[deprecate-source-assets] remove SourceAsset from assorted docs (#23718)

## Summary & Motivation

SourceAsset is a deprecated concept, but our dlt docs and some random
guides still reference them. This PR switches mentions to AssetSpec and
"external asset".

## How I Tested These Changes

docs/content/concepts/partitions-schedules-sensors/backfills.mdx

@@ -99,12 +99,12 @@ def events(context: AssetExecutionContext) -> None:
 ```python file=/concepts/partitions_schedules_sensors/backfills/single_run_backfill_io_manager.py startafter=start_marker endbefore=end_marker
 from dagster import (
     AssetExecutionContext,
+    AssetSpec,
     BackfillPolicy,
     DailyPartitionsDefinition,
     InputContext,
     IOManager,
     OutputContext,
-    SourceAsset,
     asset,
 )
 
@@ -121,7 +121,7 @@ class MyIOManager(IOManager):
 
 daily_partition = DailyPartitionsDefinition(start_date="2020-01-01")
 
-raw_events = SourceAsset("raw_events", partitions_def=daily_partition)
+raw_events = AssetSpec("raw_events", partitions_def=daily_partition)
 
 
 @asset(

docs/content/guides/dagster/enriching-with-software-defined-assets.mdx

@@ -219,11 +219,11 @@ Here's what an equivalent job looks like using asset definitions:
 ```python file=/guides/dagster/enriching_with_software_defined_assets/sda_io_manager.py
 from pandas import DataFrame
 
-from dagster import Definitions, SourceAsset, asset, define_asset_job
+from dagster import AssetSpec, Definitions, asset, define_asset_job
 
 from .mylib import s3_io_manager, snowflake_io_manager, train_recommender_model
 
-raw_users = SourceAsset(key="raw_users", io_manager_key="warehouse")
+raw_users = AssetSpec(key="raw_users", metadata={"dagster/io_manager_key": "warehouse"})
 
 
 @asset(io_manager_key="warehouse")
@@ -301,14 +301,12 @@ Here's an example of an equivalent job that uses asset definitions:
 ```python file=/guides/dagster/enriching_with_software_defined_assets/sda_nothing.py
 from pandas import read_sql
 
-from dagster import Definitions, SourceAsset, asset, define_asset_job
+from dagster import Definitions, asset, define_asset_job
 
 from .mylib import create_db_connection, pickle_to_s3, train_recommender_model
 
-raw_users = SourceAsset(key="raw_users")
 
-
-@asset(deps=[raw_users])
+@asset(deps=["raw_users"])
 def users() -> None:
     raw_users_df = read_sql("select * from raw_users", con=create_db_connection())
     users_df = raw_users_df.dropna()

docs/content/guides/dagster/software-defined-assets.mdx

@@ -29,10 +29,10 @@ Here are our asset definitions that define tables we want to materialize.
 
 ```python file=../../assets_pandas_pyspark/assets_pandas_pyspark/assets/table_assets.py startafter=start_marker endbefore=end_marker
 import pandas as pd
-from dagster import AssetKey, SourceAsset, asset
+from dagster import AssetKey, AssetSpec, Definitions, asset
 from pandas import DataFrame
 
