9.3. cmath
— Mathematical functions for complex numbers¶
This module is always available. It provides access to mathematical functions
for complex numbers. The functions in this module accept integers,
floatingpoint numbers or complex numbers as arguments. They will also accept
any Python object that has either a __complex__()
or a __float__()
method: these methods are used to convert the object to a complex or
floatingpoint number, respectively, and the function is then applied to the
result of the conversion.
Note
On platforms with hardware and systemlevel support for signed zeros, functions involving branch cuts are continuous on both sides of the branch cut: the sign of the zero distinguishes one side of the branch cut from the other. On platforms that do not support signed zeros the continuity is as specified below.
9.3.1. Conversions to and from polar coordinates¶
A Python complex number z
is stored internally using rectangular
or Cartesian coordinates. It is completely determined by its real
part z.real
and its imaginary part z.imag
. In other
words:
z == z.real + z.imag*1j
Polar coordinates give an alternative way to represent a complex number. In polar coordinates, a complex number z is defined by the modulus r and the phase angle phi. The modulus r is the distance from z to the origin, while the phase phi is the counterclockwise angle, measured in radians, from the positive xaxis to the line segment that joins the origin to z.
The following functions can be used to convert from the native rectangular coordinates to polar coordinates and back.

cmath.
phase
(x)¶ Return the phase of x (also known as the argument of x), as a float.
phase(x)
is equivalent tomath.atan2(x.imag, x.real)
. The result lies in the range [π, π], and the branch cut for this operation lies along the negative real axis, continuous from above. On systems with support for signed zeros (which includes most systems in current use), this means that the sign of the result is the same as the sign ofx.imag
, even whenx.imag
is zero:>>> phase(complex(1.0, 0.0)) 3.1415926535897931 >>> phase(complex(1.0, 0.0)) 3.1415926535897931
New in version 2.6.
Note
The modulus (absolute value) of a complex number x can be
computed using the builtin abs()
function. There is no
separate cmath
module function for this operation.

cmath.
polar
(x)¶ Return the representation of x in polar coordinates. Returns a pair
(r, phi)
where r is the modulus of x and phi is the phase of x.polar(x)
is equivalent to(abs(x), phase(x))
.New in version 2.6.

cmath.
rect
(r, phi)¶ Return the complex number x with polar coordinates r and phi. Equivalent to
r * (math.cos(phi) + math.sin(phi)*1j)
.New in version 2.6.
9.3.2. Power and logarithmic functions¶

cmath.
exp
(x)¶ Return the exponential value
e**x
.

cmath.
log
(x[, base])¶ Returns the logarithm of x to the given base. If the base is not specified, returns the natural logarithm of x. There is one branch cut, from 0 along the negative real axis to ∞, continuous from above.
Changed in version 2.4: base argument added.
9.3.3. Trigonometric functions¶

cmath.
acos
(x)¶ Return the arc cosine of x. There are two branch cuts: One extends right from 1 along the real axis to ∞, continuous from below. The other extends left from 1 along the real axis to ∞, continuous from above.

cmath.
atan
(x)¶ Return the arc tangent of x. There are two branch cuts: One extends from
1j
along the imaginary axis to∞j
, continuous from the right. The other extends from1j
along the imaginary axis to∞j
, continuous from the left.Changed in version 2.6: direction of continuity of upper cut reversed

cmath.
cos
(x)¶ Return the cosine of x.

cmath.
sin
(x)¶ Return the sine of x.

cmath.
tan
(x)¶ Return the tangent of x.
9.3.4. Hyperbolic functions¶

cmath.
acosh
(x)¶ Return the inverse hyperbolic cosine of x. There is one branch cut, extending left from 1 along the real axis to ∞, continuous from above.

cmath.
asinh
(x)¶ Return the inverse hyperbolic sine of x. There are two branch cuts: One extends from
1j
along the imaginary axis to∞j
, continuous from the right. The other extends from1j
along the imaginary axis to∞j
, continuous from the left.Changed in version 2.6: branch cuts moved to match those recommended by the C99 standard

cmath.
atanh
(x)¶ Return the inverse hyperbolic tangent of x. There are two branch cuts: One extends from
1
along the real axis to∞
, continuous from below. The other extends from1
along the real axis to∞
, continuous from above.Changed in version 2.6: direction of continuity of right cut reversed

cmath.
cosh
(x)¶ Return the hyperbolic cosine of x.

cmath.
sinh
(x)¶ Return the hyperbolic sine of x.

cmath.
tanh
(x)¶ Return the hyperbolic tangent of x.
9.3.5. Classification functions¶

cmath.
isinf
(x)¶ Return
True
if the real or the imaginary part of x is positive or negative infinity.New in version 2.6.

cmath.
isnan
(x)¶ Return
True
if the real or imaginary part of x is not a number (NaN).New in version 2.6.
9.3.6. Constants¶

cmath.
pi
¶ The mathematical constant π, as a float.

cmath.
e
¶ The mathematical constant e, as a float.
Note that the selection of functions is similar, but not identical, to that in
module math
. The reason for having two modules is that some users aren’t
interested in complex numbers, and perhaps don’t even know what they are. They
would rather have math.sqrt(1)
raise an exception than return a complex
number. Also note that the functions defined in cmath
always return a
complex number, even if the answer can be expressed as a real number (in which
case the complex number has an imaginary part of zero).
A note on branch cuts: They are curves along which the given function fails to be continuous. They are a necessary feature of many complex functions. It is assumed that if you need to compute with complex functions, you will understand about branch cuts. Consult almost any (not too elementary) book on complex variables for enlightenment. For information of the proper choice of branch cuts for numerical purposes, a good reference should be the following:
See also
Kahan, W: Branch cuts for complex elementary functions; or, Much ado about nothing’s sign bit. In Iserles, A., and Powell, M. (eds.), The state of the art in numerical analysis. Clarendon Press (1987) pp165–211.