Filters and Tests¶

Why This Exists¶

Filters transform a value (| default, | join); tests answer a yes/no question about a value (is defined, is failed) — together they're most of what makes a Jinja2 expression in a real playbook longer than a bare variable name.

What It Will Cover¶

• | default('fallback') and | default([], true) (the second argument treats an empty string/list as undefined too)
• | join(', '), | length, | unique, | sort
• | dict2items / | items2dict for looping over dictionaries with loop
• | to_json / | from_json, | to_yaml / | from_yaml
• | select/| selectattr/| map for filtering and projecting lists of dictionaries
• Tests: is defined, is undefined, is none, is failed, is success, is changed
• The difference between a filter (|) and a test (is) syntactically and conceptually

Common Mistakes¶

• Using | default('') and still hitting an "undefined variable" error because the variable is genuinely undefined in a nested lookup Jinja2 evaluates before the filter applies.
• Confusing is defined (variable exists) with a falsy-value check — an empty string or 0 is still "defined."

Interview Questions¶

• What does | default([], true) do differently from plain | default([])?
• How would you filter a list of dictionaries down to only the ones matching a condition, without a full for loop?

Next¶

Continue to Templates for Config Generation.