Frappe Impl Controllers

NEED full Python (imports, classes, generators)?     → Controller
NEED external libraries (requests, pandas)?          → Controller
NEED try/except with rollback?                       → Controller
NEED frappe.enqueue() for background jobs?           → Controller
NEED to extend standard ERPNext DocType?             → Controller
Quick validation without custom app?                 → Server Script
Simple auto-fill or notification?                    → Server Script

apps/myapp/myapp/{module}/doctype/{doctype_name}/{doctype_name}.py

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

class MyDocType(Document):
    def validate(self):
        self.validate_dates()
        self.calculate_totals()

    def validate_dates(self):
        if self.from_date and self.to_date and self.from_date > self.to_date:
            frappe.throw(_("From Date cannot be after To Date"))

    def calculate_totals(self):
        self.total = sum(item.amount for item in self.items)

WHAT DO YOU WANT?
├── Validate data / calculate fields before save?
│   └── validate — changes to self ARE saved
│
├── Action AFTER save (emails, linked docs, logs)?
│   └── on_update — changes to self NOT saved (use db_set)
│
├── Only for NEW documents?
│   └── after_insert
│
├── Before/after SUBMIT?
│   ├── Check before submit → before_submit
│   └── Ledger entries after → on_submit
│
├── Before/after CANCEL?
│   ├── Prevent cancel → before_cancel
│   └── Reverse entries → on_cancel
│
├── Before DELETE?
│   └── on_trash (throw to prevent)
│
├── Custom document naming?
│   └── autoname
│
└── Detect ANY change (including db_set)?
    └── on_change

# WRONG — changes in on_update are NOT saved
def on_update(self):
    self.status = "Completed"  # LOST!

# CORRECT — use db_set
def on_update(self):
    frappe.db.set_value(self.doctype, self.name, "status", "Completed")

def validate(self):
    errors = []
    if not self.items:
        errors.append(_("At least one item is required"))
    for item in self.items:
        if item.qty <= 0:
            errors.append(_("Row {0}: Qty must be positive").format(item.idx))
    if self.from_date > self.to_date:
        errors.append(_("From Date cannot be after To Date"))
    if errors:
        frappe.throw("<br>".join(errors))

def validate(self):
    old = self.get_doc_before_save()
    if old and old.status != self.status:
        self.flags.status_changed = True
        self.status_changed_on = frappe.utils.now()

def on_update(self):
    if self.flags.get('status_changed'):
        self.notify_status_change()

from frappe.model.naming import getseries

def autoname(self):
    # Format: PRJ-CUST-2025-001
    code = (self.customer or "GEN")[:4].upper()
    year = frappe.utils.getdate(self.start_date or frappe.utils.today()).year
    prefix = f"PRJ-{code}-{year}-"
    self.name = getseries(prefix, 3)

def before_naming(self):
    if self.is_priority:
        self.naming_series = "PRIORITY-.#####"
    else:
        self.naming_series = "STD-.#####"

DRAFT (docstatus=0) → submit() → SUBMITTED (docstatus=1) → cancel() → CANCELLED (docstatus=2)

submit():  validate → before_submit → [DB: docstatus=1] → on_update → on_submit
cancel():  before_cancel → [DB: docstatus=2] → on_cancel

class PurchaseOrder(Document):
    def validate(self):
        self.validate_items()
        self.calculate_totals()

    def before_submit(self):
        # ONLY submit-specific checks here
        if self.total > 100000 and not self.manager_approval:
            frappe.throw(_("Manager approval required for POs over 100,000"))

    def on_submit(self):
        self.update_ordered_qty()
        self.create_purchase_receipt_draft()

    def before_cancel(self):
        if frappe.db.exists("Purchase Invoice",
                {"purchase_order": self.name, "docstatus": 1}):
            frappe.throw(_("Cancel linked invoices first"))

    def on_cancel(self):
        self.reverse_ordered_qty()

# hooks.py
override_doctype_class = {
    "Sales Invoice": "myapp.overrides.CustomSalesInvoice"
}

# myapp/overrides.py
from erpnext.accounts.doctype.sales_invoice.sales_invoice import SalesInvoice

class CustomSalesInvoice(SalesInvoice):
    def validate(self):
        super().validate()  # ALWAYS call parent first
        self.custom_validation()

# hooks.py
doc_events = {
    "Sales Invoice": {
        "validate": "myapp.events.validate_sales_invoice",
    }
}

# myapp/events.py
def validate_sales_invoice(doc, method=None):
    if doc.grand_total < 0:
        frappe.throw(_("Invalid total"))

# hooks.py
extend_doctype_class = {
    "Sales Invoice": "myapp.extends.SalesInvoiceExtend"
}

# myapp/extends.py — Only methods to add/override
class SalesInvoiceExtend:
    def custom_method(self):
        pass

class Quotation(Document):
    @frappe.whitelist()
    def apply_discount(self, discount_percent):
        if discount_percent < 0 or discount_percent > 100:
            frappe.throw(_("Discount must be 0-100"))
        self.discount_amount = self.total * (discount_percent / 100)
        self.grand_total = self.total - self.discount_amount
        self.save()
        return {"grand_total": self.grand_total}

frm.call('apply_discount', { discount_percent: 10 }).then(r => {
    frm.reload_doc();
});

# Document-level flags (built-in)
doc.flags.ignore_permissions = True    # Bypass permission checks
doc.flags.ignore_validate = True       # Skip validate() hook
doc.flags.ignore_mandatory = True      # Skip required field check

# Custom flags for inter-hook communication
def validate(self):
    if self.is_urgent:
        self.flags.needs_notification = True

def on_update(self):
    if self.flags.get('needs_notification'):
        self.notify_team()

# tests/test_my_doctype.py
import frappe
from frappe.tests.utils import FrappeTestCase

class TestMyDocType(FrappeTestCase):
    def test_validate_dates(self):
        doc = frappe.get_doc({
            "doctype": "My DocType",
            "from_date": "2025-01-10",
            "to_date": "2025-01-01"  # Before from_date
        })
        self.assertRaises(frappe.ValidationError, doc.insert)

    def test_calculate_totals(self):
        doc = frappe.get_doc({
            "doctype": "My DocType",
            "items": [
                {"item": "A", "qty": 2, "rate": 100},
                {"item": "B", "qty": 3, "rate": 50}
            ]
        })
        doc.insert()
        self.assertEqual(doc.total, 350)

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

validate → before_submit → [DB: docstatus=1] →
on_update → on_submit → on_change