1.3 operators

This lesson describes Basic operators in python.

The traiditional mathematical operations can be performed on python objects just by using their symbels. This means addition, subtraction, multiplication and divitsion can be perfomred just by using +, -, * and / symbols/operators between two objects respectively.

Basic operations

+ is used for addition

a = 5 + 12
print(a)

17

If we want to add something to variable ‘a’, one way of doint it is using +=.

a +=14
print(a)

31

Above we added 10 to a and assigned the new value again to a. In this way, the value of a is updated.

Similarly we can use - for subtracting one object from another

a = 14 - 12
print(a)

2

a -= 5
print(a)

-3

* is used for for multiplication

a = 2*12
print(a)

24

a *= 2
print(a)

48

The modulo operator % returns remainder

print(14 % 5)

4

If one of the value is float, result will be a float.

print(17 % 5.0)

2.0

Above 17 was integer and 5.0 was float. Since one value was float, the result is also a float

The sign of the result will be same as sign of divider.

print(17 % -5.0)

-3.0

/ is used for division

a = 20/6
print(a)

3.3333333333333335

// is used for truncated division

print(20//6)

3

print(20//6.0)

3.0

If the answer of the truncated division is negative, the answer is rounded to the next smallest iteger (greater negative)

print(20//-6.0, -20//6.0)

-4.0 -4.0

print(-20 // -6.0)

3.0

Question What is the output of the following code

print(20//-6.0, -20//6.0)

Question What is the output of the following code

print( 20//60, 20/60)

** is used for exponentiation i.e. to raise one object over another. For example two to the power of three can be done as below

print(2**3)

8

The application of basic mathematical operators is not limited to floats or integers. We can also apply + to strings in python

a = "Materialism leads to "
b = "injustice."
print(a + b)

Materialism leads to injustice.

However we can not do same for - operator. This means we can not subtract two strings.

# uncomment following line
# a - b # -> TypeError

But when we multiply a string by an integer, the string is repeated.

print(a * 2)

Materialism leads to Materialism leads to

Above we are multiplying a string with an integer because a is a string and 2 is integer. If however, we do a +2, we will again get TypeError.

What kind of mathematical operations can be applied on an object or between two objects, depends purely upon the objects. To our surprise, we can even modify the behavior of these mathematical operators in python. More about this will come later in 3.16 magic methods.

Comparisons

If we want to compare one object with another and tell whether both are equal or not, we make use of == operator.

print(2 == 3)

False

The == operator returns either True or False depending upon the values being compared.

print(2 ==2)

True

print(2.2 == 1.1 + 1.1)

True

However, we should avoid comparing floats in this way. This is because computers can not represent accurate values of floats. 1.1 + 2.2 results in an approximated answer, so we better avoid comparing floats.

Question

Find out the type of the output returned by == operator. type(2==3)

---

print(3.3 == 1.1 + 2.2)

False

print(abs((1.1 + 2.2) - 3.3) < 1e-15)

True

> can be read as greater than or smaller than` depending upon its direction.

%% If you are getting the output as True for the above code, it is because of the subtle differences in floating point implementation in your hardware.

If we want to check whether a numerical value lies between two numbers or not we can make use of < or > operators twice.

print(8<10<12)

True

!= can be considered as opposite of == and can be read as not equal to

capitalism = 'a system based on individualism'

print(capitalism != 'justice')

True

Logical operators

not results in opposite to what comes after it.

print(not True)

False

print(not False)

True

x = 5.2
print(not x<=10)

False

capitalism = False
communism = False
justice = True
print(capitalism and communism)

False

print(capitalism or communism)

False

print(capitalism and justice)

False

Above, since one statement on left side of and is False, the output is False. For and to return True, both statements/expressions on its right and left side, must return True. If one is True and one is False, the output will be False. However, for or, even if one side evaluates to True, the output is True. This becomes clear if you understand, why the output of below code is True.

print(capitalism or justice)

True

print(capitalism is not justice)

True

Question The above code print(capitalism is not justice) printed True. This is because justice was True and thus not justice becomes False. Therefore, what we are implicitly saying False is False, which is True. Now reset the values of variables capitalism and justice in such a way that the code print(capitalism is not justice) should return False. This is shown below

capitalism =
justice =
print(capitalism is not justice)  # must print False

Default values

food = 'bread'
lunch = food or 'curry'
print(lunch)

bread

The statement on left of or was not False/None, therefore, variable lunch became bread .

food = None
lunch = food or 'curry'
print(lunch)

curry

If the first argument before or is True or not None, the value after or is discarded.

food = None
lunch = 'currey' or food
print(lunch)

currey

food = 'bread'
lunch = 'currey' or food
print(lunch)

currey

Identity

is operator compares whether both variables on its right and left side refer to same memory location or not.

a = 257
b = 257
print(a == b)

True

id(a), id(b)

(139691878961456, 139691878961456)

Because ali and hasan are stored at different location at different location, thus the answer is False

print(a is b)

True

Note If the above code prints True, it could be that the python version that you are using saves even large integers in memory.

Actually, python already stores some commonly used smaller numbers in memory, so when they are created, python refers to that same memory location and does not really create them. Thus, for smaller numbers (from -5 to 256 integers) is returns True.

a = 256
b = 254 + 2
print(a is b)

True

print(id(a), id(b))

139692357654928 139692357654928

feudalism = 'slavery'
capitalism = 'slavery'

print(feudalism is capitalism)

True

feudalism = 'a system of slavery'
capitalism = 'a system of slavery'

print(feudalism is capitalism)

True

Order of operations

The multiplication * is performed before addition + even if + appears before *.

print(20 + 4 * 10)

60

Similarly exponentiation (**) is performed before multiplication.

print(2 * 3 ** 4 * 5)

810

Question: Write the code to convert 1000 seconds into hours.

Question: Why the output of 2/3+4 and 2/(3+4) is different?

Complete order of precedence of operators in python can be found from here .