Comparing values (equality, inequality) in PyTest

Introduction

We compare values to check if things are the same or different. This helps us find mistakes in our code.

Check if a function returns the expected result.

Verify that two variables hold the same value after some operation.

Ensure that an error message is not equal to an empty string.

Confirm that a list length is not equal to zero after adding items.

Test if user input matches the required format or value.

Syntax

PyTest

assert actual == expected  # checks if values are equal
assert actual != unexpected  # checks if values are not equal

Use == to check if two values are exactly the same.

Use != to check if two values are different.

Examples

This test passes because 5 is equal to 5.

PyTest

def test_equal():
    assert 5 == 5

This test passes because 5 is not equal to 3.

PyTest

def test_not_equal():
    assert 5 != 3

Checks if two strings are the same.

PyTest

def test_string_equal():
    assert 'hello' == 'hello'

Checks if two strings are different.

PyTest

def test_string_not_equal():
    assert 'hello' != 'world'

Sample Program

This sample has two tests. The first checks if adding 2 and 3 equals 5. The second checks that adding 2 and 2 does not equal 5.

PyTest

import pytest

def add(a, b):
    return a + b

def test_add_equal():
    result = add(2, 3)
    assert result == 5

def test_add_not_equal():
    result = add(2, 2)
    assert result != 5

Important Notes

Always use assert in pytest to compare values.

Clear and simple comparisons help find bugs quickly.

Use meaningful variable names to make tests easy to read.

Summary

Use == to check if values are equal.

Use != to check if values are not equal.

Assertions help confirm your code works as expected.

Practice

1. Which operator in pytest assertions checks if two values are equal?

easy

A. <

B. !=

C. >

D. ==

Solution

1. Step 1: Understand equality operator

   The operator == is used to check if two values are equal in Python and pytest assertions.

2. Step 2: Differentiate from inequality

   The operator != checks if values are not equal, so it is not the correct choice for equality.

3. Final Answer:

   == -> Option D

4. Quick Check:

   Equality check = == [OK]

Hint: Use == for equality, != for inequality [OK]

Common Mistakes:

• Confusing == with !=
• Using > or < for equality
• Forgetting to use assertion keyword

2. Which of the following is the correct syntax to assert that variable a is not equal to b in pytest?

easy

A. assert a != b

B. assert a =! b

C. assert a ==! b

D. assert a = b

Solution

1. Step 1: Identify correct inequality syntax

   The correct operator for inequality is !=, so the assertion should be assert a != b.

2. Step 2: Check syntax correctness

   Options with =! or ==! are invalid syntax and will cause errors.

3. Final Answer:

   assert a != b -> Option A

4. Quick Check:

   Inequality assertion = assert a != b [OK]

Hint: Remember: inequality is !=, not =! or ==! [OK]

Common Mistakes:

• Using =! instead of !=
• Using = instead of ==
• Mixing equality and inequality operators

3. What will be the result of this pytest assertion?

def test_numbers():
    x = 10
    y = 5 + 5
    assert x == y

medium

A. Syntax error in assertion

B. Test fails because 10 does not equal 5

C. Test passes because 10 equals 10

D. Test fails because variables are different

Solution

1. Step 1: Calculate values of variables

   Variable x is 10, and y is calculated as 5 + 5, which equals 10.

2. Step 2: Evaluate assertion condition

   The assertion assert x == y checks if 10 equals 10, which is true, so the test passes.

3. Final Answer:

   Test passes because 10 equals 10 -> Option C

4. Quick Check:

   10 == 10 is True [OK]

Hint: Add values before comparing for equality [OK]

Common Mistakes:

• Assuming variables differ without calculation
• Confusing syntax errors with logic errors
• Ignoring that 5 + 5 equals 10

4. Identify the error in this pytest assertion:

def test_strings():
    s1 = "hello"
    s2 = "Hello"
    assert s1 != s2

medium

A. No error, test passes because strings differ in case

B. Syntax error due to missing parentheses

C. Assertion should use == instead of !=

D. Variables s1 and s2 are not defined

Solution

1. Step 1: Compare string values with case sensitivity

   Strings "hello" and "Hello" differ in case, so they are not equal.

2. Step 2: Check assertion correctness

   The assertion assert s1 != s2 correctly checks that the strings are not equal, so the test passes without error.

3. Final Answer:

   No error, test passes because strings differ in case -> Option A

4. Quick Check:

   "hello" != "Hello" is True [OK]

Hint: String comparison is case-sensitive by default [OK]

Common Mistakes:

• Assuming strings are equal ignoring case
• Expecting syntax error without parentheses in assert
• Confusing == and != in assertions

5. You want to write a pytest test to check that a function get_status() returns a value that is not equal to None. Which assertion is the best choice?

hard

A. assert get_status() == None

B. assert get_status() is not None

C. assert get_status() != None

D. assert get_status() is None

Solution

1. Step 1: Understand None comparison best practice

   In Python, to check if a value is not None, the recommended way is to use is not None rather than != None.

2. Step 2: Evaluate options for correctness

   assert get_status() == None checks equality to None (wrong for this case). assert get_status() != None uses != None (works but not best practice). assert get_status() is not None uses is not None which is best practice. assert get_status() is None checks if value is None (opposite of requirement).

3. Final Answer:

   assert get_status() is not None -> Option B

4. Quick Check:

   Use 'is not None' for None checks [OK]

Hint: Use 'is not None' to check for non-None values [OK]

Common Mistakes:

• Using == None instead of is None
• Using != None instead of is not None
• Confusing equality with identity for None