diff --git a/py/obj.c b/py/obj.c
index 0c97ee5aa3..0f36ef5690 100644
--- a/py/obj.c
+++ b/py/obj.c
@@ -218,20 +218,32 @@ mp_obj_t *mp_obj_get_array_fixed_n(mp_obj_t o_in, machine_int_t n) {
     }
 }
 
-uint mp_get_index(const mp_obj_type_t *type, machine_uint_t len, mp_obj_t index) {
-    // TODO False and True are considered 0 and 1 for indexing purposes
+// is_slice determines whether the index is a slice index
+uint mp_get_index(const mp_obj_type_t *type, machine_uint_t len, mp_obj_t index, bool is_slice) {
+    int i;
     if (MP_OBJ_IS_SMALL_INT(index)) {
-        int i = MP_OBJ_SMALL_INT_VALUE(index);
-        if (i < 0) {
-            i += len;
-        }
-        if (i < 0 || i >= len) {
-            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_IndexError, "%s index out of range", qstr_str(type->name)));
-        }
-        return i;
+        i = MP_OBJ_SMALL_INT_VALUE(index);
+    } else if (MP_OBJ_IS_TYPE(index, &bool_type)) {
+        i = (index == mp_const_true ? 1 : 0);
     } else {
         nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_TypeError, "%s indices must be integers, not %s", qstr_str(type->name), mp_obj_get_type_str(index)));
     }
+
+    if (i < 0) {
+        i += len;
+    }
+    if (is_slice) {
+        if (i < 0) {
+            i = 0;
+        } else if (i > len) {
+            i = len;
+        }
+    } else {
+        if (i < 0 || i >= len) {
+            nlr_jump(mp_obj_new_exception_msg_varg(&mp_type_IndexError, "%s index out of range", qstr_str(type->name)));
+        }
+    }
+    return i;
 }
 
 // may return MP_OBJ_NULL
diff --git a/py/obj.h b/py/obj.h
index d41db37c08..dd80b3f021 100644
--- a/py/obj.h
+++ b/py/obj.h
@@ -267,7 +267,7 @@ void mp_obj_get_complex(mp_obj_t self_in, mp_float_t *real, mp_float_t *imag);
 #endif
 //qstr mp_obj_get_qstr(mp_obj_t arg);
 mp_obj_t *mp_obj_get_array_fixed_n(mp_obj_t o, machine_int_t n);
-uint mp_get_index(const mp_obj_type_t *type, machine_uint_t len, mp_obj_t index);
+uint mp_get_index(const mp_obj_type_t *type, machine_uint_t len, mp_obj_t index, bool is_slice);
 mp_obj_t mp_obj_len_maybe(mp_obj_t o_in); /* may return NULL */
 
 // none
diff --git a/py/objarray.c b/py/objarray.c
index 30a2183115..d0b3e003b8 100644
--- a/py/objarray.c
+++ b/py/objarray.c
@@ -113,7 +113,7 @@ STATIC mp_obj_t array_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
     switch (op) {
         case RT_BINARY_OP_SUBSCR:
         {
-            uint index = mp_get_index(o->base.type, o->len, rhs);
+            uint index = mp_get_index(o->base.type, o->len, rhs, false);
             return mp_binary_get_val(o->typecode, o->items, index);
         }
 
@@ -140,7 +140,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(array_append_obj, array_append);
 
 STATIC bool array_store_item(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value) {
     mp_obj_array_t *o = self_in;
-    uint index = mp_get_index(o->base.type, o->len, index_in);
+    uint index = mp_get_index(o->base.type, o->len, index_in, false);
     mp_binary_set_val(o->typecode, o->items, index, value);
     return true;
 }
diff --git a/py/objlist.c b/py/objlist.c
index a6fbe4e423..aa082ea34b 100644
--- a/py/objlist.c
+++ b/py/objlist.c
@@ -104,7 +104,7 @@ STATIC mp_obj_t list_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
                 return res;
             }
 #endif
-            uint index = mp_get_index(o->base.type, o->len, rhs);
+            uint index = mp_get_index(o->base.type, o->len, rhs, false);
             return o->items[index];
         }
         case RT_BINARY_OP_ADD:
@@ -190,7 +190,7 @@ STATIC mp_obj_t list_pop(uint n_args, const mp_obj_t *args) {
     if (self->len == 0) {
         nlr_jump(mp_obj_new_exception_msg(&mp_type_IndexError, "pop from empty list"));
     }
-    uint index = mp_get_index(self->base.type, self->len, n_args == 1 ? mp_obj_new_int(-1) : args[1]);
+    uint index = mp_get_index(self->base.type, self->len, n_args == 1 ? mp_obj_new_int(-1) : args[1], false);
     mp_obj_t ret = self->items[index];
     self->len -= 1;
     memcpy(self->items + index, self->items + index + 1, (self->len - index) * sizeof(mp_obj_t));
@@ -383,7 +383,7 @@ void mp_obj_list_get(mp_obj_t self_in, uint *len, mp_obj_t **items) {
 
