Pythonic style

[Frank Millman]

On 2020-09-21 3:46 PM, Chris Angelico wrote:
> On Mon, Sep 21, 2020 at 11:37 PM Tim Chase
> <python.list at tim.thechases.com> wrote:
>>
>> On 2020-09-20 18:34, Stavros Macrakis wrote:
>>> Consider a simple function which returns the first element of an
>>> iterable if it has exactly one element, and throws an exception
>>> otherwise. It should work even if the iterable doesn't terminate.
>>> I've written this function in multiple ways, all of which feel a
>>> bit clumsy.
>>>
>>> I'd be interested to hear thoughts on which of these solutions is
>>> most Pythonic in style. And of course if there is a more elegant
>>> way to solve this, I'm all ears! I'm probably missing something
>>> obvious!
>>
>> You can use tuple unpacking assignment and Python will take care of
>> the rest for you:
>>
>>    >>> x, = tuple() # no elements
>>    Traceback (most recent call last):
>>      File "<stdin>", line 1, in <module>
>>    ValueError: not enough values to unpack (expected 1, got 0)
>>    >>> x, = (1, )  # one element
>>    >>> x, = itertools.repeat("hello") # 2 to infinite elements
>>    Traceback (most recent call last):
>>      File "<stdin>", line 1, in <module>
>>    ValueError: too many values to unpack (expected 1)
>>
>> so you can do
>>
>>    def fn(iterable):
>>      x, = iterable
>>      return x
>>
>> The trailing comma can be hard to spot, so I usually draw a little
>> extra attention to it with either
>>
>>    (x, ) = iterable
>>
>> or
>>
>>    x, = iterable # unpack one value
>>
>> I'm not sure it qualifies as Pythonic, but it uses Pythonic features
>> like tuple unpacking and the code is a lot more concise.
> 
> Or:
> 
> [x] = iterable
> 
> I'd definitely recommend using unpacking as the most obvious way to do
> this. Among other advantages, it gives different messages for the "too
> many" and "too few" cases.
> 

I used something similar years ago, but I made the mistake of relying on 
the error message in my logic, to distinguish between 'too few' and 'too 
many'. Guess what happened - Python changed the wording of the messages, 
and my logic failed.

After messing about with some alternatives, I ended up with the OP's 
first option (with some added comments), and have stuck with it ever 
since. It is not pretty, but it is readable and unambiguous.

Frank Millman