Magic methods in python and the str function

Question

I am trying to implement my own class for complex numbers, to better understand how classes work in python. I have been trying to replace the str magic method to print the complex number in the a+bi format.

def __str__(self):
    out="%i" % self.real
    if self.imaginary==0: return out
    if self.imaginary>=0: out+="+%ii" % self.imaginary
    else: out+="%ii" % self.imaginary
    return out

what I am interested in is the pythonic way of writing this unappealing block of code, if there is any to implement the fact that if imaginary part is negative, i should get a-bi and if imaginary part is 0 i should get a?

if you are using Python 3.6+, I strongly recommend using f-strings. — lmiguelvargasf
– lmiguelvargasf, Commented Sep 27, 2019 at 16:26
If you are using python 3+, I would recommend transitioning away from the %-method for formatting strings. You can use the str.format method for python <3.6 and literal f-strings (as mentioned above) for python >=3.6. I can also say that single line if/else-statements are not very pythonic. — SyntaxVoid
– SyntaxVoid, Commented Sep 27, 2019 at 16:29
Single line ternary statements are okay and pythonic. Anything with a : (like a normal loop, conditional statement, with statement, etc) should be multi-line — SyntaxVoid
– SyntaxVoid, Commented Sep 27, 2019 at 16:38
@Blaine, I have added an answer to your question. Let me know if that was useful. — lmiguelvargasf
– lmiguelvargasf, Commented Sep 27, 2019 at 16:43

lmiguelvargasf · Accepted Answer · 2019-09-27 17:14:07Z

1

If you are using Python 3.6+, use f-strings as follows:

def __str__(self):
    if self.imaginary == 0:
        return f'{self.real}'
    if self.real == 0:
        return f'{self.imaginary}i'

    return f'{self.real} {self.imaginary:+}i'

If you are using a version prior to Python 3.6, you should use format.

def __str__(self):
    if self.imaginary == 0: return '{}'.format(self.real)
    if self.real == 0: return '{}i'.format(self.imaginary)

    return '{} {:+}i'.format(self.real, self.imaginary)

I have also improved a bit the logic. Basically, when it has no imaginary part it is just returning the real part, if there is no real part, it is returning the imaginary part. When it has both imaginary and real part, it returns the complex number.

Notice that + specified after the two dots is the format. This format allows you to have the the sign of the imaginary part printed.

Examples

Assuming that your class is named CNumber

>>> x = CNumber(10, 1)
>>> str(x)
'10 +1i'
>>> x = CNumber(5, 0)
>>> str(x)
'5'
>>> x = CNumber(0, 3)
'3i'
>>> x = CNumber(1, -1)
'1 -1i'

If you want the following format a + bi

def __str__(self):
    if self.imaginary == 0: return f'{self.real}'
    if self.real == 0: return f'{self.imaginary}i'

    sign = '+' if self.imaginary > 0 else '-'

    return f'{self.real} {sign} {abs(self.imaginary)}i'

edited Sep 27, 2019 at 17:14

answered Sep 27, 2019 at 16:30

lmiguelvargasf

71.2k55 gold badges231 silver badges239 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

Blaine Over a year ago

this works, and it prints a +bi, what if i wanted to print a + bi , ie the space between the sign and the imaginary part?

Blaine Over a year ago

also is the pre 3.6 code valid for all versions of python?

SyntaxVoid Over a year ago

The pre 3.6 code depends on str.format which was introduced in python 2.6. Also, python 2 is reaching end of life: pythonclock.org. Be part of the transition!

norok2 Over a year ago

@Blaine PEP8 explicitly says that Compound statements (multiple statements on the same line) are generally discouraged.

lmiguelvargasf Over a year ago

@norok2, I just updated my answer, and made my code follow PEP-8

|

norok2 · Accepted Answer · 2019-09-27 17:19:25Z

1

What about just going a bit more explicit:

class Complex(object):
    IM_TOKEN = 'i'

    def __init__(self, real_part=0, imag_part=0):
        self.real = real_part
        self.imag = imag_part

    def null_imag(self):
        return self.imag == 0

    def null_real(self):
        return self.real == 0

    def __str__(self):
        if self.null_imag():
            return str(self.real)
        elif self.null_real():
            return str(self.imag) + type(self).IM_TOKEN
        else:
            return '{}{:+}{}'.format(self.real, self.imag, type(self).IM_TOKEN)

and testing it:

import itertools

for r, i in itertools.product([1, -1, 0], [1, -1, 0]):
    print(f'real: {r}, imag: {i}, complex: {Complex(r, i)}')

real: 1, imag: 1, complex: 1+1i
real: 1, imag: -1, complex: 1-1i
real: 1, imag: 0, complex: 1
real: -1, imag: 1, complex: -1+1i
real: -1, imag: -1, complex: -1-1i
real: -1, imag: 0, complex: -1
real: 0, imag: 1, complex: 1i
real: 0, imag: -1, complex: -1i
real: 0, imag: 0, complex: 0

As a side note the == does not work well with float data.

edited Sep 27, 2019 at 17:19

answered Sep 27, 2019 at 17:05

norok2

27.1k6 gold badges83 silver badges110 bronze badges

4 Comments

GZ0 Over a year ago

IM_TOKEN is a field for the class. It may be better to refer to it by Complex.IM_TOKEN instead.

norok2 Over a year ago

@GZ0 I would rather go with type(self) instead of Complex

GZ0 Over a year ago

So it would allow IM_TOKEN be overriden by subclasses?

norok2 Over a year ago

@GZ0 precisely for that

Collectives™ on Stack Overflow

Magic methods in python and the str function

2 Answers 2

9 Comments

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

9 Comments

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related