used a list of tuples to match them as pairs. To print them the for loop has two variables, student and code that takes advantage of unpacking and then prints them using the format function.

Instead of joining the students names with the codes for printing is not an ideal thing to do as it makes it harder to use the names and codes separately later.

Altogether:

import random

import string





def create_code(n, characters):

    return ''.join(random.choices(characters, k=n))





students = ["Aljoša", "Blažka", "Nana", "Kaja", "Alja", "Tjaša", "Ana",

           "Gal", "Danijela", "Alma", "Neja", "Žiga K.", "Patricija", "Aja",

           "Kristjan",  "Urban",  "Janja", "Lea", "Žana", "Aljaž", "Tilen",

           "Matic", "Marija", "Žiga T."]



# It generates the code and puts it in a list

size = 6

chars = string.ascii_uppercase + string.digits

code_list = [create_code(size, chars) for _ in range(len(students))]



# Combines the two lists (students and codes) into a list of tuples

students_plus_codes = list(zip(students, code_list))



for student, code in students_plus_codes:

    print('{} - {}'.format(code, student))