Data manipulation in Ansible: string transformation

It’s horrible. Kids, never do this!

I have a list (let’s say ‘data’), which contains stings. I need to add prefix and suffix to each string without involving Ansible loops (for example, because it’s already within a loop or two).

data:
  - string1
  - string2

We want to convert it to (with pure power of jinja2):

new_data:
  - prefix_string1_postfix
  - prefix_string2_postfix

The solution

new_data: '{{ (["prefix_"] * data|length)|zip(data)|map("join")|zip(["_suffix"] * data|length)|map("join")|list  }}'

Em… Xk’uzme?

The solution, explained

suffixes: '{{["_suffix"] * data|length }}'
prefixes: '{{["prefix_"] * data|length }}'
data_prefixes: '{{prefixes|zip(data)|map("join")|list}}'
new_data: '{{data_prefixes|zip(suffixes)|map("join")|list}}

The solution, explained, explained

suffixes: '{{["_suffix"] * data|length }}'
prefixes: '{{["prefix_"] * data|length }}'

suffixes: array of ‘_suffix’ * N times, where N == length(data).
prefixes: array of ‘prefix_’ * N times, where N == length(data).
Both uses Python trick, that [‘a’]*3 == [‘a’, ‘a’, ‘a’]. We need those arrays to have the same size as our 'data' array to use zip filter.

data_prefixes: '{{prefixes|zip(data)|map("join")|list}}'

prefixes|zip(data) creates list of list: [[‘prefix_’, ‘string1’], [‘prefix_’, ‘string2']].

map(“join”) apply ‘join’ (without arguments) to inner list. It becomes: [‘prefix_string1’, ‘prefix_string2']. Last ‘list’ is just to help debug with output (map returns python’s genexpr instead of list, so we convert it back to list). We don’t need it in full production environment.

new_data: '{{data_prefixes|zip(suffixes)|map("join")|list}}

Exactly the same trick. map data_prefixes with suffixes to create a nested list:

[['prefix_string1', '_suffix'], ['prefix_string2', '_suffix']]

Then we join and converting to list again:

['prefix_string1_suffix', 'prefix_string2_suffix'].

Postscriptum

I’d like to say ‘never use this in production’, except that I develop this horrible technique to avoid more extended horrors in my production. Sad, sad, jinja2.

Regexps?

The next day after I had wrote this article, akint showed me a simpler solution. I ashamed, because this time regexps make things simpler:

new_data: '{{ data | map("regex_replace", "^(.*)$", "prefix_\1_suffix") | list }}'