set_fact and combine¶

Why This Exists¶

Not every variable is known before a play starts — set_fact computes one at run time from other variables or registered results, and combine merges dictionaries without clobbering keys the naive +/override approach would lose.

What It Will Cover¶

• set_fact basics and its precedence, roughly alongside registered variables — see Variable Precedence
• cacheable: true on set_fact to persist a value into fact caching across runs
• Why overwriting a whole dictionary variable (my_dict: {{ new_values }}) loses keys the new value doesn't mention
• combine for a deep, non-destructive merge: my_dict | combine(new_values, recursive=True)
• A worked example: layering environment-specific overrides onto a base configuration dictionary without losing base keys

Common Mistakes¶

• Reassigning a dictionary variable directly instead of using combine, silently dropping keys that weren't in the new value.
• Forgetting recursive=True when nested dictionaries need merging, not just top-level keys.
• Overusing set_fact as a substitute for proper role variable design — every set_fact is a runtime side effect that makes a playbook's data flow harder to follow.

Interview Questions¶

• Why would my_dict: "{{ new_values }}" be wrong when you meant to merge, not replace?
• What does combine(..., recursive=True) do that a plain dictionary reassignment doesn't?

Next¶

Continue to Jinja2 & Templates.