Make str::format const (#1142)

Author: noahbkim

docs/api_core.rst

@@ -1085,7 +1085,7 @@ Wrapper classes
       *Note*: The C string will be deleted when the `str` instance is garbage
       collected.
 
-   .. cpp:function:: template <typename... Args> str format(Args&&... args)
+   .. cpp:function:: template <typename... Args> str format(Args&&... args) const
 
       C++ analog of the Python routine ``str.format``. Can be called with
       positional and keyword arguments.

include/nanobind/nb_accessor.h

@@ -250,7 +250,7 @@ detail::accessor<detail::num_item_tuple> tuple::operator[](T index) const {
     return { derived(), (Py_ssize_t) index };
 }
 
-template <typename... Args> str str::format(Args&&... args) {
+template <typename... Args> str str::format(Args&&... args) const {
     return steal<str>(
         derived().attr("format")((detail::forward_t<Args>) args...).release());
 }

include/nanobind/nb_types.h

@@ -427,7 +427,7 @@ class str : public object {
     explicit str(const char *s, size_t n)
         : object(detail::str_from_cstr_and_size(s, n), detail::steal_t{}) { }
 
-    template <typename... Args> str format(Args&&... args);
+    template <typename... Args> str format(Args&&... args) const;
 
     const char *c_str() const { return PyUnicode_AsUTF8AndSize(m_ptr, nullptr); }
 };