```
>>> import json        
>>> f = open('/tmp/t.json', 'wb')              
>>> json.dump(123, f)  
Traceback (most recent call last):             
  File "<stdin>", line 1, in <module>          
  File "/usr/lib/python3.6/json/__init__.py", line 180, in dump                               
    fp.write(chunk)    
TypeError: a bytes-like object is required, not 'str'
```

This may not a bug. But it should mention at docs https://docs.python.org/3/library/json.html#json.dump