Single parameter in PyTest - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of using a single parameter in pytest test functions?

A single parameter allows you to pass different values to the test function, enabling the same test to run multiple times with different inputs.

beginner

How do you mark a test function to run multiple times with different values for a single parameter in pytest?

Use the @pytest.mark.parametrize decorator with the parameter name and a list of values to test.

beginner

Example: What does this pytest code do?

@pytest.mark.parametrize('x', [1, 2, 3])
def test_example(x):
    assert x > 0

It runs test_example three times with x equal to 1, 2, and 3. Each time it checks that x is greater than 0.

beginner

Why is using a single parameter with @pytest.mark.parametrize helpful in testing?

It saves time by avoiding writing multiple similar test functions and helps check how code behaves with different inputs.

beginner

What happens if one of the parameter values causes the assertion to fail in a single parameter pytest test?

Only the test run with that failing value will fail; other runs with different values will still pass if their assertions hold.

Which pytest decorator is used to run a test multiple times with different values for a single parameter?

A @pytest.mark.repeat

B @pytest.mark.parametrize

C @pytest.mark.singleparam

D @pytest.mark.testcase

The @pytest.mark.parametrize decorator allows you to specify parameter names and values to run the test multiple times.

How do you specify the parameter name in @pytest.mark.parametrize for a single parameter?

A No need to specify parameter names

B As a list of parameter names

C As a dictionary of parameter names and values

D As a string with the parameter name, e.g., 'x'

You provide the parameter name as a string, like 'x', followed by a list of values to test.

What will happen if you run this test? @pytest.mark.parametrize('num', [0, 1, 2]) def test_positive(num): assert num > 0

A The test will pass for all values

B The test will fail for all values

C The test will fail for num=0 and pass for num=1 and num=2

D The test will only run once

The assertion num > 0 fails when num is 0, so that run fails; others pass.

Why might you prefer using a single parameter with @pytest.mark.parametrize instead of writing multiple test functions?

A It reduces code duplication and makes tests easier to maintain

B It makes tests run slower

C It hides test failures

D It requires more code

Using parameterization avoids repeating similar test code and helps keep tests clean.

In pytest, what type of values can you pass to a single parameter in @pytest.mark.parametrize?

A Any iterable of values like list, tuple, or set

B Only integers

C Only strings

D Only dictionaries

You can pass any iterable of values to test different inputs.

Explain how to use a single parameter with @pytest.mark.parametrize to run a test multiple times with different inputs.

Think about how to tell pytest to repeat a test with different values.

Describe what happens when one of the parameter values causes an assertion failure in a pytest test using a single parameter.

Consider how pytest handles multiple test runs with different inputs.

Practice

1. What is the purpose of using a single parameter in a pytest test function?

easy

A. To define multiple test functions at once

B. To make the test function run only once

C. To skip the test function during execution

D. To pass different values to the test function for repeated testing

Solution

1. Step 1: Understand pytest parameterization

   Pytest allows passing parameters to test functions to run the same test with different inputs.

2. Step 2: Role of a single parameter

   A single parameter lets you supply multiple values to the test, so it runs multiple times with each value.

3. Final Answer:

   To pass different values to the test function for repeated testing -> Option D

4. Quick Check:

   Single parameter = multiple test runs [OK]

Hint: Single parameter means testing with different inputs [OK]

Common Mistakes:

• Thinking single parameter defines multiple tests
• Confusing parameter with skipping tests
• Assuming single parameter runs test once only

2. Which of the following is the correct syntax to use a single parameter with pytest's @pytest.mark.parametrize decorator?

easy

A. @pytest.mark.parametrize('num', 1, 2, 3)

B. @pytest.mark.parametrize(num, [1, 2, 3])

C. @pytest.mark.parametrize('num', [1, 2, 3])

D. @pytest.mark.parametrize(['num'], 1, 2, 3)

Solution

1. Step 1: Check parameter name format

   The parameter name must be a string inside quotes, e.g., 'num'.

2. Step 2: Check values format

   The values must be inside a list or iterable, e.g., [1, 2, 3].

3. Final Answer:

   @pytest.mark.parametrize('num', [1, 2, 3]) -> Option C

4. Quick Check:

   Parametrize syntax = string + list [OK]

Hint: Parameter name in quotes, values in list brackets [OK]

Common Mistakes:

• Not quoting the parameter name
• Passing values without list brackets
• Using wrong argument structure

3. What will be the output when running this pytest code?

import pytest

@pytest.mark.parametrize('x', [2, 4])
def test_double(x):
    assert x * 2 == 4

medium

A. First test passes, second test fails

B. Both tests fail

C. First test fails, second test passes

D. Both tests pass

Solution

1. Step 1: Evaluate first test with x=2

   2 * 2 = 4, assertion is true, test passes.

2. Step 2: Evaluate second test with x=4

   4 * 2 = 8, assertion 8 == 4 is false, test fails.

3. Final Answer:

   First test passes, second test fails -> Option A

4. Quick Check:

   2*2=4 pass, 4*2=8 fail [OK]

Hint: Check each input separately for assertion [OK]

Common Mistakes:

• Assuming all tests pass without checking values
• Mixing up which input causes failure
• Ignoring assertion logic

4. Identify the error in this pytest code using a single parameter:

import pytest

@pytest.mark.parametrize('val', 1, 2, 3)
def test_positive(val):
    assert val > 0

medium

A. Parameter name should not be in quotes

B. Missing list brackets around parameter values

C. Test function must have two parameters

D. Assertion should use == instead of >

Solution

1. Step 1: Check parametrize values format

   Values must be inside a list or iterable, e.g., [1, 2, 3].

2. Step 2: Identify missing brackets

   Code passes values as separate arguments instead of a single list, causing syntax error.

3. Final Answer:

   Missing list brackets around parameter values -> Option B

4. Quick Check:

   Values need list brackets [OK]

Hint: Values must be in a list or tuple [OK]

Common Mistakes:

• Passing values without brackets
• Removing quotes from parameter name
• Wrong assertion operator

5. Given this test function, how can you modify it to test multiple string inputs using a single parameter?

def test_is_uppercase(s):
    assert s.isupper()

hard

A. Add @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) above the function

B. Change function to accept two parameters s and t

C. Use a for loop inside the test to iterate over strings

D. Call test_is_uppercase multiple times manually

Solution

1. Step 1: Understand parameterizing tests

   Using @pytest.mark.parametrize allows running the test multiple times with different inputs.

2. Step 2: Apply parametrize decorator with string list

   Adding @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) runs test_is_uppercase twice with each string.

3. Final Answer:

   Add @pytest.mark.parametrize('s', ['HELLO', 'WORLD']) above the function -> Option A

4. Quick Check:

   Parametrize decorator runs test with multiple inputs [OK]

Hint: Use @pytest.mark.parametrize with list of inputs [OK]

Common Mistakes:

• Trying to pass multiple parameters instead of one
• Using loops inside test instead of parametrize
• Calling test manually instead of using pytest features