Save a numpy array consisting of numpy arrays different shapes to a .txt file

Question

I am a beginner in Python. Here's what I am trying to do :

import numpy as np 
r10 = np.array([[i for i in range(0,10)],[i*10 for i in range(0,10)]]).T
r6 = np.array([[i for i in range(0,6)],[i*10 for i in range(0,6)]]).T
r_comb = np.array([[r10],[r6]]).T
np.savetxt('out.txt',r_comb)

Using np.savetxt gives me the following error since it only supports 1-D array :

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
~\AppData\Local\Programs\Python\Python38-32\lib\site-packages\numpy\lib\npyio.py in savetxt(fname, X, fmt, delimiter, newline, header, footer, comments, encoding)
   1433                 try:
-> 1434                     v = format % tuple(row) + newline
   1435                 except TypeError:

TypeError: only size-1 arrays can be converted to Python scalars

During handling of the above exception, another exception occurred:

TypeError                                 Traceback (most recent call last)
<ipython-input-88-c3147f076055> in <module>
----> 1 np.savetxt('out.txt',r_comb)

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

~\AppData\Local\Programs\Python\Python38-32\lib\site-packages\numpy\lib\npyio.py in savetxt(fname, X, fmt, delimiter, newline, header, footer, comments, encoding)
   1434                     v = format % tuple(row) + newline
   1435                 except TypeError:
-> 1436                     raise TypeError("Mismatch between array dtype ('%s') and "
   1437                                     "format specifier ('%s')"
   1438                                     % (str(X.dtype), format))

TypeError: Mismatch between array dtype ('object') and format specifier ('%.18e %.18e')

Is there any other way of saving the contents of the variable r_comb to a .txt file so that I can use it for other plotting programs? Basically,I want the text file to look like this :

0 0.0 0 0.0
1 0.1 1 0.1
2 0.2 2 0.2
3 0.3 3 0.3
4 0.4 4 0.4
5 0.5 5 0.5
6 0.6
7 0.7
8 0.8
9 0.9

Image showing how the contents of the text file should look

Answer 1

Hacky but works

import numpy as np 
r10 = np.array([[i for i in range(0,10)],[i*10 for i in range(0,10)]]).T
r6 = np.array([[i for i in range(0,6)],[i*10 for i in range(0,6)]]).T

# np array with nans
data = np.empty((np.max([r10.shape[0], r6.shape[0]]),4))
data[:] = np.NaN
for i in range(2):
  data[0:len(r10[:,i]), i] = r10[:, i]

for i in range(2):
  data[0:len(r6[:,i]), i+2] = r6[:, i]

# replace nans and save
data = data.astype(str)
data[data=='nan'] = ''
np.savetxt("out.txt", data, delimiter=" ", fmt="%s")

Contents of out.txt

0.0 0.0 0.0 0.0
1.0 10.0 1.0 10.0
2.0 20.0 2.0 20.0
3.0 30.0 3.0 30.0
4.0 40.0 4.0 40.0
5.0 50.0 5.0 50.0
6.0 60.0  
7.0 70.0  
8.0 80.0  
9.0 90.0