The following code causes an assertion failure: import _json def _bad_encoder(*args): return None enc = _json.make_encoder(None, None, _bad_encoder, None, 'foo', 'bar', None, None, None) enc(obj='spam', _current_indent_level=4) This is because encoder_new() (in Modules/_json.c) assumes that the received encoder() argument is a function that returns a string, and stores it in the new PyEncoderObject. When encoder_encode_string() is called (by encoder_listencode_obj()), it returns whatever the stored encoder() returned, assuming it returned a string. Then, encoder_listencode_obj() passes this value to _steal_accumulate(), which passes it to _PyAccu_Accumulate(), which asserts it is a string. Similarly, the following code also causes an assertion failure (only the obj argument is different): import _json def _bad_encoder(*args): return None enc = _json.make_encoder(None, None, _bad_encoder, None, 'foo', 'bar', None, None, None) enc(obj={'spam': 42}, _current_indent_level=4) In this case, encoder_listencode_dict() passes whatever encoder_encode_string() returned, to _PyAccu_Accumulate(), which asserts it is a string.

 New changeset 2b382dd6121bb1e4b75470fb3ef8555665df3eb6 by Serhiy Storchaka (Oren Milman) in branch 'master': bpo-31505: Fix an assertion failure in json, in case _json.make_encoder() received a bad encoder() argument. (#3643) https://github.com/python/cpython/commit/2b382dd6121bb1e4b75470fb3ef8555665df3eb6 

 New changeset 7c24e99c99c961e6633ac45390a211f65806c687 by Serhiy Storchaka (Miss Islington (bot)) in branch '3.6': [3.6] bpo-31505: Fix an assertion failure in json, in case _json.make_encoder() received a bad encoder() argument. (GH-3643) (#3777) https://github.com/python/cpython/commit/7c24e99c99c961e6633ac45390a211f65806c687