-sfo_q2_weather_sample = SourceAsset(
+sfo_q2_weather_sample = AssetSpec(
     key=AssetKey("sfo_q2_weather_sample"),
     description="Weather samples, taken every five minutes at SFO",
     metadata={"format": "csv"},
@@ -50,9 +50,12 @@ def daily_temperature_highs(sfo_q2_weather_sample: DataFrame) -> DataFrame:
 def hottest_dates(daily_temperature_highs: DataFrame) -> DataFrame:
     """Computes the 10 hottest dates."""
     return daily_temperature_highs.nlargest(10, "max_tmpf")
+
+
+defs = Definitions(assets=[sfo_q2_weather_sample, daily_temperature_highs, hottest_dates])
 ```
 
-`sfo_q2_weather_sample` represents our base temperature table. It's a <PyObject module="dagster" object="SourceAsset" />, meaning that we rely on it, but don't generate it.
+`sfo_q2_weather_sample` represents our base temperature table. It's a table that's used by the assets in our pipeline, but that we're not responsible for generating. We define it using an <PyObject object="AssetSpec" />, which allows us to specify its attributes without providing a function that materializes it.
 
 `daily_temperature_highs` represents a computed asset. It's derived by taking the `sfo_q2_weather_sample` table and applying the decorated function to it. Notice that it's defined using a pure function, a function with no side effects, just logical data transformation. The code for storing and retrieving the data in persistent storage will be supplied later on in an <PyObject object="IOManager" />. This allows us to swap in different implementations in different environments. For example, in local development, we might want to store data in a local CSV file for easy testing. However in production, we would want to store data in a data warehouse.
 

docs/content/integrations/embedded-elt/dlt.mdx

@@ -283,19 +283,19 @@ def dlt_example_assets(context: AssetExecutionContext, dlt: DagsterDltResource):
 
 In this example, we customized the translator to change how the dlt assets' names are defined. We also hard-coded the asset dependency upstream of our assets to provide a fan-out model from a single dependency to our dlt assets.
 
-### Assigning metadata to upstream source assets
+### Assigning metadata to upstream external assets
 
-A common question is how to define metadata on the source assets upstream of the dlt assets.
+A common question is how to define metadata on the external assets upstream of the dlt assets.
 
-This can be accomplished by defining a <PyObject object="SourceAsset" /> with a key that matches the one defined in the <PyObject module="dagster_embedded_elt.dlt" object="DagsterDltTranslator" method="get_deps_assets_keys" /> method.
+This can be accomplished by defining a <PyObject object="AssetSpec" /> with a key that matches the one defined in the <PyObject module="dagster_embedded_elt.dlt" object="DagsterDltTranslator" method="get_deps_assets_keys" /> method.
 
-For example, let's say we have defined a set of dlt assets named `thinkific_assets`, we can iterate over those assets and derive a <PyObject object="SourceAsset" /> with attributes like `group_name`.
+For example, let's say we have defined a set of dlt assets named `thinkific_assets`, we can iterate over those assets and derive a <PyObject object="AssetSpec" /> with attributes like `group_name`.
 
 ```python file=/integrations/embedded_elt/dlt_source_assets.py
 import dlt
 from dagster_embedded_elt.dlt import DagsterDltResource, dlt_assets
 
-from dagster import AssetExecutionContext, SourceAsset
+from dagster import AssetExecutionContext, AssetSpec
 
 
 @dlt.source
@@ -319,8 +319,7 @@ def example_dlt_assets(context: AssetExecutionContext, dlt: DagsterDltResource):
 