 void mp_obj_list_store(mp_obj_t self_in, mp_obj_t index, mp_obj_t value) {
     mp_obj_list_t *self = self_in;
-    uint i = mp_get_index(self->base.type, self->len, index);
+    uint i = mp_get_index(self->base.type, self->len, index, false);
     self->items[i] = value;
 }
 
diff --git a/py/objstr.c b/py/objstr.c
index 130c3266ac..6a2625b621 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -107,7 +107,7 @@ STATIC mp_obj_t str_binary_op(int op, mp_obj_t lhs_in, mp_obj_t rhs_in) {
             // TODO: need predicate to check for int-like type (bools are such for example)
             // ["no", "yes"][1 == 2] is common idiom
             if (MP_OBJ_IS_SMALL_INT(rhs_in)) {
-                uint index = mp_get_index(mp_obj_get_type(lhs_in), lhs_len, rhs_in);
+                uint index = mp_get_index(mp_obj_get_type(lhs_in), lhs_len, rhs_in, false);
                 if (MP_OBJ_IS_TYPE(lhs_in, &bytes_type)) {
                     return MP_OBJ_NEW_SMALL_INT((mp_small_int_t)lhs_data[index]);
                 } else {
@@ -290,10 +290,10 @@ STATIC mp_obj_t str_find(uint n_args, const mp_obj_t *args) {
     size_t end = haystack_len;
     /* TODO use a non-exception-throwing mp_get_index */
     if (n_args >= 3 && args[2] != mp_const_none) {
-        start = mp_get_index(&str_type, haystack_len, args[2]);
+        start = mp_get_index(&str_type, haystack_len, args[2], true);
     }
     if (n_args >= 4 && args[3] != mp_const_none) {
-        end = mp_get_index(&str_type, haystack_len, args[3]);
+        end = mp_get_index(&str_type, haystack_len, args[3], true);
     }
 
     const byte *p = find_subbytes(haystack + start, haystack_len - start, needle, needle_len);
@@ -487,6 +487,41 @@ STATIC mp_obj_t str_replace(uint n_args, const mp_obj_t *args) {
     return mp_obj_str_builder_end(replaced_str);
 }
 
+STATIC mp_obj_t str_count(uint n_args, const mp_obj_t *args) {
+    assert(2 <= n_args && n_args <= 4);
+    assert(MP_OBJ_IS_STR(args[0]));
+    assert(MP_OBJ_IS_STR(args[1]));
+
+    GET_STR_DATA_LEN(args[0], haystack, haystack_len);
+    GET_STR_DATA_LEN(args[1], needle, needle_len);
+
+    size_t start = 0;
+    size_t end = haystack_len;
+    /* TODO use a non-exception-throwing mp_get_index */
+    if (n_args >= 3 && args[2] != mp_const_none) {
+        start = mp_get_index(&str_type, haystack_len, args[2], true);
+    }
+    if (n_args >= 4 && args[3] != mp_const_none) {
+        end = mp_get_index(&str_type, haystack_len, args[3], true);
+    }
+
+    machine_int_t num_occurrences = 0;
+
+    // needle won't exist in haystack if it's longer, so nothing to count
+    if (needle_len > haystack_len) {
+        MP_OBJ_NEW_SMALL_INT(0);
+    }
+
+    for (machine_uint_t haystack_index = start; haystack_index + needle_len <= end; haystack_index++) {
+        if (memcmp(&haystack[haystack_index], needle, needle_len) == 0) {
+            num_occurrences++;
+            haystack_index += needle_len - 1;
+        }
+    }
+
+    return MP_OBJ_NEW_SMALL_INT(num_occurrences);
+}
+
 STATIC machine_int_t str_get_buffer(mp_obj_t self_in, buffer_info_t *bufinfo, int flags) {
     if (flags == BUFFER_READ) {
         GET_STR_DATA_LEN(self_in, str_data, str_len);
@@ -508,6 +543,7 @@ STATIC MP_DEFINE_CONST_FUN_OBJ_2(str_startswith_obj, str_startswith);
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(str_strip_obj, 1, 2, str_strip);
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR(str_format_obj, 1, str_format);
 STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(str_replace_obj, 3, 4, str_replace);
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(str_count_obj, 2, 4, str_count);
 
