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Frappe provides multiple ways to customize apps without modifying core code. This keeps your customizations maintainable and upgrade-safe.

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Customization techniques

There are several approaches to customize Frappe apps:
  1. Custom Fields: Add fields to existing DocTypes
  2. Custom Scripts: Add client-side logic to forms
  3. Server Scripts: Add server-side logic without code
  4. Hooks: Override behavior using app hooks
  5. Custom Apps: Create new apps that extend existing ones
  6. Fixtures: Export and import customizations

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Custom fields

Add fields to standard DocTypes without modifying them:
1

Navigate to Custom Field

Go to Customize > Custom Field > New
2

Configure the field

  • Select the DocType to customize
  • Set Field Name and Label
  • Choose Field Type
  • Set Insert After to position the field
3

Save

Click Save to add the field
Custom fields are stored in the database and survive updates.

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Programmatic creation

import frappe

def create_custom_field():
    if not frappe.db.exists("Custom Field", "Customer-tax_id"):
        frappe.get_doc({
            "doctype": "Custom Field",
            "dt": "Customer",
            "label": "Tax ID",
            "fieldname": "tax_id",
            "fieldtype": "Data",
            "insert_after": "customer_name"
        }).insert()

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Client scripts

Add JavaScript to forms without modifying files:
1

Create Client Script

Go to Customize > Client Script > New
2

Configure script

  • Select DocType
  • Choose Type (Form, List, etc.)
  • Write JavaScript in the Script field
3

Save and test

Save and test on the form
Example: 
frappe.ui.form.on('Customer', {
    refresh: function(frm) {
        // Add custom button
        frm.add_custom_button('Send Welcome Email', function() {
            frappe.call({
                method: 'my_app.api.send_welcome_email',
                args: {
                    customer: frm.doc.name
                },
                callback: function(r) {
                    frappe.msgprint('Email sent!');
                }
            });
        });
    },
    
    customer_type: function(frm) {
        // Auto-fill based on customer type
        if (frm.doc.customer_type === 'Company') {
            frm.set_value('tax_category', 'Corporate');
        }
    }
});

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Server scripts

Add Python logic without creating files:
1

Create Server Script

Go to Customize > Server Script > New
2

Configure script

  • Select Script Type (DocType Event, API, Permission Query, etc.)
  • For DocType events, select the Reference DocType
  • Write Python code in the Script field
3

Enable script

Check Enabled and save
Example for Before Save event: 
# Server Script for Invoice (Before Save)
if doc.customer:
    customer = frappe.get_doc('Customer', doc.customer)
    doc.customer_group = customer.customer_group
    doc.territory = customer.territory
Example for API endpoint: 
# Server Script API
# Accessible at /api/method/my_custom_api

customer = frappe.get_value('Customer', 
    filters={'email': frappe.form_dict.email},
    fieldname=['name', 'customer_name'])

frappe.response['message'] = customer

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Using hooks

Hooks allow you to override or extend framework behavior. Define hooks in hooks.py:

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Override whitelisted methods

# hooks.py
override_whitelisted_methods = {
    "frappe.desk.form.load.getdoc": "my_app.overrides.custom_getdoc"
}

# my_app/overrides.py
import frappe

@frappe.whitelist()
def custom_getdoc(doctype, name, user=None):
    # Custom logic before loading document
    doc = frappe.get_doc(doctype, name)
    # Add custom processing
    return doc

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Document events

Hook into document lifecycle: 
# hooks.py
doc_events = {
    "Customer": {
        "before_save": "my_app.api.validate_customer",
        "after_insert": "my_app.api.create_customer_portal_user",
        "on_trash": "my_app.api.check_customer_transactions"
    },
    "*": {
        "on_update": "my_app.api.log_all_updates",
        "before_save": "my_app.api.add_modified_by"
    }
}

# my_app/api.py
import frappe

def validate_customer(doc, method):
    if not doc.tax_id and doc.customer_type == "Company":
        frappe.throw("Tax ID is required for company customers")

def create_customer_portal_user(doc, method):
    if doc.email and not frappe.db.exists("User", doc.email):
        # Create portal user
        user = frappe.get_doc({
            "doctype": "User",
            "email": doc.email,
            "first_name": doc.customer_name,
            "user_type": "Website User"
        })
        user.insert(ignore_permissions=True)

def log_all_updates(doc, method):
    # Log all document updates
    frappe.log_error(f"Document {doc.doctype} {doc.name} updated")

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Request hooks

Intercept HTTP requests: 
# hooks.py
before_request = ["my_app.api.log_request"]
after_request = ["my_app.api.add_custom_header"]

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Scheduler events

Schedule background tasks: 
# hooks.py
scheduler_events = {
    "hourly": [
        "my_app.tasks.sync_data"
    ],
    "daily": [
        "my_app.tasks.send_daily_reports"
    ],
    "cron": {
        "0 9 * * *": [  # 9 AM every day
            "my_app.tasks.morning_task"
        ]
    }
}

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Extending DocType classes

Extend standard DocType controllers with mixins: 
# hooks.py
extend_doctype_class = {
    "Customer": "my_app.custom.customer.CustomerExtension"
}

# my_app/custom/customer.py
class CustomerExtension:
    def validate(self):
        # This runs in addition to standard validation
        self.validate_credit_limit()
    
    def validate_credit_limit(self):
        if self.credit_limit and self.credit_limit > 100000:
            frappe.msgprint("High credit limit requires approval")

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Fixtures

Export customizations as fixtures for version control: 
# hooks.py
fixtures = [
    "Custom Field",
    "Custom Script",
    {"dt": "Print Format", "filters": [["module", "=", "My App"]]},
    {"dt": "Role", "filters": [["name", "in", ["Sales Manager", "Sales User"]]]}
]
Export fixtures: 
bench --site sitename export-fixtures
This creates JSON files in your app’s fixtures/ directory that can be imported on other sites.

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Property setters

Modify DocType properties without customizing: 
import frappe

def customize_doctype():
    frappe.make_property_setter({
        "doctype": "Customer",
        "fieldname": "customer_name",
        "property": "read_only",
        "value": 1,
        "property_type": "Check"
    })

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Custom permissions

Define custom permission logic: 
# hooks.py
permission_query_conditions = {
    "Customer": "my_app.permissions.get_customer_query_conditions"
}

has_permission = {
    "Customer": "my_app.permissions.has_customer_permission"
}

# my_app/permissions.py
import frappe

def get_customer_query_conditions(user):
    if not user: 
        user = frappe.session.user
    
    if "Sales Manager" in frappe.get_roles(user):
        # Sales managers see all customers
        return None
    
    # Sales users see only assigned customers
    return f"""(`tabCustomer`.`owner` = {frappe.db.escape(user)} 
                 OR `tabCustomer`.`account_manager` = {frappe.db.escape(user)})"""

def has_customer_permission(doc, user):
    if "Sales Manager" in frappe.get_roles(user):
        return True
    
    return doc.owner == user or doc.account_manager == user

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Best practices

Prefer documented hooks over directly modifying framework code. Hooks are upgrade-safe.
For significant changes, create a custom app instead of modifying standard apps.
Version control your customizations by exporting fixtures after changes.
Add comments explaining why customizations were made.
Test customizations on staging before deploying to production.

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