Lesson 14 · Pytest deep dive

pytest Tutorial #14: Capturing Output & Logs

On testing real Python code with pytest. capsys, capfd, and caplog: real asserting on printed and logged output. Let's get into it. capsys intercepts…

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CoursePytest deep dive
Lesson14 of 18
FormatJupyter notebook · 7 code cells

logging pytest

What you'll learn

The Helper From Last Time
capsys - Capturing print() Output
readouterr() Reads and Clears
capfd - Capturing at the File-Descriptor Level
caplog - Capturing Log Records
caplog.atlevel - Capturing Below the Default Threshold
A Real Pattern - Testing a Function That Prints and Logs
Wrap-Up: What You Learned

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pytest Deep-Dive, Video 14: Capturing Output#

Video fourteen of an eighteen-part series on testing real Python code with pytest.
capsys, capfd, and caplog: real asserting on printed and logged output.
Let's get into it.

Part 1: The Helper From Last Time#

import pytest, tempfile, os
def run_pytest(code_str, args=None):
    with tempfile.NamedTemporaryFile(mode='w', suffix='_test.py', delete=False) as f:
        f.write(code_str)
        path = f.name
    result = pytest.main([path, *(args or [])])
    os.unlink(path)
    return result

Part 2: capsys - Capturing print() Output#

code = '''
def greet(name):
    print(f"Hello, {name}!")

def test_greet_prints_message(capsys):
    greet('Ada')
    captured = capsys.readouterr()
    assert captured.out == 'Hello, Ada!\\n'
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 1 item

..\..\..\Users\makmw\AppData\Local\Temp\tmpv3g_axq8_test.py::test_greet_prints_message PASSED [100%]

============================== 1 passed in 0.22s ==============================
0

Part 3: readouterr() Reads and Clears#

code = '''
def test_readouterr_clears_buffer(capsys):
    print('first batch')
    first = capsys.readouterr()
    assert first.out == 'first batch\\n'
    second = capsys.readouterr()
    assert second.out == ''
    print('second batch')
    third = capsys.readouterr()
    assert third.out == 'second batch\\n'
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 1 item

..\..\..\Users\makmw\AppData\Local\Temp\tmp_1apgg17_test.py::test_readouterr_clears_buffer PASSED [100%]

============================== 1 passed in 0.11s ==============================
0

Part 4: capfd - Capturing at the File-Descriptor Level#

code = '''
import os

def test_capfd_catches_raw_os_write(capfd):
    os.write(1, b'raw fd write\\n')
    captured = capfd.readouterr()
    assert 'raw fd write' in captured.out
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 1 item

..\..\..\Users\makmw\AppData\Local\Temp\tmppwyfsyqh_test.py::test_capfd_catches_raw_os_write PASSED [100%]

============================== 1 passed in 0.12s ==============================
0

Part 5: caplog - Capturing Log Records#

code = '''
import logging

def risky_operation(value):
    if value < 0:
        logging.warning('received a negative value: %s', value)
    return abs(value)

def test_warning_is_logged(caplog):
    result = risky_operation(-5)
    assert result == 5
    assert 'received a negative value' in caplog.text

def test_no_warning_for_valid_input(caplog):
    risky_operation(5)
    assert caplog.text == ""
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 2 items

..\..\..\Users\makmw\AppData\Local\Temp\tmpxp2v74zo_test.py::test_warning_is_logged PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmpxp2v74zo_test.py::test_no_warning_for_valid_input PASSED [100%]

============================== 2 passed in 0.11s ==============================
0

Part 6: caplog.at_level - Capturing Below the Default Threshold#

code = '''
import logging

def test_info_hidden_by_default(caplog):
    logging.info('this is quiet by default')
    assert 'this is quiet' not in caplog.text

def test_info_visible_with_at_level(caplog):
    with caplog.at_level(logging.INFO):
        logging.info('now this is visible')
    assert 'now this is visible' in caplog.text
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 2 items

..\..\..\Users\makmw\AppData\Local\Temp\tmp5dfcvaae_test.py::test_info_hidden_by_default PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmp5dfcvaae_test.py::test_info_visible_with_at_level PASSED [100%]

============================== 2 passed in 0.12s ==============================
0

Part 7: A Real Pattern - Testing a Function That Prints and Logs#

code = '''
import logging

def process_order(quantity):
    if quantity > 100:
        logging.warning('unusually large order: %s', quantity)
    print(f'Processing order for {quantity} units.')
    return quantity * 10

def test_process_order_large_quantity(capsys, caplog):
    total = process_order(150)
    assert total == 1500
    captured = capsys.readouterr()
    assert 'Processing order for 150 units' in captured.out
    assert 'unusually large order' in caplog.text

def test_process_order_normal_quantity(capsys, caplog):
    total = process_order(10)
    assert total == 100
    captured = capsys.readouterr()
    assert 'Processing order for 10 units' in captured.out
    assert caplog.text == ""
'''
exit_code = run_pytest(code, ['-v', '--no-header'])
print(exit_code)

============================= test session starts =============================
collecting ... collected 2 items

..\..\..\Users\makmw\AppData\Local\Temp\tmpohws5wxm_test.py::test_process_order_large_quantity PASSED [ 50%]
..\..\..\Users\makmw\AppData\Local\Temp\tmpohws5wxm_test.py::test_process_order_normal_quantity PASSED [100%]

============================== 2 passed in 0.13s ==============================
0

Wrap-Up: What You Learned#

capsys intercepts sys.stdout and sys.stderr; readouterr() returns what was printed as a named tuple.
Each readouterr() call only returns output written since the last call, then clears the buffer.
capfd captures at the OS file-descriptor level, catching raw os.write calls capsys would miss.
caplog captures Python logging output; by default it captures WARNING and above.
caplog.at_level(...), used as a with block, temporarily lowers the captured threshold to INFO or DEBUG.
capsys and caplog can be requested together to verify both printed and logged output in one test.
That wraps up capturing output. Next up: Parallel Testing With pytest-xdist.

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