complex() — Convert to Complex

complex() converts a value into a complex number.

Unlike int() and float(), Python represents complex numbers using j, not i.

What you'll learn

• Convert int, float, bool, and numeric strings
• Use j as Python’s imaginary unit
• Understand .real and .imag
• Recognize malformed complex strings

What does complex() do?

complex() converts a possible value into a complex object.

number = complex(value)

If the original value has no imaginary part, Python automatically adds 0j.

Example 1 — Integer → Complex

Every integer becomes a complex number with an imaginary part of 0.

print(complex(10))
print(complex(-12))

(10+0j)
(-12+0j)

Example 2 — Float → Complex

Floating-point numbers keep their decimal value.

print(complex(1.2))
print(complex(-1.3))

(1.2+0j)
(-1.3+0j)

Example 3 — Boolean → Complex

Booleans convert just like numbers.

Boolean	complex()
True	(1+0j)
False	0j

False becomes the zero complex number, displayed as 0j.

Example 4 — String → Complex

complex() accepts three numeric string formats:

String	Result
"12"	(12+0j)
"1.2"	(1.2+0j)
"2+3j"	(2+3j)

a = "12"
b = complex(a)

print(b)
print(b.real)
print(b.imag)

(12+0j)
12.0
0.0

Notice that .real and .imag are always stored as floats.

.real and .imag

Every complex number contains two parts.

For (2+3j):

Property	Value
.real	2.0
.imag	3.0

Invalid string examples

These strings are malformed, so Python raises ValueError.

Input	Why it fails
"True"	Not a numeric literal
"2-3i"	Python uses j, not i
"PYTHON"	Not a number

Correct

complex("2+3j")

Incorrect

complex("2-3i")

Interactive playground

Try different source types and inspect the real and imaginary parts.

Complete conversion map

Source	Example	Result
int	complex(10)	(10+0j)
float	complex(1.2)	(1.2+0j)
bool	complex(True)	(1+0j)
bool	complex(False)	0j
Integer string	complex("12")	(12+0j)
Float string	complex("1.2")	(1.2+0j)
Complex string	complex("2+3j")	(2+3j)
"True"	complex("True")	ValueError
"2-3i"	complex("2-3i")	ValueError

Practice exercises

Exercise 1 — Convert these values:

complex(-12)
complex("-23")

Expected results: (-12+0j) and (-23+0j).

Exercise 2 — Print the two properties:

z = complex("12")

print(z.real)
print(z.imag)

What data type are .real and .imag?

Exercise 3 — Uncomment this line:

complex("2-3i")

Why does Python reject it?

Quick knowledge check

1. What is complex(10)?
   (10+0j)

2. What is complex(False)?
   0j

3. Why does complex("2-3i") fail?
   Python uses j as the imaginary unit, not i.

Summary

• complex() converts integers, floats, booleans, and valid numeric strings into complex numbers.
• Real-only values automatically receive an imaginary part of 0j.
• Use j, never i, in complex strings.
• .real and .imag let you inspect the two parts; both are stored as floats.