Stack two pandas data frames

In [5]: a = pd.DataFrame(data=np.random.randint(0,100,(2,5)),columns=list('ABCDE'))

In [6]: b = pd.DataFrame(data=np.random.randint(0,100,(2,5)),columns=list('ABCDE'))

In [7]: c = pd.concat([a,b],ignore_index=True)

In [8]: c
Out[8]: 
    A   B   C   D   E
0  12  56  62  35  20
1  10  71  63   0  70
2  61  72  29  10  71
3  88  82  39  73  94