Frappe Syntax Controllers

import frappe
from frappe import _
from frappe.model.document import Document

class SalesOrder(Document):
    def autoname(self):
        """Custom naming logic. Sets self.name."""
        self.name = f"SO-{self.customer_code}-{frappe.utils.now_datetime().year}"

    def validate(self):
        """MAIN validation — runs on EVERY save (insert and update).
        Changes to self ARE saved to database."""
        if not self.items:
            frappe.throw(_("Items are required"))
        self.total = sum(item.amount for item in self.items)

    def on_update(self):
        """After save — changes to self are NOT saved.
        Use frappe.db.set_value() for post-save field changes."""
        self.notify_linked_docs()

    def on_submit(self):
        """After submit (docstatus 0 -> 1). Create ledger entries here."""
        self.create_gl_entries()

    def on_cancel(self):
        """After cancel (docstatus 1 -> 2). Reverse ledger entries here."""
        self.reverse_gl_entries()

    @frappe.whitelist()
    def recalculate(self):
        """Exposed to client JS via frm.call('recalculate')."""
        self.total = sum(item.amount for item in self.items)
        return {"total": self.total}

before_insert -> before_naming -> autoname -> before_validate -> validate
-> before_save -> [db_insert] -> after_insert -> on_update -> on_change

before_validate -> validate -> before_save -> [db_update]
-> on_update -> on_change

before_validate -> validate -> before_submit -> [db_update]
-> on_submit -> on_update -> on_change

before_cancel -> [db_update] -> on_cancel -> on_change

before_update_after_submit -> [db_update]
-> on_update_after_submit -> on_change

on_trash -> [db_delete] -> after_delete

before_discard -> [db_set docstatus=2] -> on_discard

What do you need to do?
|
+-- Validate data or calculate fields?
|   +-- validate (changes to self ARE saved)
|
+-- Action AFTER save (emails, sync, linked docs)?
|   +-- on_update (changes to self are NOT saved)
|
+-- Only for NEW documents?
|   +-- after_insert (runs once on first save only)
|
+-- Custom document name?
|   +-- autoname (set self.name)
|
+-- Before/after SUBMIT?
|   +-- Validate before submit? -> before_submit
|   +-- Create entries after submit? -> on_submit
|
+-- Before/after CANCEL?
|   +-- Check linked docs? -> before_cancel
|   +-- Reverse entries? -> on_cancel
|
+-- Cleanup before delete?
|   +-- on_trash
|
+-- React to ANY value change (including db_set)?
|   +-- on_change (MUST be idempotent)

# WRONG - change is lost after on_update
def on_update(self):
    self.status = "Completed"  # NOT saved to database

# CORRECT - use db_set or frappe.db.set_value
def on_update(self):
    self.db_set("status", "Completed")

# WRONG - breaks Frappe transaction management
def validate(self):
    frappe.db.commit()  # Can cause partial updates on error

# CORRECT - Frappe commits automatically at end of request
def validate(self):
    self.update_related()  # No commit needed

# WRONG - parent validation is skipped entirely
def validate(self):
    self.custom_check()

# CORRECT - parent logic preserved
def validate(self):
    super().validate()
    self.custom_check()

def on_update(self):
    if self.flags.get("from_linked_doc"):
        return
    linked = frappe.get_doc("Linked Doc", self.linked_doc)
    linked.flags.from_linked_doc = True
    linked.save()

# WRONG - document is already saved when this throws
def on_update(self):
    if self.total < 0:
        frappe.throw("Invalid total")  # Too late!

# CORRECT - validate BEFORE save
def validate(self):
    if self.total < 0:
        frappe.throw("Invalid total")  # Blocks save

from frappe.model.naming import getseries

class Project(Document):
    def autoname(self):
        prefix = f"P-{self.customer[:3].upper()}-"
        self.name = getseries(prefix, 3)
        # Result: P-ACM-001, P-ACM-002, etc.

When to use UUID:              When to use traditional naming:
- Cross-system sync            - User-facing references (SO-00001)
- Bulk record creation         - Sequential numbering required
- Global uniqueness needed     - Auditing requires readable names

# hooks.py
override_doctype_class = {
    "Sales Order": "custom_app.overrides.CustomSalesOrder"
}

# custom_app/overrides.py
from erpnext.selling.doctype.sales_order.sales_order import SalesOrder

class CustomSalesOrder(SalesOrder):
    def validate(self):
        super().validate()  # ALWAYS call super()
        self.custom_validation()

# hooks.py
extend_doctype_class = {
    "Address": ["custom_app.extensions.address.GeocodingMixin"],
    "Contact": [
        "custom_app.extensions.common.ValidationMixin",
        "custom_app.extensions.contact.PhoneMixin"
    ]
}

# custom_app/extensions/address.py
from frappe.model.document import Document

class GeocodingMixin(Document):
    @property
    def full_address(self):
        return f"{self.address_line1}, {self.city}, {self.country}"

    def validate(self):
        super().validate()
        self.geocode_address()

# hooks.py
doc_events = {
    "Sales Order": {
        "validate": "custom_app.events.validate_sales_order",
        "on_submit": "custom_app.events.on_submit_sales_order"
    },
    "*": {  # ALL DocTypes
        "after_insert": "custom_app.events.log_creation"
    }
}

# custom_app/events.py
def validate_sales_order(doc, method=None):
    if doc.total > 100000:
        doc.requires_approval = 1

Need full class replacement?     -> override_doctype_class [all versions]
Need to add methods/properties?  -> extend_doctype_class [v16+]
Need to hook one or two events?  -> doc_events [all versions]
Need to extend in v14/v15?       -> override_doctype_class or doc_events

class SalesOrder(Document):
    @frappe.whitelist()
    def send_email(self, recipient):
        """Callable from JS: frm.call('send_email', {recipient: '...'})"""
        frappe.sendmail(recipients=[recipient], message="Order confirmed")
        return {"status": "sent"}

// Client-side call
frm.call('send_email', { recipient: 'customer@example.com' })
    .then(r => frappe.msgprint(r.message.status));