__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
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.
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)
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
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
class Data: pass x = Data() res = complex(x) print(res)
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
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
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)
Where to Go From Here?
Enough theory. Let’s get some practice!
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While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com. He’s author of the popular programming book Python One-Liners (NoStarch 2020), coauthor of the Coffee Break Python series of self-published books, computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.