[Tutor] Type annotation errors

[Mats Wichmann]

On 6/3/20 10:30 PM, boB Stepp wrote:
> As I continue to integrate type annotations with David Beazley's
> recently released course, "Practical Python Programming", I ran into
> another issue.  I have pared down a larger function to just what seems
> to be giving the type annotation complaint:
> 
> def parse_csv(data_rows: List, has_headers: bool = True) -> List:
>     """Parse a CSV file into a list of records."""
>     data_rows_iter = iter(data_rows)
>     if has_headers:
>         headers = next(data_rows_iter)
>     records = []
>     for row in data_rows_iter:
>         if has_headers:
>             # Make a dictionary
>             record = dict(zip(headers, row))
>         else:
>             # Otherwise make a tuple
>             record = tuple(row)
>     records.append(record)
> 
>     return records
> 
> The complaint is:  test.py|19 col 22 error| Incompatible types in
> assignment (expression has type "Tuple[Any, ...]", variable has type
> "Dict[Any, Any]")
> 
> One of the exercises is to return a list of dictionaries if a csv file
> has headers, but if not to return a list of tuples.  mypy apparently
> does not like this. 

I don't like it either :)   (I know, who am I to criticize dabeaz - even
if I did so professionally for a very short period as one of his
technical editors on Python Essential Reference 2:e way back in 2000).

Functions/methods that return different types under different calling
scenarios  are certainly possible in Python, but that doesn't mean it's
a great idea for usability.  A contrived learning example is fine; In
Real Life, I don't see why on earth you'd want this awkward setup. So
worrying too much about twisting typing so mypy is happy about this
doesn't seem terribly worthwhile... just sayin'.