PERF: Speed-up common case of loadtxt()ing non-hex floats. (#19598)

* PERF: Speed-up common case of loadtxt()ing non-hex floats.

`python runtests.py --bench bench_io` reports a ~5-10% perf gain.

* TST: Add tests to check fromhex not called unintentionally.

Adds regression tests to check that the logic surrounding when
the default floatconv applies the fromhex conversion does not
change.

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

2 files changed, 31 insertions, 4 deletions

diff --git a/numpy/lib/npyio.py b/numpy/lib/npyio.py
index 9ae5d0b59..3b6a1c563 100644
--- a/numpy/lib/npyio.py
+++ b/numpy/lib/npyio.py
@@ -732,10 +732,15 @@ def _getconv(dtype):
     """ Find the correct dtype converter. Adapted from matplotlib """
 
     def floatconv(x):
-        x.lower()
-        if '0x' in x:
-            return float.fromhex(x)
-        return float(x)
+        try:
+            return float(x)  # The fastest path.
+        except ValueError:
+            if '0x' in x:  # Don't accidentally convert "a" ("0xa") to 10.
+                try:
+                    return float.fromhex(x)
+                except ValueError:
+                    pass
+            raise  # Raise the original exception, which makes more sense.
 
     typ = dtype.type
     if issubclass(typ, np.bool_):
diff --git a/numpy/lib/tests/test_io.py b/numpy/lib/tests/test_io.py
index 534ab683c..d97ad76df 100644
--- a/numpy/lib/tests/test_io.py
+++ b/numpy/lib/tests/test_io.py
@@ -984,6 +984,28 @@ class TestLoadTxt(LoadTxtBase):
             res = np.loadtxt(c, dtype=dt)
             assert_equal(res, tgt, err_msg="%s" % dt)
 
+    def test_default_float_converter_no_default_hex_conversion(self):
+        """
+        Ensure that fromhex is only used for values with the correct prefix and
+        is not called by default. Regression test related to gh-19598.
+        """
+        c = TextIO("a b c")
+        with pytest.raises(
+            ValueError, match="could not convert string to float"
+        ):
+            np.loadtxt(c)
+
+    def test_default_float_converter_exception(self):
+        """
+        Ensure that the exception message raised during failed floating point
+        conversion is correct. Regression test related to gh-19598.
+        """
+        c = TextIO("qrs tuv")  # Invalid values for default float converter
+        with pytest.raises(
+            ValueError, match="could not convert string to float"
+        ):
+            np.loadtxt(c)
+
     def test_from_complex(self):
         tgt = (complex(1, 1), complex(1, -1))
         c = TextIO()