How can I exclude a certain value from an iterative list comprehension in Python?

Question

I am trying to exclude a certain value (specifically a space " ") from an iterative list comprehension and i'm not sure how to do that within the list comprehension.

I am building a simple password generator and I know how to do it later down by just doing an if else statement but i'm not quite sure how I would do it in the passcharacters.append(c) part. I want it to get all the ASCI characters apart from a space.

import random as rd

passlength = int(input('How long should the password be?:\n'))

passcharacters = []

for c in (chr(i) for i in range(32,127)):
    passcharacters.append(c)
print(passcharacters)

secure_random = rd.SystemRandom()

password = ""
for x in range(passlength):
    character = secure_random.choice(passcharacters)
    password = password + character

print("\nYour password is: \n"+password)

uhm.. i see a generator expression, that is iterated through and appended to a list. That should literally just be a list comprehension to begin with. Also, you can put if statements inside list comprehensions/generator expressions. — Paritosh Singh
– Paritosh Singh, Commented Feb 17, 2019 at 19:57

iz_ · Accepted Answer · 2019-02-17 20:05:17Z

3

You should do a direct list comprehension instead of appending. List comprehensions and generator expressions both support conditionals:

passcharacters = [chr(i) for i in range(32,127) if chr(i) != ' ']

Another more readable way is this:

import string

passcharacters = list(string.printable.strip())

It's the same characters that you want, just in a different order. You can confirm this with

set(string.printable.strip()) == {chr(i) for i in range(32,127) if chr(i) != ' '}

which is True.

edited Feb 17, 2019 at 20:05

answered Feb 17, 2019 at 19:56

iz_

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5 Comments

nicolos Over a year ago

Thanks! Super helpful :D ... I forgot about that!

nicolos Over a year ago

Anecdotally, do you happen to know what the difference between random.randomchoice and random.SystemRandom actually is?

iz_ Over a year ago

@coconido Please avoid asking a different question; post another one of needed. Anyway, choice is in Python and SystemRandom is the system RNG. Also consider the secrets module if you want cryptographically strong RNG.

Jean-François Fabre Over a year ago

use set comprehension instead of list comprehension fed to a set

iz_ Over a year ago

@Jean-FrançoisFabre Edited, though you won't use that code in this scenario. It's just for double checking that the two are the same.

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