Document currency code capability, in README and example

README.md

@@ -151,8 +151,8 @@ For a batch transform, use `Client`. You should only need the following methods:
 * `find_file_path(file_name)` – Resolve a bundled file (placed under `payload/files/`) to a `pathlib.Path` that exists. Works the same locally and inside Data Cloud — see [Bundled file resolution](#bundled-file-resolution) below for the full lookup order. Raises `FileNotFoundError` if the file isn't found.
 * `read_dlo(name)` – Read from a Data Lake Object by name
 * `read_dmo(name)` – Read from a Data Model Object by name
-* `write_to_dlo(name, spark_dataframe, write_mode)` – Write to a Data Model Object by name with a Spark dataframe
-* `write_to_dmo(name, spark_dataframe, write_mode)` – Write to a Data Lake Object by name with a Spark dataframe
+* `write_to_dlo(name, spark_dataframe, write_mode)` – Write to a Data Lake Object by name with a Spark dataframe
+* `write_to_dmo(name, spark_dataframe, write_mode)` – Write to a Data Model Object by name with a Spark dataframe
 
 For a streaming (delta) transform, use `StreamingClient`, which exposes the streaming counterparts:
 * `read_dlo_deltas()` – Read the streaming change feed (deltas) of a Data Lake Object as a streaming DataFrame.
@@ -176,6 +176,27 @@ client.write_to_dlo('output_DLO')
 > [!WARNING]
 > Currently we only support reading from DMOs and writing to DMOs or reading from DLOs and writing to DLOs, but they cannot mix.
 
+### Writing Currency fields
+
+A DLO or DMO with a Currency field has a system field, `cdp_sys_record_currency__c`, that holds each record's ISO currency code. Data Explorer UI uses it to display currency values, and shows a blank value if it's null (Query Editor still shows it).
+
+Code Extensions don't set `cdp_sys_record_currency__c` for you, since only your script knows what currency its amounts are in. When writing to an object with Currency fields, set it yourself:
+
+```python
+from pyspark.sql.functions import lit
+
+# Either source has Currency fields: keep its cdp_sys_record_currency__c in your output
+df = source.select("id__c", "amount__c", "cdp_sys_record_currency__c")
+
+# OR amounts are in a known currency
+df = df.withColumn("cdp_sys_record_currency__c", lit("EUR"))
+```
+
+- Use an ISO code that exists in your org's configured currency rates.
+- When aggregating, group by the currency code so amounts in different currencies aren't summed together.
+
+See [`docs/examples/script/currency_totals`](docs/examples/script/currency_totals) for a complete example.
+
 ### Streaming (delta) transforms
 
 Streaming BYOC transforms process a Data Lake Object's Change Data Feed continuously instead of reading a bounded snapshot. Use a `StreamingClient` and its `*_deltas` methods in place of the batch `Client` read/write methods:

docs/examples/script/currency_totals/README.md

@@ -0,0 +1,33 @@
+# Example: Totals that keep their currency code
+
+A Code Extension for Batch Transform that sums `Payment__dll` amounts per customer and writes them to `Customer_Payment_Totals__dll`, a DLO with a Currency field, setting the `cdp_sys_record_currency__c` system field so Data Explorer UI can display the totals.
+
+## What it demonstrates
+
+- **Carrying `cdp_sys_record_currency__c` through** from the source rows to the output. Code Extensions don't auto-populate `cdp_sys_record_currency__c`. And if it's null, Data Explorer shows the Currency value as blank. See [Writing Currency fields](../../../../README.md#writing-currency-fields).
+- **Grouping by currency** so EUR and USD amounts are never summed together. Customer `C1` gets one EUR row and one USD row.
+- If all amounts are in one known currency, one can also use `lit("EUR")` instead of copying a column.
+
+## Prerequisites
+
+Input DLO `Payment__dll` in the target dataspace:
+
+| Column                       | Type     | Notes                                                                  |
+|------------------------------|----------|------------------------------------------------------------------------|
+| `id__c`                      | text     | Primary key.                                                           |
+| `customer_id__c`             | text     |                                                                        |
+| `amount__c`                  | Currency |                                                                        |
+| `cdp_sys_record_currency__c` | text     | System field, added with the Currency field. From CSV `currency_code`. |
+
+Data should be loaded via `sample_data/payment.csv`.  See [loading sample data](../README.md#loading-sample-data).
+
+Output DLO `Customer_Payment_Totals__dll` in the same dataspace:
+
+| Column                       | Type     | Notes                                            |
+|------------------------------|----------|--------------------------------------------------|
+| `id__c`                      | text     | Primary key (`<customer_id>_<currency>`).        |
+| `customer_id__c`             | text     |                                                  |
+| `total_amount__c`            | Currency |                                                  |
+| `cdp_sys_record_currency__c` | text     | System field, added with the Currency field.     |
+
+The ISO codes in `sample_data/payment.csv` must exist in your org's currency rates.

docs/examples/script/currency_totals/payload/config.json

@@ -0,0 +1,13 @@
+{
+  "sdkVersion": "6.1.0",
+  "entryPoint": "entrypoint.py",
+  "dataspace": "default",
+  "permissions": {
+    "read": {
+      "dlo": ["Payment__dll"]
+    },
+    "write": {
+      "dlo": ["Customer_Payment_Totals__dll"]
+    }
+  }
+}

docs/examples/script/currency_totals/payload/entrypoint.py

@@ -0,0 +1,35 @@
+from pyspark.sql.functions import (
+    col,
+    concat_ws,
+    sum as _sum,
+)
+
+from datacustomcode.client import Client
+from datacustomcode.io.writer.base import WriteMode
+
+
+def main():
+    client = Client()
+
+    payments = client.read_dlo("Payment__dll").select(
+        "customer_id__c", "amount__c", "cdp_sys_record_currency__c"
+    )
+
+    totals = (
+        payments.groupBy("customer_id__c", "cdp_sys_record_currency__c")
+        .agg(_sum("amount__c").alias("total_amount__c"))
+        .select(
+            concat_ws(
+                "_", col("customer_id__c"), col("cdp_sys_record_currency__c")
+            ).alias("id__c"),
+            col("customer_id__c"),
+            col("total_amount__c"),
+            col("cdp_sys_record_currency__c"),
+        )
+    )
+
+    client.write_to_dlo("Customer_Payment_Totals__dll", totals, WriteMode.OVERWRITE)
+
+
+if __name__ == "__main__":
+    main()

docs/examples/script/currency_totals/requirements.txt

@@ -0,0 +1 @@
+# Packages required for the custom code

docs/examples/script/currency_totals/sample_data/payment.csv

@@ -0,0 +1,7 @@
+id,customer_id,amount,currency_code
+P1,C1,100.00,EUR
+P2,C1,250.50,EUR
+P3,C1,40.00,USD
+P4,C2,1200.00,USD
+P5,C2,300.00,USD
+P6,C3,75.25,GBP