How can I make many empty lists without manually typing the following?
list1=[], list2=[], list3=[]
Is there a for loop that will make me 'n' number of such empty lists?
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1See also: stackoverflow.com/questions/1373164/…Karl Knechtel– Karl Knechtel2022-07-07 18:18:40 +00:00Commented Jul 7, 2022 at 18:18
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8 Answers
A list comprehension is easiest here:
>>> n = 5
>>> lists = [[] for _ in range(n)]
>>> lists
[[], [], [], [], []]
Be wary not to fall into the trap that is:
>>> lists = [[]] * 5
>>> lists
[[], [], [], [], []]
>>> lists[0].append(1)
>>> lists
[[1], [1], [1], [1], [1]]
10 Comments
inspectorG4dget
Holy wow! we posted this at EXACTLY The same time (mind=blown)
Eric
@inspectorG4dget: There should be a badge for that
alemangui
This creates a list of lists though, not separate variables with different names. Although it is definitely shorter.
redbeam_
Why does the "trap" happen?
Antoine
I did fall into the trap and you saved me :) Thanks!
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If you want to create different lists without a "list of lists", try this:
list1, list2, list3, list4 = ([] for i in range(4))
Look up list comprehensions:
listOfLists = [[] for i in range(N)]
Now, listOfLists has N empty lists in it.
More links on list comprehensions:
Comments
I see that many answers here get you many lists with no names assigned (as elements of a larger list). This may be not always what's needed.
It is possible to create multiple empty lists, each with a different name/value, using a tuple:
a,b,c = ([],[],[])
Changing this structure should get you what you want.
1 Comment
juanbretti
In Python 3, you could also do
a, b, c = [], [], [].The code below will dynamically 'create' itself. For every iteration, the following command will issued:
listnumber = []
Where number is the value of i in the loop.
x = 3 # Amount of lists you want to create
for i in range(1, x+1):
command = "" # This line is here to clear out the previous command
command = "list" + str(i) + " = []"
exec(command)
The result of this particular piece of code is three variables: list1, list2 and list3 being created and each assigned an empty list.
You can generalize this to make practically anything you want. Do note that you cannot put this into a function as far as I know, because of how variables and global variables work.
name = "my_var" # This has to be a string, variables like my_var1, my_var2 will be created.
value = "[]" # This also has to be a string, even when you want to assign integers! When you want to assign a string "3", you'd do this: value = "'3'"
amount = 5 # This must be an integer. This many variables will be created (my_var1, my_var2 ... my_var5).
for i in range(1, amount+1):
command_variable = ""
command_variable = name + str(i) + " = " + value
exec(command_variable)
1 Comment
Eric
exec is almost always a really bad idea. If you think about using this code, then change your mind and don't. At the very least, use globals()[varname] = val, if you must dynamically create variablesUse this:
def mklist(n):
for _ in range(n):
yield []
Usage:
list(mklist(10))
[[], [], [], [], [], [], [], [], [], []]
a, b, c = mklist(3) # a=[]; b=[]; c=[]
Comments
Or as simple as
added, modified, removed, inactive = ([],) * 4
2 Comments
Robert Alexander
This is nice. A little bit more cryptic than "added, mod, rem, inact = [],[],[],[]" but for sure more compact !!!
patrick7
Becareful, when I use this method, my lists are "connected" in the sense that changing items in "added" will also change "modified and "removed" ... etc. I would suggest the for loop way as suggested by inspectorG4dget
You can also add a list of list of list of list... like this too. It won't return any error. But I haven't got any idea what it's used for.
n = 5
>>> lists = [[[]]] for _ in range(n)]
>>> lists
[[[[]]], [[[]]], [[[]]], [[[]]], [[[]]]]
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