This guide covers coding standards, best practices, and formatting rules for Millennium Dawn mod development.

Quick Tools:

Run python3 tools/standardization/standardize.py focus to auto-format focus trees

Run python3 tools/standardization/standardize.py event to auto-format events

Run python3 tools/standardization/standardize.py decision to auto-format decisions

Run python3 tools/standardization/standardize.py idea to auto-format ideas

Quick Reference

Feature	Key Rules
Focus Trees	Use `relative_position_id`, include logging, `ai_will_do` last
Decisions	Include logging, use `fire_only_once` sparingly
Events	Use `is_triggered_only = yes`, log only if effects exist
Ideas	Remove `allowed = { always = no }`, use `allowed_civil_war`
Formatting	Tabs (not spaces), 1 line between elements

Performance Tips

These guidelines help keep the mod running smoothly:

Division vs Multiplication: Use multiplication instead of division when possible (e.g., * 0.01 instead of / 100)
Logging: Only log when there are meaningful effects. Logging causes I/O overhead
MTTH Events: Avoid open-fire MTTH events (those without is_triggered_only). They continuously evaluate and hurt performance
On Actions: Use tag-specific variants (on_daily_TAG) instead of global triggers
Dynamic Modifiers: Use sparingly. Avoid force_update_dynamic_modifier as it causes lag
Arrays: Replace every_country/random_country with specific array triggers
Cleanup: Remove unused code and commented-out blocks

Focus Trees

Reference: HOI4 Wiki - National Focus Modding (opens in new tab)

File Naming

Prefix	Usage
`00_`	System requirements only
`01-04_`	Shared/joint trees (EU, African Union)
`05_`	Country-specific trees

The number forces load order (shared trees load before country-specific).

Required Order Within a Focus

1. id (first line)
2. icon (second line)
3. x, y coordinates
4. relative_position_id
5. cost
6. allow_branch
7. prerequisite / mutually_exclusive
8. search_filters
9. available / bypass / cancel
10. completion_reward / select_effect / bypass_effect
11. ai_will_do (LAST)

Best Practices

Use relative_position_id for tree positioning
Add logging: log = "[GetDateText]: [Root.GetName]: Focus TAG_focus_name"
Omit default values: cancel_if_invalid = yes, continue_if_invalid = no
Include ai_will_do with game options checks
Add search_filters for all focuses
Remove unused/commented code
Limit permanent effects to 5, use timed ideas for more

Example Focus

focus = {
    id = SER_free_market_capitalism
    icon = blr_market_economy

    x = 5
    y = 3
    relative_position_id = SER_free_elections

    cost = 5

    # allow_branch = { }
    prerequisite = { focus = SER_western_approach }
    # mutually_exclusive = { }
    search_filters = { FOCUS_FILTER_POLITICAL }

    available = {
        western_liberals_are_in_power = yes
    }
    # bypass = { }
    # cancel = { }

    completion_reward = {
        log = "[GetDateText]: [Root.GetName]: Focus SER_free_market_capitalism"
        add_ideas = SER_free_market_idea
    }
    # bypass_effect = { }

    ai_will_do = {
        base = 1
    }
}

Decisions

Best Practices

Use fire_only_once only when necessary
Include logging in complete_effect
Structure with clear visible and available conditions
Include ai_will_do

Example Decision

URA_world_opr = {
    allowed = { original_tag = URA }
    icon = GFX_decision_sovfed_button

    cost = 50
    days_remove = 400

    visible = {
        country_exists = OPR
        OPR = {
            OR = {
                has_autonomy_state = autonomy_republic_rf
                has_autonomy_state = autonomy_kray_rf
            }
        }
    }

    complete_effect = {
        log = "[GetDateText]: [Root.GetName]: Decision URA_world_opr"
        OPR = { country_event = { id = subject_rus.121 days = 1 } }
    }

    ai_will_do = {
        factor = 10
    }
}

Events

Best Practices

Use is_triggered_only = yes for triggered events
Log only if there are actual effects in the block
Use major = yes sparingly (for news events)
Trigger date-based events via common/scripted_effects/00_yearly_effects.txt

Example Event

country_event = {
    id = france_md.504
    title = france_md.504.t
    desc = france_md.504.d
    picture = GFX_france_mcdonalds_bombing
    is_triggered_only = yes

    option = {
        name = france_md.504.a
        log = "[GetDateText]: [This.GetName]: france_md.504.a executed"
        set_party_index_to_ruling_party = yes
        set_temp_variable = { party_popularity_increase = -0.01 }
        add_relative_party_popularity = yes

        ai_chance = {
            base = 1
        }
    }

    option = {
        name = france_md.504.b
        ai_chance = {
            base = 0
        }
    }
}

Ideas

Best Practices

Include allowed_civil_war = { always = yes } for civil war tags
Remove allowed = { always = no } - this is the default and hurts performance
Remove cancel = { always = no } - checked hourly, never true
Remove empty on_add = { log = "" } unless you’re actually doing something
Log in on_add only when making changes

Example Idea

BRA_idea_higher_minimum_wage_1 = {
    name = BRA_idea_higher_minimum_wage
	picture = gold
    allowed_civil_war = { always = yes }

    modifier = {
        political_power_factor = 0.1
        stability_factor = 0.05
        consumer_goods_factor = 0.075
        population_tax_income_multiplier_modifier = 0.05
    }
}

Code Formatting

Indentation

Use tabs, not spaces
Increase indent on {, decrease on }
Keep consistent throughout

Brackets

Place closing brackets on the same line as the keyword
Avoid excessive whitespace
Keep simple checks on one line when appropriate

# Good
available = { has_country_flag = some_flag }

# Avoid
available = {
    has_country_flag = some_flag
}

Subideology Localization

Format for political party localization:

TAG.ideology: "£PARTY_ICON (ABBRV) - Party Name"
TAG.ideology_icon: "£PARTY_ICON"
TAG.ideology_desc: "(Ideology Group) - Party Name (Native Name, ABBRV)\n\nDescription"

Example:

MOR.conservatism: "£MOR_NRI (RNI) - National Rally of Independents"
MOR.conservatism_icon: "£MOR_NRI"
MOR.conservatism_desc: "(Classic Liberalism) - National Rally of Independents (Arabic: ..., RNI)\n\nParty description here."

Military-Industrial Organizations (MIO)

Company Structure

CHI_norinco_manufacturer = {
    allowed = { original_tag = CHI }
    icon = GFX_idea_Norinco_CHI

    task_capacity = 18

    equipment_type = {
        infantry_weapons_type
        artillery_equipment
        mio_cat_all_armor
    }

    research_categories = {
        CAT_infrastructure
        CAT_armor
        CAT_artillery
    }

    initial_trait = {
        name = CHI_norinco_company_trait
        equipment_bonus = {
            reliability = 0.03
            build_cost_ic = -0.03
        }
    }
}

Trait Guidelines

Maximum grid: y = 0 - 9
Use relative positioning within the grid

Code Resources - Modifiers and effects
Game Rules - Game rule reference

Quick Reference

Performance Tips

Focus Trees

File Naming

Required Order Within a Focus

Best Practices

Example Focus

Decisions

Best Practices

Example Decision

Events

Best Practices

Example Event

Ideas

Best Practices

Example Idea

Code Formatting

Indentation

Brackets

Subideology Localization

Military-Industrial Organizations (MIO)

Company Structure

Trait Guidelines

Related Resources