Comparison operators

Operator Name
== Equal
!= Not equal
> Greater than
< Less than
>= Greater than or equal to
<= Less than or equal to

Floating point comparisons

a = 0.15 + 0.15
b = 0.10 + 0.20

print(a == b)
print(a >= b)
False
False

What is happening here? How might we perform a reasonable comparison between these floating point numbers?

One way to solve: use math.isclose():

import math

a = 0.15 + 0.15
b = 0.10 + 0.20

print(math.isclose(a,b))
True

Sequence comparison

  • For equality and non-equality:
a = "Hi"
b = "Hi"
c = "Hello"
print(a == b)
print(a == c)
print(a != b)
print(a != c)
True
False
False
True
a = [1,2]
b = [1,2]
print(a == b)
True

However, order matters:

a = [1,2]
b = [2,1]
print(a == b)
False

(after all, with strings it is also easy to see that Hi is different than iH).

If we need to ignore order, and compare only elements, we can compare the sorted lists:

a = [1,2]
b = [2,1]
print(sorted(a) == sorted(b))
True

Greater and smaller than

Sequences follow a lexicographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted.

For example, all comparisons below are True:

(1, 2, 3)              < (1, 2, 4)
[1, 2, 3]              < [1, 2, 4]
'ABC' < 'C' < 'Pascal' < 'Python'
(1, 2, 3, 4)           < (1, 2, 4)
(1, 2)                 < (1, 2, -1)
(1, 2, 3)             == (1.0, 2.0, 3.0)
(1, 2, ('aa', 'ab'))   < (1, 2, ('abc', 'a'), 4)

Comparison operator chaining

Differently from most other languages, Python supports operator chaining, like 2 < x < 5.

  • Chaining compares left to right, evaluating a < b first
  • If the result is true, then b < c is evaluated next
age = int(input('Please enter your age: '))
if 12 < age < 20:
  print('You are a teenager')