I don’t think, “other languages do that too” is a good argument here. This would apply if behaving differently would break user expectation. But here we would do nothing more than explicitly inform the user of a relevant operation. If they already expected that behaviour, they can disregard the warning.

I don’t see how `parse_int`would help me here, I would need a `parse_str=int`, but then it would try to parse every string, and I don’t see the use case for that.

I would suggest a warning similar to this:

--- json/encoder.py
+++ json/encoder.py
@@ -1,6 +1,7 @@
 """Implementation of JSONEncoder
 """
 import re
+import warnings
 
 try:
     from _json import encode_basestring_ascii as c_encode_basestring_ascii
@@ -353,7 +354,9 @@
             items = sorted(dct.items(), key=lambda kv: kv[0])
         else:
             items = dct.items()
+        non_str_key = False
         for key, value in items:
+            non_str_key = non_str_key or not isinstance(key, str)
             if isinstance(key, str):
                 pass
             # JavaScript is weakly typed for these, so it makes sense to
@@ -403,6 +406,8 @@
                 else:
                     chunks = _iterencode(value, _current_indent_level)
                 yield from chunks
+        if non_str_key:
+            warnings.warn("Encountered non-string key(s), converted to string.", RuntimeWarning)
         if newline_indent is not None:
             _current_indent_level -= 1
             yield '\n' + _indent * _current_indent_level