How to use cmp() in Python 3?

Question

I cannot get the command cmp() to work.

Here is the code:

a = [1,2,3]
b = [1,2,3]
c = cmp(a,b)
print (c)

I am getting the error:

Traceback (most recent call last):
  File "G:\Dropbox\Code\a = [1,2,3]", line 3, in <module>
    c = cmp(a,b)
 NameError: name 'cmp' is not defined
[Finished in 0.1s]

Answer 1

As mentioned in the comments, cmp doesn't exist in Python 3. If you really want it, you could define it yourself:

def cmp(a, b):
    return (a > b) - (a < b) 

which is taken from the original What's New In Python 3.0. It's pretty rare -- though not unheard of -- that it's really needed, though, so you might want to think about whether it's actually the best way to do whatever it is you're up to.

Answer 2

In Python 3.x you can import operator and use operator module's eq(), lt(), etc... instead of cmp()

Answer 3

When the sign is needed, probably safest alternative is using math.copysign:

import math
ang = -2
# alternative for cmp(ang, 0):
math.copysign(1, ang)

# Result: -1

In particular if ang is of np.float64 type because of depreciation of the '-' operator. Example:

import numpy as np

def cmp_0(a, b):
    return (a > b) - (a < b)

ang = np.float64(-2)
cmp_0(ang, 0)

# Result:
# DeprecationWarning: numpy boolean subtract, the `-` operator, is deprecated, 
# use the bitwise_xor, the `^` operator, or the logical_xor function instead.

instead one could use:

def cmp_0(a, b):
    return bool(a > b) - bool(a < b)

ang = np.float64(-2)
cmp(ang, 0)
# Result: -1

Answer 4

adding to @maxin's answer, in python 3.x, if you want to compare two lists of tuples a and b

import operator

a = [(1,2),(3,4)]
b = [(3,4),(1,2)]
# convert both lists to sets before calling the eq function
print(operator.eq(set(a),set(b))) #True

Answer 5

While in the general case, these are all good replacements for cmp(), for the actual use case given by the original poster, surely

a = [1,2,3]
b = [1,2,3]
c = a != b
print(c)

or just

a = [1,2,3]
b = [1,2,3]
print(a != b)

would work quite well.

Answer 6

If a or b is a class object, then the above answers will have the compilation error as below: For example: a is Class Clock:

  File "01_ClockClass_lab16.py", line 14, in cmp
    return (a > b) - (a < b)
TypeError: '>' not supported between instances of 'Clock' and 'Clock'

Change the type with int() to remove the error:

def cmp(a, b):
    return (int(a) > int(b)) - (int(a) < int(b))  

Answer 7

This cmp() function works only on Python version 2.x, if you try to use it in version 3.x it will give an error:

NameError: name 'cmp' is not defined
[Finished in 0.1s with exit code 1]

See the code below:

a=60
b=90
print(cmp(a,b))

output:

-1

While comparing integers cmp() just performs subtraction of its argument i.e in this case a-b, if subtraction is -ve it returns -1 i.e a<b if subtraction is +ve it returns 1 i.e a>b

a=90
b=60
print(cmp(a,b))

output:

1

Again:

a="abc"
b="abc"
print(cmp(a,b))

output:

0

when both the arguments are equal i.e a=b, it returns 0 as output. Here, we have passed two string type of values. Here, cmp() compares both the strings character by character and if found same then returns 0.