gh-95605: Fix float(s) error message when s contains only whitespace (GH-95665)

This PR fixes the error message from float(s) in the case where s contains only whitespace.

Lib/test/test_float.py

@@ -137,6 +137,10 @@ def check(s):
         check('123\xbd')
         check('  123 456  ')
         check(b'  123 456  ')
+        # all whitespace (cf. https://github.com/python/cpython/issues/95605)
+        check('')
+        check(' ')
+        check('\t \n')
 
         # non-ascii digits (error came from non-digit '!')
         check('\u0663\u0661\u0664!')

Misc/NEWS.d/next/Core and Builtins/2022-08-04-18-46-54.gh-issue-95605.FbpCoG.rst

@@ -0,0 +1,2 @@
+Fix misleading contents of error message when converting an all-whitespace
+string to :class:`float`.

Objects/floatobject.c

@@ -162,11 +162,18 @@ float_from_string_inner(const char *s, Py_ssize_t len, void *obj)
     double x;
     const char *end;
     const char *last = s + len;
-    /* strip space */
+    /* strip leading whitespace */
     while (s < last && Py_ISSPACE(*s)) {
         s++;
     }
+    if (s == last) {
+        PyErr_Format(PyExc_ValueError,
+                     "could not convert string to float: "
+                     "%R", obj);
+        return NULL;
+    }
 
+    /* strip trailing whitespace */
     while (s < last - 1 && Py_ISSPACE(last[-1])) {
         last--;
     }