gh-95605: Fix `float(s)` error message when `s` contains only whitespace (GH-95665) (GH-95859)

This PR fixes the error message from float(s) in the case where s contains only whitespace.
(cherry picked from commit 97e9cfa75a80b54a0630b7371f35e368a12749d1)

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/Objects/floatobject.c b/Objects/floatobject.c
index 5af2678..064ba2e 100644
--- a/Objects/floatobject.c
+++ b/Objects/floatobject.c
@@ -150,11 +150,18 @@ 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--;
     }