How to use cmp() in Python 3?

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]

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.

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

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

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

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.

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))  

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 ab

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

1

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

0