^=, -= and += symbols in Python

As almost any modern language, Python has assignment operators so they can use them every time you want to assign a value to a variable after doing some arithmetic or logical operation, both (assignment and operation) are expressed in a compact way in one statement...

Table from Tutorials Point:

Operator	Description	Example
=	Assigns values from right side operands to left side operand	c = a + b assigns value of a + b into c
+= Add AND	It adds right operand to the left operand and assign the result to left operand	c += a is equivalent to c = c + a
-= Subtract AND	It subtracts right operand from the left operand and assign the result to left operand	c -= a is equivalent to c = c - a
*= Multiply AND	It multiplies right operand with the left operand and assign the result to left operand	c *= a is equivalent to c = c * a
/= Divide AND	It divides left operand with the right operand and assign the result to left operand	c /= a is equivalent to c = c / a
%= Modulus AND	It takes modulus using two operands and assign the result to left operand	c %= a is equivalent to c = c % a
**= Exponent AND	Performs exponential (power) calculation on operators and assign value to the left operand	c **= a is equivalent to c = c ** a
//= Floor Division	It performs floor division on operators and assign value to the left operand	c //= a is equivalent to c = c // a

When you compute X = X + Y you are actually returning the sum of X and Y into a new variable, which, in your example, overwrites the previous value of X. When you use an assignment operator in the form of X += 1, the value 1 is directly summed on the current value of X, without returning the result in a new variable. Take a look at the code below:

>>> V = np.arange(10)
>>> view = V[3:]        # view is just a subspace (reference) of the V array
>>> print(V);print(view)
[0 1 2 3 4 5 6 7 8 9]
[3 4 5 6 7 8 9] 
>>> view = view + 3     # add view to a constant in a new variable 
>>> print(V);print(view)
[0 1 2 3 4 5 6 7 8 9]
[ 6  7  8  9 10 11 12]
>>> view = V[3:]
>>> view += 3           # here you actually modify the value of V
>>> print(V);print(view)
[ 0  1  2  6  7  8  9 10 11 12]
[ 6  7  8  9 10 11 12]

You can also look for the documentation of numpy.ndarray.base to check if an array is actually a reference of another array.