py: normalize bounds for string find and count

Author: ac1982

py/string.go

@@ -619,6 +619,26 @@ func (s String) M__contains__(item Object) (Object, error) {
 	return NewBool(strings.Contains(string(s), string(needle))), nil
 }
 
+// stringSearchBounds adjusts negative indices without clamping a start
+// beyond the end, which must not match even an empty substring.
+func stringSearchBounds(start, end, length int) (int, int) {
+	if start < 0 {
+		start += length
+		if start < 0 {
+			start = 0
+		}
+	}
+	if end < 0 {
+		end += length
+		if end < 0 {
+			end = 0
+		}
+	} else if end > length {
+		end = length
+	}
+	return start, end
+}
+
 func (s String) Count(args Tuple) (Object, error) {
 	var (
 		pysub Object
@@ -636,14 +656,9 @@ func (s String) Count(args Tuple) (Object, error) {
 		end  = int(pyend.(Int))
 		size = s.len()
 	)
-	if beg > size {
-		beg = size
-	}
-	if end < 0 {
-		end = size
-	}
-	if end > size {
-		end = size
+	beg, end = stringSearchBounds(beg, end, size)
+	if beg > end {
+		return Int(0), nil
 	}
 
 	var (
@@ -670,14 +685,9 @@ func (s String) find(args Tuple) (Object, error) {
 		end  = int(pyend.(Int))
 		size = s.len()
 	)
-	if beg > size {
-		beg = size
-	}
-	if end < 0 {
-		end = size
-	}
-	if end > size {
-		end = size
+	beg, end = stringSearchBounds(beg, end, size)
+	if beg > end {
+		return Int(-1), nil
 	}
 
 	var (

py/string_search_bounds_test.go

@@ -0,0 +1,56 @@
+// Copyright 2026 The go-python Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package py
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestStringSearchBounds(t *testing.T) {
+	// Expected results were checked against CPython.
+	for _, tc := range []struct {
+		s, sub                  string
+		start, end, find, count int
+	}{
+		{"abcabc", "a", -3, 6, 3, 1},
+		{"abcabc", "a", 0, -1, 0, 2},
+		{"abcabc", "c", 0, -1, 2, 1},
+		{"abcabc", "a", -100, 100, 0, 2},
+		{"abcabc", "a", 0, -100, -1, 0},
+		{"abcabc", "", 7, 100, -1, 0},
+		{"abcabc", "", 4, 2, -1, 0},
+		{"abcabc", "", 6, 6, 6, 1},
+		{"abcabc", "", 0, -1, 0, 6},
+		{"é世界é", "é", -1, 4, 3, 1},
+		{"é世界é", "界", 0, -1, 2, 1},
+		{"é世界é", "", -3, -1, 1, 3},
+		{"", "", 0, 0, 0, 1},
+		{"", "", 1, 1, -1, 0},
+	} {
+		for _, method := range []string{"find", "count"} {
+			t.Run(fmt.Sprintf("%s/%q/%q/%d:%d", method, tc.s, tc.sub, tc.start, tc.end), func(t *testing.T) {
+				defer func() {
+					if p := recover(); p != nil {
+						t.Errorf("unexpected panic: %v", p)
+					}
+				}()
+				args := Tuple{String(tc.sub), Int(tc.start), Int(tc.end)}
+				var got Object
+				var err error
+				want := tc.find
+				if method == "find" {
+					got, err = String(tc.s).find(args)
+				} else {
+					got, err = String(tc.s).Count(args)
+					want = tc.count
+				}
+				if err != nil || got != Int(want) {
+					t.Errorf("got %v, %v; want %d", got, err, want)
+				}
+			})
+		}
+	}
+}

py/tests/string.py

@@ -955,3 +955,11 @@ def __index__(self):
 
 
 doc="finished"
+
+# Search bounds count Unicode characters and accept negative indices.
+assert 'abcabc'.find('a', -3) == 3
+assert 'abcabc'.count('c', 0, -1) == 1
+assert 'abc'.find('', 4) == -1
+assert 'abc'.count('', 4) == 0
+assert 'é世界é'.find('é', -1) == 3
+assert 'é世界é'.count('', -3, -1) == 3