Move BehaviorChangeWarning to first array Quantity creation

3 files changed, 34 insertions, 11 deletions

diff --git a/pint/compat.py b/pint/compat.py
index 254a7b1..abe5126 100644
--- a/pint/compat.py
+++ b/pint/compat.py
@@ -25,17 +25,21 @@ def tokenizer(input_string):
 # TODO: remove this warning after v0.10
 class BehaviorChangeWarning(UserWarning):
     pass
-_msg = """The way pint handles numpy operations has changed with the implementation of NEP 18.
-Unimplemented numpy operations will now fail instead of making assumptions about units. Some 
-functions, eg concat, will now return Quanties with units, where they returned ndarrays 
-previously. See  https://github.com/hgrecco/pint/pull/905. 
+array_function_change_msg = """The way Pint handles NumPy operations has changed with the
+implementation of NEP 18. Unimplemented NumPy operations will now fail instead of making
+assumptions about units. Some functions, eg concat, will now return Quanties with units, where
+they returned ndarrays previously. See https://github.com/hgrecco/pint/pull/905.
 
-To hide this warning use the following code to import pint:
+To hide this warning, wrap your first creation of an array Quantity with
+warnings.catch_warnings(), like the following:
 
+import numpy as np
 import warnings
+from pint import Quantity
+
 with warnings.catch_warnings():
     warnings.simplefilter("ignore")
-    import pint
+    Quantity([])
 
 To disable the new behavior, see
 https://www.numpy.org/neps/nep-0018-array-function-protocol.html#implementation
@@ -74,9 +78,7 @@ try:
             return False
 
     HAS_NUMPY_ARRAY_FUNCTION = _test_array_function_protocol()
-
-    if HAS_NUMPY_ARRAY_FUNCTION:
-        warnings.warn(_msg, BehaviorChangeWarning)
+    SKIP_ARRAY_FUNCTION_CHANGE_WARNING = not HAS_NUMPY_ARRAY_FUNCTION
 
     NP_NO_VALUE = np._NoValue
 
@@ -91,6 +93,7 @@ except ImportError:
     NUMPY_VER = '0'
     NUMERIC_TYPES = (Number, Decimal)
     HAS_NUMPY_ARRAY_FUNCTION = False
+    SKIP_ARRAY_FUNCTION_CHANGE_WARNING = True
     NP_NO_VALUE = None
 
     def _to_magnitude(value, force_ndarray=False):
diff --git a/pint/quantity.py b/pint/quantity.py
index ca6f25f..6d57e42 100644
--- a/pint/quantity.py
+++ b/pint/quantity.py
@@ -26,7 +26,9 @@ from .formatting import (remove_custom_flags, siunitx_format_unit, ndarray_to_la
 from .errors import (DimensionalityError, OffsetUnitCalculusError, PintTypeError,
                      UndefinedUnitError, UnitStrippedWarning)
 from .definitions import UnitDefinition
-from .compat import Loc, NUMPY_VER, ndarray, np, _to_magnitude, is_upcast_type, eq
+from .compat import (Loc, NUMPY_VER, SKIP_ARRAY_FUNCTION_CHANGE_WARNING,
+                     BehaviorChangeWarning, ndarray, np, _to_magnitude, is_upcast_type, eq,
+                     array_function_change_msg)
 from .util import (PrettyIPython, logger, UnitsContainer, SharedRegistryObject,
                    to_units_container, infer_base_unit, iterable, sized)
 from .numpy_func import (HANDLED_UFUNCS, copy_units_output_ufuncs, get_op_output_unit,
@@ -118,6 +120,8 @@ class Quantity(PrettyIPython, SharedRegistryObject):
         return _unpickle, (Quantity, self.magnitude, self._units)
 
     def __new__(cls, value, units=None):
+        global SKIP_ARRAY_FUNCTION_CHANGE_WARNING
+
         if units is None:
             if isinstance(value, str):
                 if value == '':
@@ -156,6 +160,10 @@ class Quantity(PrettyIPython, SharedRegistryObject):
         inst.__used = False
         inst.__handling = None
 
+        if not SKIP_ARRAY_FUNCTION_CHANGE_WARNING and isinstance(inst._magnitude, ndarray):
+            warnings.warn(array_function_change_msg, BehaviorChangeWarning)
+            SKIP_ARRAY_FUNCTION_CHANGE_WARNING = True
+
         return inst
 
     @property
diff --git a/pint/testsuite/test_quantity.py b/pint/testsuite/test_quantity.py
index d07e229..dc82ff2 100644
--- a/pint/testsuite/test_quantity.py
+++ b/pint/testsuite/test_quantity.py
@@ -4,12 +4,14 @@ import copy
 import datetime
 import math
 import operator as op
+import warnings
 
 from pint import DimensionalityError, OffsetUnitCalculusError, UnitRegistry
 from pint.unit import UnitsContainer
-from pint.compat import np
+from pint.compat import BehaviorChangeWarning, np
 from pint.testsuite import QuantityTestCase, helpers
 from pint.testsuite.parameterized import ParameterizedTestCase
+from unittest.mock import patch
 
 
 class TestQuantity(QuantityTestCase):
@@ -402,6 +404,16 @@ class TestQuantity(QuantityTestCase):
         with self.assertRaises(TypeError):
             iter(x)
 
+    @helpers.requires_array_function_protocol()
+    @patch('pint.quantity.SKIP_ARRAY_FUNCTION_CHANGE_WARNING', False)
+    def test_array_function_warning_on_creation(self):
+        # Test that warning is raised on first creation, but not second
+        with self.assertWarns(BehaviorChangeWarning):
+            self.Q_([])
+        with warnings.catch_warnings():
+            warnings.filterwarnings('error')
+            self.Q_([])
+
 
 class TestQuantityToCompact(QuantityTestCase):