import numpy as np
A = np.arange(12).reshape(4,3)
for a in A:
    a = a + 1
B = np.arange(12).reshape(4,3)
for b in B:
    b += 1
print('A:\n',A)
print('B:\n',B)

 A:
 [[ 0  1  2]
 [ 3  4  5]
 [ 6  7  8]
 [ 9 10 11]]
B:
 [[ 1  2  3]
 [ 4  5  6]
 [ 7  8  9]
 [10 11 12]]

These methods are called to implement the augmented arithmetic assignments (+=, -=, *=, @=, /=, //=, %=, **=, <<=, >>=, &=, ^=, |. These methods should attempt to do the operation in-place (modifying self) and return the result (which could be, but does not have to be, self). If a specific method is not defined, the augmented assignment falls back to the normal methods. For instance, if x is an instance of a class with an __iadd__() method, x += y is equivalent to x = x.__iadd__(y) . Otherwise, x.__add__(y) and y.__radd__(x) are considered, as with the evaluation of x + y. In certain situations, augmented assignment can result in unexpected errors (see Why does a_tuple += raise an exception when the addition works?), but this behavior is in fact part of the data model.

PEHDOM
тоесть в ситуации a+=b если в а не определен __iadd__ тогда интерпретатор вызовет a__add__(b) отсюда и непонятки

 >>> t = (1, 2, 3)
>>> 
>>> t  + (4,)
(1, 2, 3, 4)
>>> 
>>> t += (4,)
>>> t
(1, 2, 3, 4)
>>>

  
>>> t = (1, 2, 3)
>>> 
>>> t  + 'abc'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate tuple (not "str") to tuple
>>> 
>>> t += 'abc'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate tuple (not "str") to tuple
>>>

4kpt_V
Типа как в последнем пайтоне словари, которые хранят последовательность.