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Topic: “Filter numbers with a list comprehension” (PyBite #107)
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Topic: “Filter numbers with a list comprehension” (PyBite #107)
Here is the task I am working on: Write a script which receives a list of numbers and returns only the even numbers that are > 0 and even (divisible by 2). Use list comprehension.

Here was my final valiant attempt:

 numbers = list(range(-10, 11))
# numbers = [2, 4, 51, 44, 47, 10]
# numbers = [0, -1, -3, -5]
result = []
result = [result.append(num) for num in numbers if  num % 2 == 0 or num <= 0]
For lines 1-3, you can comment/uncomment them to test different list inputs.

I was close but slightly off. The problem with my script is that the result appends None for every list item generated. The reason why this is wrong is because result when originally defined is empty (None Type) and for every iteration through the comprehensive list, it just keeps adding None. The solution that the online coursewhere presents is similar to mine (very close) but instead of using the append class method, it’s just result. Here is the one line solution: result = [n for n in numbers if n > 0 and n % 2 == 0].

My basic, simple pointed question for all of you is this: Do all list comprehensions automatically somehow know to append list items automatically? Are the append/extend class methods implied or assumed?

This is my effort to solve PyBite #107.
Not really sure what you are after but for the "None", this works.
numbers = list(range(-10, 11))
# numbers = [2, 4, 51, 44, 47, 10]
# numbers = [0, -1, -3, -5]
result = []
result.append([num for num in numbers if  num % 2 == 0 or num <= 0])
[[-10, -9, -8, -7, -6, -5, -4, -3, -2, -1, 0, 2, 4, 6, 8, 10]]
(Jun-11-2020, 03:55 PM)Drone4four Wrote: Do all list comprehensions automatically somehow know to append list items automatically?

Sorta, yes. They are list comprehensions, so they return a list of the elements. You don't have to create it by manually appending anything. Likewise a dict comprehension will populate a dict, and a set comprehension will append to a set.

r = range(-5, 6)
l = [x for x in r if x % 2 == 0] # all the even numbers.  l is created here, no appending to other list.
[-4, -2, 0, 2, 4]
In your attempt you are appending the entire list generated into an empty list. So you just get an extra level of nesting.
I think bowlofred did an excellent job answering this so there really is not much for me to add. However, I wanted to say that when I first was introduced to comprehension they confused me as well. Until I realized they are just a fancy way of writing a for loop. For example result = [n for n in numbers if n > 0 and n % 2 == 0] could be rewritten as

result = list()
for n in numbers:
    if n > 0 and n % 2 == 0:

One thing that I think should be mentioned is. You are creating a list of numbers in a range with numbers = list(range(-10, 11)) which is perfectly valid and does work. However, note that a comprehension just like a for loop can iterate over a sequence. Range returns an sequence type so you could simplify this with result = [n for n in range(-10, 11) if n > 0 and n % 2 == 0]
I actually initially jumped into this exercise writing a regular for loop and came up with something slightly different (but created the same result) to what you came up with, @knackwurstbagel:

def filtering(numbers):
    for num in numbers:
        if num <= 0:
        if num % 2 == 0:
    return list_of_numbers
Mine is a little less elegant, but after I came up with that, I then proceeded to achieve the same result with list comprehension (or almost).

Sharing alternate script solutions with someone else helps. So thank you, @knackwurstbagel, @menator01, and @bowlofred Big Grin

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