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

Author: jorisvandenbossche

.github/workflows/unit-tests.yml

@@ -57,6 +57,10 @@ jobs:
             # Also install zh_CN (its encoding is gb2312) but do not activate it.
             # It will be temporarily activated during tests with locale.setlocale
             extra_loc: "zh_CN"
+          - 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"
@@ -75,6 +79,7 @@ jobs:
       LANG: ${{ matrix.lang || 'C.UTF-8' }}
       LC_ALL: ${{ matrix.lc_all || '' }}
       PANDAS_CI: '1'
+      PANDAS_FUTURE_INFER_STRING: ${{ matrix.pandas_future_infer_string || '0' }}
       TEST_ARGS: ${{ matrix.test_args || '' }}
       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

@@ -858,7 +858,7 @@ def register_converter_cb(key: str) -> 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).",