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How to Assert That Nested Function Called In Pytest?

5 minutes read

To assert that a nested function is called in pytest, you can use the mock library to create a mock object for the nested function. Then, you can use the assert_called_once() method on the mock object to check if the function was called exactly once. Alternatively, you can also use assert_called_with() to check if the nested function was called with the expected arguments. This can help you ensure that the nested function is being called correctly in your test cases.


How to assert a specific nested function call sequence in pytest?

In pytest, you can use the mock library to assert specific nested function call sequences. Here is an example of how you can do this:

  1. First, install the pytest-mock plugin by running the following command:
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pip install pytest-mock


  1. Now, let's say you have a function outer_function that calls another function inner_function:
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# module.py

def inner_function():
    pass

def outer_function():
    inner_function()


  1. In your test file, you can use the MagicMock from the mock library to mock the inner_function and assert the call sequence like this:
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# test_module.py

import pytest
from unittest.mock import MagicMock

from module import outer_function, inner_function

def test_outer_function_calls_inner_function(mocker):
    mock_inner = mocker.patch('module.inner_function', MagicMock())

    outer_function()

    mock_inner.assert_called_once()


  1. In this test, the mocker.patch method is used to mock the inner_function in the module module. Then, when outer_function is called, the mocked inner_function is also called. Finally, we use assert_called_once to assert that the inner_function was called exactly once.
  2. Now you can run the test using pytest and it will assert the specific nested function call sequence.


How to check if a nested function is called in pytest?

In order to check if a nested function is called in pytest, you can use the pytest-mock library.


Here's an example of how you can use pytest-mock to check if a nested function is called in your test:

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# Code to be tested
def nested_function():
    print("This is a nested function")

def main_function():
    nested_function()

# Test code using pytest and pytest-mock
import pytest
from unittest.mock import patch

def test_main_function(mock):
    with patch('__main__.nested_function') as mock_nested_function:
        main_function()

    mock_nested_function.assert_called_once()


In this example, we are using patch from the unittest.mock library to mock the nested_function. We then call the main_function and check if the nested_function was called once using the assert_called_once method.


Make sure to install pytest-mock library using pip install pytest-mock before running the test.


How to handle exceptions when asserting a nested function call in pytest?

When asserting a nested function call in pytest, you can handle exceptions by using the pytest.raises context manager. Here is an example of how you can handle exceptions when asserting a nested function call in pytest:

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import pytest

def nested_function(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

def test_nested_function():
    with pytest.raises(ValueError):
        result = nested_function(10, 0)

    assert nested_function(10, 2) == 5


In the above example, we are testing the nested_function and asserting that it raises a ValueError when dividing by zero. We handle the ValueError exception using the pytest.raises context manager.


You can also use specific exception types like ZeroDivisionError or any custom exception you have defined in your code. Just replace ValueError with the proper exception type in the pytest.raises context manager.


Remember to always test for both the expected exception and the correct behavior when asserting a nested function call in pytest.


How to confirm a nested function was called in pytest?

To confirm that a nested function was called in pytest, you can use the pytest-mock library. This library provides tools for mocking and asserting function calls in your tests.


Here is an example of how you can confirm a nested function was called in a pytest test using pytest-mock:

  1. Install the pytest-mock library:
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pip install pytest-mock


  1. Create your test function using pytest and pytest-mock:
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# test_nested_function.py

import pytest
from unittest.mock import Mock

def nested_function():
    print("Nested function was called")

def main_function():
    nested_function()

def test_nested_call(mocker):
    mocker.patch('__main__.nested_function')
    
    main_function()
    
    assert nested_function.called


In this test, we are using the mocker.patch method to mock the nested_function in the test. Then, we call the main_function, which calls the nested_function. Finally, we use the assert statement to check if the nested_function was called.


When you run this test using pytest, it will confirm if the nested function was called successfully.


How to assert a nested function call that raises an exception in pytest?

To assert a nested function call that raises an exception in pytest, you can use the pytest.raises() context manager. Here's an example:


Let's say you have a function nested_function() that calls another function inner_function() which raises an exception:

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def inner_function():
    raise ValueError("Something went wrong")

def nested_function():
    inner_function()


In your test function, you can assert that calling nested_function() raises a ValueError like this:

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import pytest

def test_nested_function_raises_exception():
    with pytest.raises(ValueError):
        nested_function()


When you run this test, pytest will pass the test if nested_function() raises a ValueError, and fail the test if it does not raise an exception or raises a different type of exception.


How to assert a nested function call within a loop in pytest?

To assert a nested function call within a loop in pytest, you can use the assert statement within the loop to check the expected result of the nested function call. Here is an example of how you can do this:

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# Function to be tested
def nested_func(x):
    return x * 2

# Test function
def test_nested_func():
    test_data = [1, 2, 3, 4, 5]

    for data in test_data:
        result = nested_func(data)
        assert result == data * 2


In the above example, the test_nested_func() function iterates over a list of test data, calls the nested_func() with each data point, and asserts that the result of the nested function call is equal to the expected value (data * 2). If the result of the nested function call does not match the expected value for any data point, the test will fail and pytest will provide information on the failed assertion.

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