ENH: Make concat succeed on empty list.

Is your feature request related to a problem?

Yes, pd.concat([]) currently raises the exception, ValueError(“No objects to concatenate”).

Describe the solution you’d like

To be correct on all input sizes, it should return an empty DataFrame.

API breaking implications

Obviously, concat will no longer detect empty data. However, empty data will detect itself.

Additional context

The concatenation operation is for lists what addition is for numbers. Concatenation of an empty list should produce an empty dataframe, because the empty dataframe represents 0. Consider

sum([])

which python evaluates to 0.

This makes concatenation and sum commute, sum([1] + []) == sum([1]) + sum([]).

This already works for an empty dataframe:

pd.concat([pd.DataFrame({'A':[1]}), pd.DataFrame()])

if is very odd for concat([]) not to produce a DataFrame, since its input has the correct type.

[mroeschke]

I find the current behavior correct as is.

For reference np.concatenate also raises a similar error as pd.concat

In [1]: np.concatenate([])
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-1-7806ca832c55> in <module>
----> 1 np.concatenate([])

<__array_function__ internals> in concatenate(*args, **kwargs)

ValueError: need at least one array to concatenate

And I believe it’s more explicit for the user to decide what to do in code flow if they get to a pd.concat([]) situation.

So I’d be -1 on any changes here.

[mroeschke]

Thanks for the suggestion, but also from the review on a proposal PR https://github.com/pandas-dev/pandas/pull/41355#pullrequestreview-654681587, I think this behavior is best left for the user to decide manually what the result of empty concatenation should be. Closing.