dict_update.py

before = {
    "id": "sub_sched_KppC1fgG2c95PyN47xCK2Fdv",
    "object": "subscription_schedule",
    "application": None,
    "canceled_at": None,
    "completed_at": None,
    "created": 1690985828.495943,
    "current_phase": None,
    "customer": "cust_D2las0ShUdDNJh5GeudIakfm",
    "default_settings": {
        "application_fee_percent": None,
        "automatic_tax": {"enabled": False},
        "billing_cycle_anchor": "automatic",
        "billing_thresholds": None,
        "collection_method": "charge_automatically",
        "default_payment_method": None,
        "description": None,
        "invoice_settings": None,
        "on_behalf_of": None,
        "transfer_data": None,
    },
    "end_behavior": "release",
    "livemode": False,
    "metadata": {},
    "phases": [
        {
            "add_invoice_items": [],
            "application_fee_percent": None,
            "billing_cycle_anchor": None,
            "billing_thresholds": None,
            "collection_method": None,
            "coupon": None,
            "currency": "usd",
            "default_payment_method": None,
            "default_tax_rates": [],
            "description": None,
            "end_date": 1722626228.495858,
            "invoice_settings": None,
            "items": [
                {
                    "billing_thresholds": None,
                    "metadata": {},
                    "price": "price_1M3OSoIxQImiSx1FgPckhikZ",
                    "quantity": 1,
                    "tax_rates": [],
                }
            ],
            "metadata": {},
            "on_behalf_of": None,
            "proration_behavior": "create_prorations",
            "start_date": 1691003828.495858,
            "transfer_data": None,
            "trial_end": None,
        }
    ],
    "released_at": None,
    "released_subscription": None,
    "status": "active",
    "subscription": "sub_DRctGmkR4CJIyE8G7rSX8tQR",
    "test_clock": None,
}

after =  {
    "id": "sub_sched_KppC1fgG2c95PyN47xCK2Fdv",
    "object": "subscription_schedule",
    "application": None,
    "canceled_at": None,
    "completed_at": None,
    "created": 1690985828.495943,
    "current_phase": None,
    "customer": "cust_D2las0ShUdDNJh5GeudIakfm",
    "default_settings": {
        "application_fee_percent": None,
        "automatic_tax": {"enabled": False},
        "billing_cycle_anchor": "automatic",
        "billing_thresholds": None,
        "collection_method": "charge_automatically",
        "default_payment_method": None,
        "description": None,
        "invoice_settings": None,
        "on_behalf_of": None,
        "transfer_data": None,
    },
    "end_behavior": "release",
    "livemode": False,
    "metadata": {},
    "phases": [
        {
            "add_invoice_items": [],
            "application_fee_percent": None,
            "billing_cycle_anchor": None,
            "billing_thresholds": None,
            "collection_method": None,
            "coupon": None,
            "currency": "usd",
            "default_payment_method": None,
            "default_tax_rates": [],
            "description": None,
            "end_date": 1722626228.495858,
            "invoice_settings": None,
            "items": [
                {
                    "billing_thresholds": None,
                    "metadata": {},
                    "price": "price_1M3OSoIxQImiSx1FgPckhikZ",
                    "quantity": 1,
                    "tax_rates": [],
                }
            ],
            "metadata": {},
            "on_behalf_of": None,
            "proration_behavior": "create_prorations",
            "start_date": 1691003828.495858,
            "transfer_data": None,
            "trial_end": None,
        }
    ],
    "released_at": None,
    "released_subscription": None,
    "status": "active",
    "subscription": "sub_DRctGmkR4CJIyE8G7rSX8tQR",
    "test_clock": None,
}

params = {
    "phases[0]": {"end_date": "1693253412", "start_date": "1690575012"},
    "phases[0][items][0]": {"price": "price_1M3OSoIxQImiSx1FgPckhikZ"},
    "phases[1]": {"start_date": "1693253412"},
    "phases[1][items][0]": {"price": "price_1M3OSoIxQImiSx1FgPckhikZ"},
}


def recursive_update_dict(data: dict, data_to_update: dict):
    for k, v in data_to_update.items():
        if "[" in k:
            d = data
            for part in [x if not x.isnumeric() else int(x) for x in k.replace(']', '').split('[')]:
                d = d[part]
            d.update(**v)

        elif "__" in k:
            name, sub = k.split("__", maxsplit=1)
            recursive_update_dict(data[name], {sub: v})
        else:
            data[k] = v

recursive_update_dict(d, params)

if you want to include the missing parts instead of skipping

def recursive_update_dict(data: dict, data_to_update: dict):
    for k, v in data_to_update.items():
        if "[" in k:
            d = data
            for part in [x if not x.isnumeric() else int(x) for x in k.replace(']', '').split('[')]:
                try:
                    d = d[part]
                except IndexError:
                    d.append({})
                    d = d[part]
                except KeyError:
                    d[part] = []
                    d = d[part]
            d.update(**v)

        elif "__" in k:
            name, sub = k.split("__", maxsplit=1)
            recursive_update_dict(data[name], {sub: v})
        else:
            data[k] = v