complex() is a built-in function in Python 3 that hanldles the creation of a complex number according to the arguments passed. It returns a complex number with the value real + imag*1j when called in form of complex(real, imag) or converts a string or number to a complex number.

This function takes two types of arguments which are discussed below :

When called with two arguments, both being numbers of any numeric type (including complex), this function returns a complex number using both arguments. complex(x, y) function returns a complex number with the value x + y*1j

>>> complex(3, 2)
(3+2j)
>>> complex(3+4j, 6)
(3+10j)

When called with one numeric argument, it returns a complex number using the argument as the real part and an imaginary part of zero.

>>> complex(4)
(4+0j)

If called with no arguments, it returns a complex zero.

>>> complex()
0j

If the first argument is a string, it will be interpreted as a complex number and the function must be called without a second parameter. It will return the interpreted complex number.

>>> complex('3+4j')
(3+4j)

The string must be in the form of a valid complex number, otherwise the function will raise a ValueError exception and it must not contain whitespace around the central + or - operator. For example, complex('1+2j') is fine, but complex('1 + 2j') raises ValueError.

print(complex(4, 5))      # prints (4+5j)
print(complex(6))         # prints (6+0j)
print(complex())          # prints 0j
print(complex(2.5,5))     # prints (2.5+5j)
print(complex(5,6+8j))    # prints (-3+6j) as 5+(6+8j)*1j = -3+6j
print(complex('10+20j'))  # prints (10+20j)

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