 STATIC const mp_method_t str_type_methods[] = {
     { "find", &str_find_obj },
@@ -517,6 +553,7 @@ STATIC const mp_method_t str_type_methods[] = {
     { "strip", &str_strip_obj },
     { "format", &str_format_obj },
     { "replace", &str_replace_obj },
+    { "count", &str_count_obj },
     { NULL, NULL }, // end-of-list sentinel
 };
 
diff --git a/py/objtuple.c b/py/objtuple.c
index d39b36d9fa..827441f704 100644
--- a/py/objtuple.c
+++ b/py/objtuple.c
@@ -111,7 +111,7 @@ mp_obj_t tuple_binary_op(int op, mp_obj_t lhs, mp_obj_t rhs) {
                 return res;
             }
 #endif
-            uint index = mp_get_index(o->base.type, o->len, rhs);
+            uint index = mp_get_index(o->base.type, o->len, rhs, false);
             return o->items[index];
         }
         case RT_BINARY_OP_ADD:
diff --git a/py/sequence.c b/py/sequence.c
index d3c3e32854..1723bb416c 100644
--- a/py/sequence.c
+++ b/py/sequence.c
@@ -149,9 +149,9 @@ mp_obj_t mp_seq_index_obj(const mp_obj_t *items, uint len, uint n_args, const mp
     uint stop = len;
 
     if (n_args >= 3) {
-        start = mp_get_index(type, len, args[2]);
+        start = mp_get_index(type, len, args[2], true);
         if (n_args >= 4) {
-            stop = mp_get_index(type, len, args[3]);
+            stop = mp_get_index(type, len, args[3], true);
         }
     }
 
diff --git a/tests/basics/list_index.py b/tests/basics/list_index.py
index f28263fba6..a669e69c49 100644
--- a/tests/basics/list_index.py
+++ b/tests/basics/list_index.py
@@ -3,6 +3,16 @@ print(a.index(1))
 print(a.index(2))
 print(a.index(3))
 print(a.index(3, 2))
+print(a.index(1, -100))
+print(a.index(1, False))
+
+try:
+    print(a.index(1, True))
+except ValueError:
+    print("Raised ValueError")
+else:
+    print("Did not raise ValueError")
+
 try:
     print(a.index(3, 2, 2))
 except ValueError:
diff --git a/tests/basics/string_count.py b/tests/basics/string_count.py
new file mode 100644
index 0000000000..bac99e78d8
--- /dev/null
+++ b/tests/basics/string_count.py
@@ -0,0 +1,22 @@
+print("asdfasdfaaa".count("asdf", -100))
+print("asdfasdfaaa".count("asdf", -8))
+print("asdf".count('s', True))
+print("asdf".count('a', True))
+print("asdf".count('a', False))
+print("asdf".count('a', 1 == 2))
+print("hello world".count('l'))
+print("hello world".count('l', 5))
+print("hello world".count('l', 3))
+print("hello world".count('z', 3, 6))
+print("aaaa".count('a'))
+print("aaaa".count('a', 0, 3))
+print("aaaa".count('a', 0, 4))
+print("aaaa".count('a', 0, 5))
+print("aaaa".count('a', 1, 5))
+print("aaaa".count('a', -1, 5))
+print("abbabba".count("abba"))
+
+def t():
+    return True
+
+print("0000".count('0', t()))
diff --git a/tests/basics/string_find.py b/tests/basics/string_find.py
index 90063228f8..df65fd6e5a 100644
--- a/tests/basics/string_find.py
+++ b/tests/basics/string_find.py
@@ -9,3 +9,15 @@ print("hello world".find("ll", 1, 2))
 print("hello world".find("ll", 1, 3))
 print("hello world".find("ll", 1, 4))
 print("hello world".find("ll", 1, 5))
+print("hello world".find("ll", -100))
+print("0000".find('0'))
+print("0000".find('0', 0))
+print("0000".find('0', 1))
+print("0000".find('0', 2))
+print("0000".find('0', 3))
+print("0000".find('0', 4))
+print("0000".find('0', 5))
+print("0000".find('-1', 3))
+print("0000".find('1', 3))
+print("0000".find('1', 4))
+print("0000".find('1', 5))