Getting pieces from a list with ansible

[Dan C]

Hi, I know I am provably misunderstanding somthing, but I am not able to find a solution to my (rather simple) problem.

What I want is quite easy. I have a list and I want to get the last (or first) X elements of it:

Example:

- set_fact: mylist={{["l10","j82","b83","8jk","j83"]|list}}
- debug: var=mylist
- set_fact: max_n={{1|int}}
- debug: var=max_n
- debug: var=mylist[-{{max_n}}:]

Since here everything seems to work ok with the output:

PLAY [deploy] ******************************************************************


TASK [setup] *******************************************************************
ok: [localhost]


TASK [cleanup_s3_folder : set_fact] ********************************************
ok: [localhost]


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "mylist": [
        "l10",
        "j82",
        "b83",
        "8jk",
        "j83"
    ]
}


TASK [cleanup_s3_folder : set_fact] ********************************************
ok: [localhost]


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "max_n": "1"
}


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "mylist[-1:]": [
        "j83"
    ]
}


PLAY RECAP *********************************************************************
localhost                  : ok=6    changed=0    unreachable=0    failed=0

Now, this last list, containing in this case only the value "j83", I want it on a variable to use further on, so I try with:

- set_fact: elements=mylist[-{{max_n}}:]
- debug: var=elements

- set_fact: elements2={{ mylist[-{{max_n}}:] }}
- debug: var=elements2

And I am not getting what I want:

PLAY [deploy] ******************************************************************


TASK [setup] *******************************************************************
ok: [localhost]


TASK [cleanup_s3_folder : set_fact] ********************************************
ok: [localhost]


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "mylist": [
        "l10",
        "j82",
        "b83",
        "8jk",
        "j83"
    ]
}


TASK [cleanup_s3_folder : set_fact] ********************************************
ok: [localhost]


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "max_n": "1"
}


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "mylist[-1:]": [
        "j83"
    ]
}


TASK [cleanup_s3_folder : set_fact] ********************************************
ok: [localhost]


TASK [cleanup_s3_folder : debug] ***********************************************
ok: [localhost] => {
    "elements": "mylist[-1:]"
}


TASK [cleanup_s3_folder : set_fact] ********************************************
fatal: [localhost]: FAILED! => {"failed": true, "msg": "ERROR! template error while templating string: expected token ':', got '}'"}


PLAY RECAP *********************************************************************
localhost                  : ok=8    changed=0    unreachable=0    failed=1

Is there an easy way to get a VARIABLE piece of a list into a new variable?

[Brian Coca]

first thing, this is not needed:

- set_fact: mylist={{["l10","j82","b83","8jk","j83"]|list}}

this should be enough:
- set_fact:

mylist: ["l10","j82","b83","8jk","j83"]

another issue is that moustaches do not stack! and you need moustaches
for variables


- set_fact: elements={{mylist[-max_n:]}}


the first way you did it you had a string mylist, .not a variable, the
second you were putting {{ }} inside of {{}} which does not work
(moustaches do not stack!).

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--
Brian Coca

[Dan C]

Thanks Brian,

You are right about not needing the "|list", it works perfectly without it.

But I am yet not able to assign the variable, using what you suggested I am now getting a different error:

- set_fact: mylist={{["l10","j82","b83","8jk","j83"]}}

- debug: var=mylist
- set_fact: max_n={{1|int}}
- debug: var=max_n
- debug: var=mylist[-{{max_n}}:]

- set_fact: elements={{mylist[-max_n:]}}
- debug: var=elements

[...]


TASK [debug] *******************************************************************

ok: [localhost] => {
    "mylist[-1:]": [
        "j83"
    ]
}

TASK [set_fact] ****************************************************************
fatal: [localhost]: FAILED! => {"failed": true, "msg": "ERROR! an unexpected type error occurred. Error was bad operand type for unary -: 'unicode'"}

Thanks!

[Dan C]

Hi again,

So, it is not possible to get a "variable" piece of a "variable" list?

What I need is to maintain a fixed number of objects in a storage, I want to have always only the "last X" objects and delete all the rest.

Is there any other way to do so?