Python Subtraction Operator

Python provides the subtraction operator - to subtract one object from another. The semantics of the subtraction depends on the operands’ data types. For example, subtracting two integers performs the arithmetic difference operation whereas subtracting two sets performs the set difference operation. The specific return value of the minus operator is defined in a data types’ __sub__() magic method.

Have a look at the following examples!

Examples

The – operator on integer operands yields another integer—the mathematical difference of both operands:

>>> 2 - 2
0
>>> 2 - 3
-1
>>> -99 - (-1)
-98

If at least one of the operands is a float value, the result is also a float—float is infectious!

>>> 2.0 - 1
1.0
>>> 1 - 2.2
-1.2000000000000002
>>> 44.0 - 2.0
42.0

You can also perform the subtraction operator on Python sets. In this case, it calculates the set difference, i.e., it creates a new set with elements in the first but not in the second operand.

Here’s an example:

>>> {1, 2, 3} - {1, 2}
{3}
>>> {'Alice', 'Bob'} - {1, 'Bob'}
{'Alice'}
>>> {1, 2, 3} - {1, 2, 3, 4, 5}
set()

What if two operands have an incompatible data type? For example, if you try to subtract a set from a string?

>>> 'hello' - {1, 2, 3}
Traceback (most recent call last):
  File "<pyshell#10>", line 1, in <module>
    'hello' - {1, 2, 3}
TypeError: unsupported operand type(s) for -: 'str' and 'set'

The result of incompatible addition is a TypeError. You can fix it by using only compatible data types for the operation.

Can you use the subtraction operator on custom objects? Yes!

Python Subtraction Magic Method

To use the subtraction operator on custom objects, define the __sub__() dunder method that takes two arguments: self and other and returns the result of self - other. You can define the specific behavior by using the attributes (data) maintained in this object.

In the following code, you create a basket from {'coffee', 'banana', 'bred'} but then you remove the contents in another basket {'bred'} from it—for example to prevent double-purchasing:

class Basket:
    def __init__(self, goods):
        self.goods = goods

    def __sub__(self, other):
        return Basket(self.goods - other.goods)

my_basket = Basket({'coffee', 'banana', 'bred'})
to_remove = Basket({'bred'})

updated_basket = my_basket - to_remove
print(updated_basket.goods)

The output of this code snippet is the new basket:

{'banana', 'coffee'}

The code consists of the following steps:

  • Create the class Basket that holds the list contents to store some goods.
  • Define the magic method __sub__ that creates a new Basket by combining the sets of goods from the two operands’ baskets. Note that we rely on the already implemented subtraction operator on sets, i.e. set difference, to actually implement the subtraction operator for baskets.
  • We create two baskets my_basket and to_remove, and calculate the difference between them to a new basket updated_basket.

Can You Subtract Lists in Python?

Python doesn’t allow built-in support for the list difference operation, i.e., creating a new list with elements from the first list operand but without the elements from the second list operand. Instead, to subtract lst_2 from list lst_1, use the list comprehension statement as a filter [x for x in lst_1 if x not in lst_2].

Here’s a code example:

lst_1 = [1, 2, 3, 4, 5, 6]
lst_2 = [1, 2, 3]
difference = [x for x in lst_1 if not x in lst_2]

The output is:

print(difference)
# Output: [4, 5, 6]

This code makes use of list comprehension that is a compact way of creating lists. The simple formula is [expression + context].

  • Expression: What to do with each list element?
  • Context: What elements to select? The context consists of an arbitrary number of for and if statements.

You can learn more about list comprehension in this in-depth tutorial with video:

*** List Comprehension – Ultimate Guide ***


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Python Subtraction Program with User Input

To create a simple subtraction program in Python taking the user’s input and subtracting the provided numbers, you can use the following four steps:

  1. Get the user input as a string using the built-in input() function, and store the result in variables num_1 and num_2.
  2. Convert the string user inputs to numerical types using, for example, the int() or float() constructors.
  3. Subtract the numerical values using the subtraction operator num_1 - num_2.
  4. Print the result to the Python shell.

Here are those four steps in Python code:

# Python subtraction program with user input

# 1. Get string user inputs representing integers
num_1 = input('First number: ')
num_2 = input('Second number: ')

# 2. Converting strings to ints
num_1 = int(num_1)
num_2 = int(num_2)

# 3. Subtracting numbers
result = num_1 - num_2

# 4. Display the result
print(num_1, '-', num_2, '=', result)

Here’s an example code execution where I put in integers 44 and 2 and calculated the difference using the subtraction operator:

First number: 44
Second number: 2
44 - 2 = 42

Python Subtraction Operator Chaining

You can chain together two subtraction operators. For example, the expression x - y - z would first calculate the difference between x and y and then subtract z from the resulting object. Thus, it is semantically identical to ((x - y) - z).

Here’s a minimal example:

>>> x = 10
>>> y = 5
>>> z = 2
>>> x - y - z
3
>>> ((x - y) - z)
3

Arithmetic Operators

Arithmetic operators are syntactical shortcuts to perform basic mathematical operations on numbers.

OperatorNameDescriptionExample
+AdditionCalculating the sum of the two operands3 + 4 == 7
SubtractionSubtracting the second operand from the first operand4 - 3 == 1
*MultiplicationMultiplying the first with the second operand3 * 4 == 12
/DivisionDividing the first by the second operand3 / 4 == 0.75
%ModuloCalculating the remainder when dividing the first by the second operand7 % 4 == 3
//Integer Division, Floor DivisionDividing the first operand by the second operand and rounding the result down to the next integer8 // 3 == 2
**ExponentRaising the first operand to the power of the second operand2 ** 3 == 8