BUG: Append the empty frame with columns, #3121

[waitingkuo]

From issue3121

Bug

In [1]: import pandas as pd

In [2]: df1 = pd.DataFrame(columns=['test'])

In [3]: df2 = pd.DataFrame()

In [4]: df1.append(df2)

ValueError: need at least one array to concatenate

Reason

numpy cannot concatenate nothing:

In [10]: import numpy as np

In [11]: np.concatenate([])

ValueError: need at least one array to concatenate

Fix

Return an empty numpy array if we find that the arrays to be concatenated are empty

# return the empty np array, if nothing to concatenate, #3121
    if not to_concat: return np.array([], dtype=object)

Test

Test case test_append_empty_frame is added in test_frame.py

[hayd]

This bug fix looks great, but what are the other datetime tests at the end of the file? (or am I missing something?)

[waitingkuo]

Sorry for accidentally adding some duplicated test cases. Have removed them.

[hayd]

Awesome!