Q:

How can I vertically concatenate 1x2 arrays in numpy?

How can I concatenate 1x2 arrays produced in another function?
I have a for loop that produces xa as output, which is a float64 (1L,2L).
xa = [[ 1.17281823  1.210732  ]]

The code I tried for concatenate is 
A = []
for i in range(5): 
    # actually xa = UglyCalculation(**Inputs[i])
    xa = np.array([[ i, i+1 ]]) # for our example here
    # do something

All I want to do is concatenate/join/append these xa values vertically.
Basically the desired output should be
 0 1
 1 2
 2 3 
 3 4 
 4 5 

I have tried the following lines of code, but it's not working. Can you help?
 A = np.concatenate((A, xa)) # Concatenate the matrices 
 A = np.concatenate((A,xa),axis=0)
 A=np.vstack((A,xa))

A:

Setting a shape on A allows concatenation of similar column sized arrays:
import numpy as np
A = np.array([])
A.shape=(0,2) # set A to be 0 rows x 2 cols matrix
for i in range(5):
    xa = np.array([[i, i+1]])
    A = np.concatenate( (A,xa) )
print A

output
[[ 0.  1.]
 [ 1.  2.]
 [ 2.  3.]
 [ 3.  4.]
 [ 4.  5.]]

without A.shape = (0,2) an Exception will be thrown:
Traceback (most recent call last):
  File "./arr.py", line 5, in <module>
    A = np.concatenate( (A,xa) )
ValueError: all the input arrays must have same number of dimensions

