Python __ipow__() Magic Method | Finxter

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Python Equal To | Finxter


Syntax

object.__ipow__(self, other)

The Python __ipow__() magic method implements the in-place exponentiation x **= y that calculates the result of the power function x ** y, and assignsit to the first operands’ variable x. This type of in-place operation is also called augmented arithmetic assignment. The method simply returns the new value to be assigned to the first operand.

  • When you call x **= y, Python first attempts to call x.__ipow__(y).
  • If this is not implemented, it tries the normal exponentiation operation x.__pow__(y).
  • If this is not implemented either, it tries reverse exponentiation operation y.__rpow__(x) with swapped operands.

The result is then assigned to the first operand x. If none of those operations is implemented, Python raises a TypeError.

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Basic Example Overriding __ipow__

In the following code example, you create a class Data and define the magic method __ipow__(self, other).

  • The “self” argument is the default argument of each method and it refers to the object on which it is called—in our case, the first operand of the in-place operation.
  • The “other” argument of the in-place method refers to the second operand, i.e., y in the in-place operation x **= y.

The return value of the operation returns a dummy string 'finxter 42' to be assigned to the first operand. In practice, this would be the result of the in-place exponentiation operation.

class Data:
    def __ipow__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x **= y

print(x)
# finxter 42

In-Place Exponentiation **= without __ipow__()

To support the in-place power function on a custom class, you don’t have to overwrite the __ipow__() method. Because if the method is not defined, Python will fall back to the normal __pow__() method and assign its result to the first operand.

Here’s an example:

class Data:
    def __pow__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x **= y

print(x)
# finxter 42

Even though the __ipow__() method is not defined, the in-place exponentiation operation x **= y still works due to the __pow__() “fallback” magic method!

In-Place Exponentiation **= without __ipow__() and __pow__()

To support in-place exponentiation x **= y on a custom class, you don’t even have to overwrite any of the x.__ipow__(y) or x.__pow__(y) methods. If both are not defined, Python falls back to the reverse y.__rpow__(x) method and assigns its result to the first operand.

Here’s an example where you create a custom class for the first operand that doesn’t support the exponentiation operation. Then you define a custom class for the second operand that defines the __rpow__() method. For the in-place operation, Python falls back to the __rpow__() method defined on the second operand and assigns it to the first operand x:

class Data_1:
    pass

class Data_2:
    def __rpow__(self, other):
        return 'finxter 42'

x = Data_1()
y = Data_2()

x **= y

print(x)
# finxter 42

TypeError: unsupported operand type(s) for **=

If you try to perform in-place exponentiation x **= y but neither x.__ipow__(y), nor x.__pow__(y), nor y.__rpow(x) is defined, Python raises a “TypeError: unsupported operand type(s) for **=". To fix this error, simply define any of those methods before performing the in-place operation.

class Data:
    pass      # ... you should define __ipow__ here to prevent error! ... #


x = Data()
y = Data()

x **= y

Output:

Traceback (most recent call last):
  File "C:UsersxcentDesktopcode.py", line 8, in <module>
    x **= y
TypeError: unsupported operand type(s) for ** or pow(): 'Data' and 'Data'

Related Video

References:

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