EX294 Transform data with filters and plugins Practice Question
An administrator must parse an inventory file where hostnames are stored in YAML format under a list 'nodes'. The task needs to extract only hostnames that contain 'prod' in the name, then sort them in reverse order. Which combination of filters in a single Ansible expression achieves this?
⚠ Common exam trap
Watch out — candidates often confuse `select` (which keeps matching items) with `reject` (which removes matching items), or mistakenly use `regex_search` or `map` thinking they will filter the list, when in fact those filters return substrings or transformed values, not the original list elements.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
nodes | select('match', '.*prod.*') | sort(reverse=True)
The `select` filter with the `match` test returns only list items that match the regex `'.*prod.*'`, i.e., hostnames containing 'prod'. The `sort(reverse=True)` then sorts the resulting list in descending alphabetical order, fulfilling both requirements in a single Ansible expression.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
nodes | select('match', '.*prod.*') | sort(reverse=True)
Why this is correct
The select filter with match applies the regex '.*prod.*' to each node, keeping only hostnames containing prod, and sort(reverse=True) orders the survivors descending. This single chained expression satisfies both the filtering and reverse-sorting constraints in one evaluation.
- ✗
nodes | regex_search('prod') | sort(True)
Why it's wrong here
regex_search returns the matched substring or None, not a filtered list, so sort receives a string or null and fails. It is tempting because regex_search detects the pattern, but selecting matching elements requires select with match or selectattr, then sort(reverse=True).
- ✗
nodes | map('regex_search', 'prod') | sort(reverse=True)
Why it's wrong here
map applies regex_search to every element, yielding matches or None rather than the hostnames themselves, so the filtered list is wrong. It is tempting because map transforms lists, but the task needs select to retain matching hostnames before reverse sorting.
- ✗
nodes | reject('match', '.*prod.*') | sort(reverse=True)
Why it's wrong here
The reject filter removes every hostname matching '.*prod.*', leaving exactly the non-production hosts — the inverse of what is required. It is tempting because reject is the natural filter for excluding entries, and it would be correct if the task were to discard production nodes rather than keep them.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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