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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →Python’s built-in complex() converts a supported string or number into a complex value, or constructs one from real and imaginary parts. Use complex(3, 4) for (3+4j), or complex('3+4j') to parse a string. This guide follows the Python 3.14 built-in-functions reference.
What does Python complex() do?
complex() is both a conversion function and a constructor. A complex value has a real part and an imaginary part; Python writes the imaginary unit with a j or J suffix. The components are available through the .real and .imag attributes.
z = complex(3, 4)
print(z) # (3+4j)
print(z.real) # 3.0
print(z.imag) # 4.0Python documents complex numbers as having real and imaginary parts that are each floating-point numbers. See the Python 3.14.8 numeric types reference.
Choose the right call form
| Call form | Use it for | Example | What can go wrong |
|---|---|---|---|
| One string argument | Parsing a complex number written as text | complex('3+4j') gives (3+4j) |
Malformed syntax, including disallowed internal spaces, raises ValueError. |
| One numeric argument | Converting a number to a complex value with zero imaginary part | complex(1.23) gives (1.23+0j) |
The object must support the documented conversion protocol. |
| Two numeric arguments | Supplying the real part first and imaginary part second | complex(-1.23, 4.5) gives (-1.23+4.5j) |
Use numeric arguments; the two-argument form is not the string-parsing form. |
| No arguments | Creating the zero complex value | complex() gives 0j |
None for this form. |
Parse a complex number from a string
A single string may represent a real value, an imaginary value, or both. For a combined value, write an explicit sign before the imaginary component. The suffix for the imaginary component is j or J.
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complex('3') # (3+0j)
complex('4j') # 4j
complex('3+4j') # (3+4j)
complex('3-4J') # (3-4j)
complex(' (3+4j) ') # (3+4j)Whitespace around the entire expression and parentheses around it are allowed. Arbitrary whitespace inside the expression is not: complex('1 + 2j') raises ValueError. Keep the numeric expression together, as in '1+2j'.
Why does complex(‘1 + 2j’) raise ValueError?
The string parser does not accept spaces inserted between the real component, operator, and imaginary component. Removing those internal spaces fixes this case: complex('1+2j'). Outer whitespace, such as in ' 1+2j ', is a different matter and is permitted.
Convert numbers and other objects
With one numeric argument, complex(x) converts the value and supplies a zero imaginary part when appropriate. For a general object, Python tries conversion methods in this order:
x.__complex__(), if available.x.__float__(), if no complex conversion method is available.x.__index__(), as the final fallback.
The __index__() fallback is documented as a change introduced in Python 3.8. For custom classes, implementing __complex__() is the direct way to define complex conversion; the fallback methods let suitable objects provide a real-valued conversion instead.
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Complex values support ordinary arithmetic, including addition, subtraction, multiplication, division, and exponentiation. They do not support ordering comparisons such as < or >; operations such as floor division are also unavailable for complex values. If a calculation needs a magnitude or a comparison, work with an explicitly chosen property or derived quantity rather than trying to order the complex values themselves.
Python 3.14 note: complex.from_number()
The Python 3.14 built-in-functions reference also documents complex.from_number(x), added in Python 3.14. It is a separate class method, not another spelling of the built-in call forms described above. Consult the versioned Python 3.14 reference when using it, and check the Python version in an environment that may run an earlier release.
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