[GitHub] [arrow] pitrou commented on a change in pull request #9948: ARROW-12150: [Python] Correctly infer type of mixed-precision Decimals

[GitBox <gi...@apache.org>]

pitrou commented on a change in pull request #9948:
URL: https://github.com/apache/arrow/pull/9948#discussion_r625185741



##########
File path: python/pyarrow/tests/test_convert_builtin.py
##########
@@ -1501,6 +1501,42 @@ def test_sequence_decimal_too_high_precision():
         pa.array([decimal.Decimal('1' * 80)])
 
 
+def test_sequence_decimal_infer():
+    for data, typ in [
+        # simple case
+        (decimal.Decimal('1.234'), pa.decimal128(4, 3)),
+        # trailing zeros
+        (decimal.Decimal('12300'), pa.decimal128(5, 0)),
+        (decimal.Decimal('12300.0'), pa.decimal128(6, 1)),
+        # scientific power notation
+        (decimal.Decimal('123E+2'), pa.decimal128(3, -2)),

Review comment:
       What about `decimal.Decimal('0.0123')`?

##########
File path: python/pyarrow/tests/test_convert_builtin.py
##########
@@ -1501,6 +1501,42 @@ def test_sequence_decimal_too_high_precision():
         pa.array([decimal.Decimal('1' * 80)])
 
 
+def test_sequence_decimal_infer():
+    for data, typ in [
+        # simple case
+        (decimal.Decimal('1.234'), pa.decimal128(4, 3)),
+        # trailing zeros
+        (decimal.Decimal('12300'), pa.decimal128(5, 0)),
+        (decimal.Decimal('12300.0'), pa.decimal128(6, 1)),
+        # scientific power notation
+        (decimal.Decimal('123E+2'), pa.decimal128(3, -2)),
+    ]:
+        assert pa.infer_type([data]) == typ
+        arr = pa.array([data])
+        assert arr.type == typ
+        assert arr.to_pylist()[0] == data
+
+
+def test_sequence_decimal_infer_mixed():
+    # ARROW-12150 - ensure mixed precision gets correctly inferred to
+    # common type that can hold all input values
+    cases = [
+        ([decimal.Decimal('1.234'), decimal.Decimal('3.456')],
+         pa.decimal128(4, 3)),
+        ([decimal.Decimal('1.234'), decimal.Decimal('456.7')],
+         pa.decimal128(6, 3)),
+        ([decimal.Decimal('123.4'), decimal.Decimal('4.567')],
+         pa.decimal128(6, 3)),
+        ([decimal.Decimal('123e2'), decimal.Decimal('4567e3')],
+         pa.decimal128(5, -2)),

Review comment:
       Hmm... can you add a test for `[decimal.Decimal('123e4'), decimal.Decimal('4567e2')]`, and also one for `[decimal.Decimal('0.123'), decimal.Decimal('0.04567')]`?




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