How to format complex number in Python

Question

In my Python script for the line

result = sp.solve(eqn)

I get the following output in the format similar to

result = 0.0100503358535014*I

I gather this means the solution is a complex number. So in this case it could also be seen as result = 0.01j.

I want to add formatting to my code to change result = 0.0100503358535014*I to something more like result = 0.01j. However I am finding issues trying to do this as I was trying to use isinstance to check if result was complex

if isinstance(result, complex):
        print "Result is complex!"
        ... my code here...

However this loop is never entered (i.e 0.0100503358535014*I isn't classified as complex here).

What way should I write an if statement to check if result is given in the manner xxxxxxx*I correctly?

What is I here? Some special constant? If you're not working with complex, what are you working with? — ShadowRanger
– ShadowRanger, Commented Mar 14, 2017 at 16:10
I too think that I is an actual symbol (it's probably a SymPy library), and not an indication of a complex number. — leovp
– leovp, Commented Mar 14, 2017 at 16:11

Josh Lee · Accepted Answer · 2017-03-14 16:34:41Z

4

SymPy supports Python's built-in function complex():

>>> import sympy
>>> x = sympy.sqrt(-2)
>>> x
sqrt(2)*I
>>> complex(x)
1.4142135623730951j

There are some examples in http://docs.sympy.org/latest/modules/evalf.html

A Python complex number can be formatted similar to a float:

>>> format(complex(x), 'e')
'0.000000e+00+1.414214e+00j'
>>> format(complex(x), 'f')
'0.000000+1.414214j'
>>> format(complex(x), 'g')
'0+1.41421j'

If you want to format the real and imag parts separately you can do it yourself.

The conversion would raise a TypeError if it can't be done:

>>> complex(sympy.Symbol('x'))
Traceback (most recent call last):
TypeError: can't convert expression to float

answered Mar 14, 2017 at 16:34

Josh Lee

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Comments

Community · Accepted Answer · 2017-05-23 12:17:19Z

Here's an alternative which incidentally indicates how to check whether one or both parts of a complex number are available.

>>> from sympy import *
>>> var('x')
x
>>> expr = (x+0.012345678*I)*(x-0.2346678934)*(x-(3-2.67893455*I))
>>> solve(expr)
[0.234667893400000, -0.012345678*I, 3.0 - 2.67893455*I]
>>> roots = solve(expr)
>>> for root in roots:
...     r, i = root.as_real_imag()
...     '%10.3f %10.3f i' % (r,i)
...     
'     0.235      0.000 i'
'     0.000     -0.012 i'
'     3.000     -2.679 i'

You could check the sign of the complex part to decide whether to put a plus sign in. I would like to have been able to use the newfangled formatting but fell afoul of the bug which comes with Py3.4+ mentioned in Python TypeError: non-empty format string passed to object.__format__ for which I have no remedy.

Stelios · Accepted Answer · 2017-03-14 16:21:45Z

0

The following checks whether the sympy object result represents a complex number and truncates the float value to 2 decimal points.

import sympy as sp

result = 0.0100503358535014*sp.I
print(result.is_complex)
print(sp.N(result,2))

True
0.01*I

answered Mar 14, 2017 at 16:21

Stelios

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2 Comments

Catherine Over a year ago

I have tried this way and it works however if I use an if statement like if (result.is_complex) then this return true for both type(result) = <class 'sympy.core.mul.Mul'> and type(result) = <class 'sympy.core.numbers.Float'> which isn't what I want. How do I get a specific if statement only for when the result has an *I part at the end?

asmeurer Over a year ago

is_complex will return True for a large class of SymPy expressions, including symbolic expressions, and even real numbers. x.is_complex just checks if x is a complex number.

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