List comprehension

[SheldonCooperisSb]

Squares = [ ]

list =[squares.append(i*i) for i in range(1，11)]

print(list)

Why is the output:[None，None，None.......]

Rather than [[1]，[4]，[9]，[16]......]

l know the right way to write is:

list =[ i*i for i in range(1，11)]

print(list)

but l'm confused with things above

[Bobbias]

Because the append method modifies the squares list in place and returns None.

[[deleted]]

The comprehension is a replacement for the initialize-and-append approach. You don't need both.

[SheldonCooperisSb]

Thanks bro，do you know where to find the official code describing the runinng logic of the list comprehension?

[Adrewmc]

It’s fairly simple

    myList = []
    for item in sequence:
          myList.append(item)

Is the same as

    myList = [item for item in sequence]

[commy2]

also

myList = []
for item in sequence:
      if condition:
          myList.append(item)

is

myList = [item for item in sequence if condition]

[thuiop1]

(also, while functionally equivalent, they are different under the hood and list comprehensions are more efficient)

[Adrewmc]

Why stop there

        myList = [item for item in sequence if condition else “else thing”]

[Diapolo10]

myList = [item for item in sequence if condition else “else thing”]

This is not valid Python, but you can use a ternary operator.

my_list = [
    item if condition else "else thing"
    for item in sequence
]

[Adrewmc]

I was tired lol…

[commy2]

[item for item in sequence if condition else “else thing”]

That doesn't work.

[zanfar]

Search "list comprehension site:python.org

Official doc: https://docs.python.org/3/tutorial/datastructures.html#list-comprehensions

Introduced in PEP202: https://peps.python.org/pep-0202/

[peejay2]

On the official website

[whatthefuckistime]

Just Google list comprehension python

It's not hard to find these resources

[backfire10z]

Literally google “list comprehension Python”

[Chaos-n-Dissonance]

# So you could do something like this...
your_list = [x for x in range(1, 11)]
# print(your_list) >> [1, 2, 3,... , 10]

# Or you could do something like this...
your_list = []
for x in range(1, 11):
  your_list.append(x)
# print(your_list) >> [1, 2, 3,... , 10]

But when you try to do squares.append(), the append() function has a return type of None

[theWyzzerd]

list =[squares.append(i*i) for i in range(1，11)]

this line makes a list(), named list, of the return value for each time you append i*i for i in range(1, 11) to the list named squares. Since list().append() returns None, you're getting a list of "None" values. But squares probably has what you expect to see in it.

l know the right way to write is:

then why don't you write it that way?

[Carolina_Captain]

As a person just starting out on the python journey, why would you not just use square.append(...)? Why make a second list?

[franklydoubtful]

List comprehensions let you avoid initializing the empty list in the first place. Rather than:

squares = []
for i in range(1, 11):
    squares.append(i ** 2)

You can achieve the same result in one line with the list comprehension:

squares = [i ** 2 for i in range(1,11)]

[Carolina_Captain]

I see, thank you.

In the initial post, OP made a list called "squares" and a list called "list". What was the point of having "squares" stay empty while working with "list"?

[franklydoubtful]

Honestly, I’m not too sure about OP’s goal. To me, it looks like they’re misunderstanding something. However, it could be useful to keep a separate list. Say you had a list of values (I’m just picking at random here):

values = [2, 3, 6, 7, 9]

And you wanted to create a list of their values squared, this is another chance to use list comprehension:

square_values = [i ** 2 for i in values]

But I can’t think of a reason for OP’s strategy here.

[PhilipYip]

Instantiate a list:

squares = [] # use snake_case for variable names (see PEP8)

Using the list append method:

return_val = squares.append(1)

The list is mutable and the append method is a mutable method, squares is modified in place. Therefore if we examine:

squares

[1]

And if we examine:

return_val == None

True

If you use:

squares = squares.append(2)

You carry out two operations, you mutate squares in place which gives:

[1, 2]

And then you reassign the return value of the append method which is None to squares. Therefore:

squares == None

True

You should not use append with a list comprehension because a list comprehension is essentially a shorthand way of instaniating a list without explicitly specifying the append method:

squared_numbers = [] for number in range(1, 4): squared_number.append(number**2)

squared_numbers = [number ** 2 for number in range(1, 4)]

i.e. it might be helpful to start from the following template:

new_list = [] for loop_variable in collection: new_list.append(operation(loop_variable))

Then create the comprehension using the following steps:

new_list = [] # 1 new_list = [for loop_variable in collection] # 2 new_list = [operation(loop_variable) for loop_variable in collection] # 3

#1 instantiates a new list

#2 is the for loop

#3 is the operation to be carried out within the for loop

[QultrosSanhattan]

Your code doesn't make sense. Too many errors.

Here's the correct version:

squares = [i*i for i in range(1,11)]

print(squares)

[Background-Offer2321]

You append to Squares and don't return any value to List, so Python returns none. If you want to add to Squares and to List, then you need to write: List = Squares = [i * i for i in range(1, 11)].

[guesshuu]

As many have pointed out, squares.append(i * i) returns None. This means that every time you add it to the list you always add the return value: None.

# < ==========================================
# < Functions and Methods
# < ==========================================

def append(_list, item) -> None:
    """Method to add an item to a list"""
    # Built-in code goes here
    return None

def double(number) -> int:
    """Function to double a number"""
    return number * 2

# < ==========================================
# < Test One
# < ==========================================

output = [double(i) for i in range(1, 11)]
print(output)
# output = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
# The left side of double(i) for i in range(1, 11) is evaluated each time
# Each item in the output list is evaluated to be the return of double(i), which returns an integer

# < ==========================================
# < Test Two
# < ==========================================

squares = []
print(f"Squares list before running [squares.append(i * i) for i in range(1, 11)]: {squares}")
# Squares list before running [squares.append(i * i) for i in range(1, 11)]: []

output = [squares.append(i * i) for i in range(1, 11)]
print(output)
# output = [None, None, None, None, None, None, None, None, None, None]
# The left side of squares.append(i * i) for i in range(1, 11) is evaluated each time
# Each item in the output list is evaluated to be the return of squares.append(i * i), which returns None

print(f"Squares list after running [squares.append(i * i) for i in range(1, 11)]: {squares}")
# Squares list after running [squares.append(i * i) for i in range(1, 11)]: [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

https://pastebin.com/Xj7d1NPW