type hint additional message

[bluefrog]

I'm attempting to type hint on a temperature conversion function.
The function has a deliberate mistake in that it returns a Number instead of a dictionary, but the
return statement is for a Dictionary.

#!/home/bluefrog/anaconda3/bin/python3
from decimal import Decimal
from typing import Optional, Union, Dict
Number = Union[int, float, Decimal]

def convert_temperature(*, f_temp: Optional[Number]=None, c_temp: Optional[Number]=None) -> Number:
  if c_temp is None:
    c_temp = 5*(f_temp-32)/9
  elif f_temp is None:
    f_temp = 32+9*c_temp/5
  else:
    raise Exception( "Logic Design Problem" )
    result: Dict[str, Number] = {'c_temp': c_temp,
                                 'f_temp': f_temp}
  return result

When I run mypy I get the expected message on the incorrect return, but I also get 2 other messages that I do not understand:

$ mypy celcius_to_fahrenheit.py 
celcius_to_fahrenheit.py:8: error: Unsupported operand types for - ("None" and "int")
celcius_to_fahrenheit.py:8: note: Left operand is of type "Union[int, float, Decimal, None]"
celcius_to_fahrenheit.py:15: error: Incompatible return value type (got "Dict[str, Union[int, float, Decimal]]", expected "Union[int, float, Decimal]")

Could somebody explain why the 2 messages for line 8 appear. I was only expecting a message for line 15, which does appear. I am using 3.7 along with mypy

Thanks

[Larz60+]

please post error traceback verbatim

[bluefrog]

celcius_to_fahrenheit.py:8: error: Unsupported operand types for - ("None" and "int")
celcius_to_fahrenheit.py:8: note: Left operand is of type "Union[int, float, Decimal, None]"
celcius_to_fahrenheit.py:15: error: Incompatible return value type (got "Dict[str, Union[int, float, Decimal

This is the complete output. There is no other output.

[gruntfutuk]

What mypy configuration are you using?

https://mypy.readthedocs.io/en/latest/config_file.html

[bluefrog]

I've created a mypy.ini as suggested in the link you posted:

$ pwd
/home/bluefrog/anaconda3/pkgs/mypy-0.620-py37_0
$ cat > mypy.ini <<EOF
> [mypy]
> python_version = 3.7
> warn_return_any = True
> warn_unused_configs = True
> EOF

$ cd ~/sample_code/python/functions/

$ ~/anaconda3/pkgs/mypy-0.620-py37_0/bin/mypy celcius_to_fahrenheit2.py 
celcius_to_fahrenheit2.py:8: error: Unsupported operand types for - ("Union[int, float, Decimal, None]" and "int")
celcius_to_fahrenheit2.py:15: error: Incompatible return value type (got "Dict[str, Union[int, float, Decimal]]", expected "Union[int, float, Decimal]")

So the first message is now gone, but the second still does not make sense.
Perhaps it is because mypy is still new?

[micseydel]

It looks like the minus operator is unsupported because the left hand side may be None, according to the type hints. That's because f_temp is optional. Before trying to use minus with it, you should verify that it is non-None; that should fix your issue.