Mysteriously a tuple

When a result is inexplicably of the wrong type

Not the first time I've been stung by this, so it's worth recording the problem.


A function or call inexplicably returns not as the expected type but as a tuple containing the said type. After a furious amount of debugging, you determine that everything is acting as it should and returning the types that it should. Yet, somehow, the value you capture from a particular line is a tuple.

Your code will look something like this:

f = radio_input ('Pathogen', 'pathogen', pathogen_choices,
        env=d, ... ...
        helptext="Which organism are you looking for matches with?",


You may have spotted this already but it's the trailing comma. Easy to miss when you're scanning complex code and a series of nested calls, but instead of this:

x = long_complicated_stuff

you're doing this:

x = long_complicated_stuff,

which evalulates to this:

x = (long_complicated_stuff, )

i.e. an implicit one-element tuple


Remove the comma, obviously.