TST / string dtype: add env variable to enable future_string and add test build (#58459)

Author: jorisvandenbossche

.github/workflows/unit-tests.yml

@@ -85,6 +85,10 @@ jobs:
             env_file: actions-39.yaml
             pattern: "not slow and not network and not single_cpu"
             pandas_copy_on_write: "warn"
+          - name: "Future infer strings"
+            env_file: actions-311.yaml
+            pattern: "not slow and not network and not single_cpu"
+            pandas_future_infer_string: "1"
           - name: "Pypy"
             env_file: actions-pypy-39.yaml
             pattern: "not slow and not network and not single_cpu"
@@ -103,7 +107,8 @@ jobs:
       LANG: ${{ matrix.lang || 'C.UTF-8' }}
       LC_ALL: ${{ matrix.lc_all || '' }}
       PANDAS_COPY_ON_WRITE: ${{ matrix.pandas_copy_on_write || '0' }}
-      PANDAS_CI: ${{ matrix.pandas_ci || '1' }}
+      PANDAS_CI: '1'
+      PANDAS_FUTURE_INFER_STRING: ${{ matrix.pandas_future_infer_string || '0' }}
       TEST_ARGS: ${{ matrix.test_args || '' }}
       PYTEST_WORKERS: ${{ matrix.pytest_workers || 'auto' }}
       PYTEST_TARGET: ${{ matrix.pytest_target || 'pandas' }}

ci/run_tests.sh

@@ -16,5 +16,11 @@ if [[ "$PATTERN" ]]; then
   PYTEST_CMD="$PYTEST_CMD -m \"$PATTERN\""
 fi
 
+# temporarily let pytest always succeed (many tests are not yet passing in the
+# build enabling the future string dtype)
+if [[ "$PANDAS_FUTURE_INFER_STRING" == "1" ]]; then
+  PYTEST_CMD="$PYTEST_CMD || true"
+fi
+
 echo $PYTEST_CMD
 sh -c "$PYTEST_CMD"

pandas/core/config_init.py

@@ -905,7 +905,7 @@ def register_converter_cb(key) -> None:
 with cf.config_prefix("future"):
     cf.register_option(
         "infer_string",
-        False,
+        True if os.environ.get("PANDAS_FUTURE_INFER_STRING", "0") == "1" else False,
         "Whether to infer sequence of str objects as pyarrow string "
         "dtype, which will be the default in pandas 3.0 "
         "(at which point this option will be deprecated).",