docs(python): More examples for read_csv

Addition to pola-rs#13545 with unnecessary doctest skips removed.
Part of pola-rs#13161.

py-polars/polars/io/csv/functions.py

@@ -196,16 +196,16 @@ def read_csv(
 
     Reproducible example using BytesIO object, parsing dates.
 
-    >>> import io  # doctest: +SKIP
+    >>> import io
     >>> source = io.BytesIO(
     ...     (
     ...         "ID,Name,Birthday\n"
     ...         "1,Alice,1995-07-12\n"
     ...         "2,Bob,1990-09-20\n"
     ...         "3,Charlie,2002-03-08"
     ...     ).encode()
-    ... )  # doctest: +SKIP
-    >>> pl.read_csv(source, try_parse_dates=True)  # doctest: +SKIP
+    ... )
+    >>> pl.read_csv(source, try_parse_dates=True)
     shape: (3, 3)
     ┌─────┬─────────┬────────────┐
     │ ID  ┆ Name    ┆ Birthday   │
@@ -216,6 +216,46 @@ def read_csv(
     │ 2   ┆ Bob     ┆ 1990-09-20 │
     │ 3   ┆ Charlie ┆ 2002-03-08 │
     └─────┴─────────┴────────────┘
+
+    Consider specifying `dtypes` or `schema` to avoid wrong schema inference.
+    Alternatively, `infer_schema_length` can be adapted.
+
+    >>> pl.read_csv("sparse_data.csv")  # doctest: +SKIP
+    shape: (102, 2)
+    ┌────────┬────────────┐
+    │ row_nr ┆ sparse_int │
+    │ ---    ┆ ---        │
+    │ i64    ┆ str        │
+    ╞════════╪════════════╡
+    │ 0      ┆ null       │
+    │ 1      ┆ null       │
+    │ 2      ┆ null       │
+    │ 3      ┆ null       │
+    │ …      ┆ …          │
+    │ 98     ┆ null       │
+    │ 99     ┆ null       │
+    │ 100    ┆ null       │
+    │ 101    ┆ 1          │
+    └────────┴────────────┘
+    >>> pl.read_csv(
+    ...     "sparse_data.csv", infer_schema_length=None, dtypes={"row_nr": pl.Int8}
+    ... )  # doctest: +SKIP
+    shape: (102, 2)
+    ┌────────┬────────────┐
+    │ row_nr ┆ sparse_int │
+    │ ---    ┆ ---        │
+    │ i8     ┆ i64        │
+    ╞════════╪════════════╡
+    │ 0      ┆ null       │
+    │ 1      ┆ null       │
+    │ 2      ┆ null       │
+    │ 3      ┆ null       │
+    │ …      ┆ …          │
+    │ 98     ┆ null       │
+    │ 99     ┆ null       │
+    │ 100    ┆ null       │
+    │ 101    ┆ 1          │
+    └────────┴────────────┘
     """
     _check_arg_is_1byte("separator", separator, can_be_empty=False)
     _check_arg_is_1byte("quote_char", quote_char, can_be_empty=True)