Python __complex__() Magic Method

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Syntax

object.__complex__(x)

The Python __complex__() method implements the built-in complex() function. So, when you cal complex(x), Python attempts to call x.__complex__(). If the return value is not a complex number or the x.__complex__() method is not defined for an object on which you call complex(x), Python will raise 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.

Background complex()

Python’s built-in complex() method returns a complex number object. You can either pass a string argument to convert the string to a complex number, or you provide the real and imaginary parts to create a new complex number from those.

Here are some examples:

>>> complex(1, -2)
(1-2j)
>>> complex(2, -1)
(2-1j)
>>> complex(2, 2)
(2+2j)
>>> complex(1)
(1+0j)
>>> complex(2)
(2+0j)
>>> complex('42-21j')
(42-21j)
Python Complex() is Dead 😲 Long Live Python Complex()!

Example Custom __complex__()

In the following example, you create a custom class Data and overwrite the __complex__() magic method so that it returns a complex number (42+21j) when trying to call complex(x) on a custom Data object.

class Data:
    def __complex__(self):
        return (42+21j)


x = Data()
res = complex(x) 

print(res)
# (42+21j)

TypeError: complex() first argument must be a string or a number, not …

If you hadn’t defined the __complex__() magic method, Python would’ve raised a TypeError:

class Data:
    pass


x = Data()
res = complex(x)

print(res)

Output:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 6, in <module>
    res = complex(x)
TypeError: complex() first argument must be a string or a number, not 'Data'

To fix this error, define the x.__complex__() method for an object x before calling the built-in complex(x) method passing this object as an argument:

class Data:
    def __complex__(self):
        return 1+2j


x = Data()
res = complex(x)

print(res)
# (1+2j)

TypeError: __bool__ should return bool, returned …

Consider the following code snippet where you try to return an integer, i.e., non-complex number in the dunder method __complex__():

class Data:
    def __complex__(self):
        return 42


x = Data()
res = complex(x)

print(res)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in <module>
    res = complex(x)
TypeError: __complex__ returned non-complex (type int)

The reason for the TypeError: __complex__ returned non-complex (type ...) error is that the __complex__() method must return a complex number. So, to resolve the error, return a complex number, for example, by using the syntax x+yj for two integers x and y in your method definition as shown previously:

class Data:
    def __complex__(self):
        return 1+2j # This is a complex number


x = Data()
res = complex(x)

print(res)
# (1+2j)

References:

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