Append a new item to a list within a list

python list append

21,815

Solution 1

lst = [[] for _ in xrange(2)] (or just [[], []]). Don't use multiplication with mutable objects — you get the same one X times, not X different ones.

Solution 2

list_list = [[] for Null in range(2)]

dont call it list, that will prevent you from calling the built-in function list().

The reason that your problem happens is that Python creates one list then repeats it twice. So, whether you append to it by accessing it either with list_list[0] or with list_list[1], you're doing the same thing so your changes will appear at both indexes.

Solution 3

You can do in simplest way....

>>> list = [[]]*2
>>> list[1] = [2.5]
>>> list
[[], [2.5]]

Solution 4

list = [[]]
list.append([2.5])

list = [[],[]]
list[1].append(2.5)

Solution 5

As per @Cat Plus Plus dont use multiplication.I tried without it.with same your code.

>> list = [[],[]]
>> list[1].append(2.5)
>> list
>> [[],[2.5]]

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Author by

Admin

Updated on October 21, 2020

Comments

Admin over 3 years
I'm trying to append a new float element to a list within another list, for example:
```
list = [[]]*2
list[1].append(2.5)
```
And I get the following:
```
print list
[[2.5], [2.5]]
```
When I'd like to get:
```
[[], [2.5]]
```
How can I do this?

Thanks in advance.
agf almost 13 years

I'd reccomend not using _ as apparently it can mess up some interactive interpreters. Also, don't call it list, you're masking the builtin. Also, you'll get the same one X times for immutable objects too, it just doesn't often matter, unless you later want to check object identity.
Cat Plus Plus almost 13 years

@ag: _ is standard "I don't care about this variable" variable. It is used by the interactive mode to store the last result, but it certainly doesn't mess anything up.
agf almost 13 years

I know what it's used for. I forget now, but I believe it messes up tracebacks in one of the IDEs.
orlp almost 13 years

@agf: Then that is a bug of that IDE. Common codestyle is more important, to me, at least.
Niklas R almost 13 years

But allows you to create a list of any dimension.
Oben Sonne almost 13 years

Of course! Over-engineered stuff usually lets you do "cool" things, but most of it is not needed with regard to the initial requirements ;-)
scraplesh over 12 years

@CatPlusPlus underscore often used as translator in multilingual applications, so it's bad practice to use it unless you never work on such a projects
Dani over 7 years

C'mon you can find so many more options. At the biggest case, use... I don't know.. max. (disclaimer: docs.python.org/2/library/functions.html#max)
Vinícius Figueiredo over 6 years

A note since this question has many views: in Python3+, use range(2) instead of xrange(2).