__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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