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()