 
 thinkific_source_assets = [
-    SourceAsset(key, group_name="thinkific")
-    for key in example_dlt_assets.dependency_keys
+    AssetSpec(key, group_name="thinkific") for key in example_dlt_assets.dependency_keys
 ]
 ```
 

examples/assets_pandas_pyspark/assets_pandas_pyspark/assets/table_assets.py

@@ -1,9 +1,9 @@
 # start_marker
 import pandas as pd
-from dagster import AssetKey, SourceAsset, asset
+from dagster import AssetKey, AssetSpec, Definitions, asset
 from pandas import DataFrame
 
-sfo_q2_weather_sample = SourceAsset(
+sfo_q2_weather_sample = AssetSpec(
     key=AssetKey("sfo_q2_weather_sample"),
     description="Weather samples, taken every five minutes at SFO",
     metadata={"format": "csv"},
@@ -23,4 +23,6 @@ def hottest_dates(daily_temperature_highs: DataFrame) -> DataFrame:
     return daily_temperature_highs.nlargest(10, "max_tmpf")
 
 
+defs = Definitions(assets=[sfo_q2_weather_sample, daily_temperature_highs, hottest_dates])
+
 # end_marker

examples/assets_pandas_pyspark/assets_pandas_pyspark_tests/test_assets.py

@@ -1,4 +1,4 @@
-from dagster import load_assets_from_modules, materialize
+from dagster import materialize
 from dagster._core.test_utils import instance_for_test
 
 from assets_pandas_pyspark.assets import spark_asset, table_assets
@@ -9,7 +9,7 @@ def test_weather_assets():
 
     with instance_for_test() as instance:
         assert materialize(
-            load_assets_from_modules([table_assets]),
+            table_assets.defs.assets,
             instance=instance,
             resources={"io_manager": LocalFileSystemIOManager()},
         ).success
@@ -20,7 +20,7 @@ def test_spark_weather_assets():
 
     with instance_for_test() as instance:
         assert materialize(
-            load_assets_from_modules([table_assets, spark_asset]),
+            [*table_assets.defs.assets, spark_asset.daily_temperature_high_diffs],
             instance=instance,
             resources={"io_manager": LocalFileSystemIOManager()},
         ).success

examples/docs_snippets/docs_snippets/concepts/partitions_schedules_sensors/backfills/single_run_backfill_io_manager.py

@@ -1,12 +1,12 @@
 # start_marker
 from dagster import (
     AssetExecutionContext,
+    AssetSpec,
     BackfillPolicy,
     DailyPartitionsDefinition,
     InputContext,
     IOManager,
     OutputContext,
-    SourceAsset,
     asset,
 )
 
@@ -23,7 +23,7 @@ def handle_output(self, context: OutputContext, obj):
 
 daily_partition = DailyPartitionsDefinition(start_date="2020-01-01")
 
-raw_events = SourceAsset("raw_events", partitions_def=daily_partition)
+raw_events = AssetSpec("raw_events", partitions_def=daily_partition)
 
 
 @asset(

examples/docs_snippets/docs_snippets/guides/dagster/enriching_with_software_defined_assets/sda_io_manager.py

@@ -1,10 +1,10 @@
 from pandas import DataFrame
 
-from dagster import Definitions, SourceAsset, asset, define_asset_job
+from dagster import AssetSpec, Definitions, asset, define_asset_job
 
 from .mylib import s3_io_manager, snowflake_io_manager, train_recommender_model
 
-raw_users = SourceAsset(key="raw_users", io_manager_key="warehouse")
+raw_users = AssetSpec(key="raw_users", metadata={"dagster/io_manager_key": "warehouse"})
 
 
 @asset(io_manager_key="warehouse")

examples/docs_snippets/docs_snippets/guides/dagster/enriching_with_software_defined_assets/sda_nothing.py

@@ -1,13 +1,11 @@
 from pandas import read_sql
 
-from dagster import Definitions, SourceAsset, asset, define_asset_job
+from dagster import Definitions, asset, define_asset_job
 
 from .mylib import create_db_connection, pickle_to_s3, train_recommender_model
 
-raw_users = SourceAsset(key="raw_users")
 
-
-@asset(deps=[raw_users])
+@asset(deps=["raw_users"])
 def users() -> None:
     raw_users_df = read_sql("select * from raw_users", con=create_db_connection())
     users_df = raw_users_df.dropna()

examples/docs_snippets/docs_snippets/integrations/embedded_elt/dlt_source_assets.py

@@ -1,7 +1,7 @@
 import dlt
 from dagster_embedded_elt.dlt import DagsterDltResource, dlt_assets
 
-from dagster import AssetExecutionContext, SourceAsset
+from dagster import AssetExecutionContext, AssetSpec
 
 
 @dlt.source
@@ -25,6 +25,5 @@ def example_dlt_assets(context: AssetExecutionContext, dlt: DagsterDltResource):
 
 
 thinkific_source_assets = [
-    SourceAsset(key, group_name="thinkific")
-    for key in example_dlt_assets.dependency_keys
+    AssetSpec(key, group_name="thinkific") for key in example_dlt_assets.dependency_keys
 ]