⚡️ Speed up function get_stderr_fileno by 15%

[codeflash-ai]

📄 15% (0.15x) speedup for get_stderr_fileno in src/_pytest/faulthandler.py

⏱️ Runtime : 802 microseconds → 699 microseconds (best of 250 runs)

📝 Explanation and details

The optimization improves performance by restructuring control flow to avoid redundant exception handling and caching the sys.stderr reference.

Key optimizations:

1. Single sys.stderr lookup: Stores stderr = sys.stderr once instead of accessing sys.stderr multiple times, reducing attribute lookups.

2. Streamlined exception handling: Moves the fileno == -1 check outside the try/except block. In the original code, when fileno() succeeds but returns -1, it raises AttributeError() just to be caught immediately. The optimized version eliminates this unnecessary exception raising/catching cycle.

3. Reduced try/except scope: The optimized version only catches actual exceptions from stderr.fileno(), not artificially raised ones for control flow.

Performance impact by test case:

• Normal cases (valid fileno): Slight improvement from cached stderr reference
• Twisted Logger cases (fileno returns -1): 40.9% faster - eliminates the raise/catch cycle that was used purely for control flow
• Exception cases (AttributeError/ValueError): Similar performance, as these still follow the exception path

Context significance:
The function is called during pytest configuration setup (pytest_configure) where it's used to capture stderr file descriptors for fault handling. While not in a tight loop, the 14% overall speedup reduces pytest startup time, and the 40.9% improvement for Twisted Logger scenarios directly benefits projects using that logging framework.

The optimization maintains identical behavior while making the common Twisted Logger case significantly faster by using proper conditional logic instead of exception-based control flow.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 1921 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	✅ 1 Passed
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import io
import sys

from _pytest.faulthandler import get_stderr_fileno

# imports
import pytest


# unit tests

# ----------- BASIC TEST CASES -----------


def test_default_stderr_fileno():
    """Test with default sys.stderr (should be an open file object)."""
    # Save original sys.stderr
    orig_stderr = sys.stderr
    try:
        # sys.stderr is usually sys.__stderr__ at startup
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_stderr_is_stringio():
    """Test when sys.stderr is replaced with StringIO (no fileno method)."""
    orig_stderr = sys.stderr
    sys.stderr = io.StringIO()
    try:
        # StringIO has no fileno method, should fallback to sys.__stderr__
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_stderr_is_file_with_fileno():
    """Test when sys.stderr is a file-like object with a valid fileno."""
    orig_stderr = sys.stderr
    with open(__file__) as f:
        sys.stderr = f
        try:
            codeflash_output = get_stderr_fileno()
        finally:
            sys.stderr = orig_stderr


# ----------- EDGE TEST CASES -----------


def test_stderr_missing_fileno_attribute():
    """Test when sys.stderr lacks a fileno method entirely."""

    class NoFileno:
        pass

    orig_stderr = sys.stderr
    sys.stderr = NoFileno()
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_raises_valueerror():
    """Test when sys.stderr.fileno raises ValueError."""

    class ValueErrorFileno:
        def fileno(self):
            raise ValueError("No fileno available")

    orig_stderr = sys.stderr
    sys.stderr = ValueErrorFileno()
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_returns_minus_one():
    """Test when sys.stderr.fileno returns -1 (Twisted Logger case)."""

    class MinusOneFileno:
        def fileno(self):
            return -1

    orig_stderr = sys.stderr
    sys.stderr = MinusOneFileno()
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_raises_attributeerror():
    """Test when sys.stderr.fileno raises AttributeError directly."""

    class AttributeErrorFileno:
        def fileno(self):
            raise AttributeError("No fileno available")

    orig_stderr = sys.stderr
    sys.stderr = AttributeErrorFileno()
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_sys_stderr_is_none():
    """Test when sys.stderr is set to None."""
    orig_stderr = sys.stderr
    sys.stderr = None
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_sys_stderr_is_integer():
    """Test when sys.stderr is set to an integer."""
    orig_stderr = sys.stderr
    sys.stderr = 42
    try:
        codeflash_output = get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


# ----------- LARGE SCALE TEST CASES -----------


def test_many_stderr_substitutions():
    """Test performance and correctness with many different sys.stderr objects."""
    orig_stderr = sys.stderr

    class DummyFileno:
        def __init__(self, val):
            self.val = val

        def fileno(self):
            return self.val

    # Generate 1000 different DummyFileno objects, half with valid fileno, half with -1
    for i in range(500):
        sys.stderr = DummyFileno(i)
        codeflash_output = get_stderr_fileno()  # 121μs -> 122μs (0.467% slower)
    for i in range(500):
        sys.stderr = DummyFileno(-1)
        codeflash_output = get_stderr_fileno()  # 236μs -> 168μs (40.9% faster)

    sys.stderr = orig_stderr


def test_large_batch_of_edge_cases():
    """Test with a large batch mixing edge cases and valid file objects."""
    orig_stderr = sys.stderr

    class NoFileno:
        pass

    class ValueErrorFileno:
        def fileno(self):
            raise ValueError()

    class AttributeErrorFileno:
        def fileno(self):
            raise AttributeError()

    class MinusOneFileno:
        def fileno(self):
            return -1

    class ValidFileno:
        def __init__(self, val):
            self.val = val

        def fileno(self):
            return self.val

    test_objects = (
        [NoFileno() for _ in range(100)]
        + [ValueErrorFileno() for _ in range(100)]
        + [AttributeErrorFileno() for _ in range(100)]
        + [MinusOneFileno() for _ in range(100)]
        + [ValidFileno(i) for i in range(100)]
    )

    for idx, obj in enumerate(test_objects):
        sys.stderr = obj
        if hasattr(obj, "fileno") and callable(getattr(obj, "fileno", None)):
            try:
                val = obj.fileno()
                if val == -1:
                    codeflash_output = get_stderr_fileno()
                else:
                    codeflash_output = get_stderr_fileno()
            except (AttributeError, ValueError):
                codeflash_output = get_stderr_fileno()
        else:
            codeflash_output = get_stderr_fileno()
    sys.stderr = orig_stderr


# ----------- ADDITIONAL EDGE CASES -----------


def test_sys___stderr___is_missing():
    """Test behavior if sys.__stderr__ is missing (should raise AttributeError)."""
    orig_stderr = sys.stderr
    orig___stderr__ = getattr(sys, "__stderr__", None)
    sys.stderr = io.StringIO()
    try:
        # Remove sys.__stderr__ temporarily
        if hasattr(sys, "__stderr__"):
            del sys.__stderr__
        with pytest.raises(AttributeError):
            get_stderr_fileno()
    finally:
        # Restore sys.__stderr__ if it was present
        if orig___stderr__ is not None:
            sys.__stderr__ = orig___stderr__
        sys.stderr = orig_stderr


def test_sys___stderr___has_no_fileno():
    """Test behavior if sys.__stderr__ exists but has no fileno method."""
    orig_stderr = sys.stderr
    orig___stderr__ = getattr(sys, "__stderr__", None)

    class NoFileno:
        pass

    sys.stderr = io.StringIO()
    sys.__stderr__ = NoFileno()
    try:
        with pytest.raises(AttributeError):
            get_stderr_fileno()
    finally:
        if orig___stderr__ is not None:
            sys.__stderr__ = orig___stderr__
        sys.stderr = orig_stderr


# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.

import io
import sys

from _pytest.faulthandler import get_stderr_fileno

# imports
import pytest


# ---------------------- UNIT TESTS ----------------------

# Basic Test Cases


def test_stderr_is_standard_stream():
    """
    Basic: sys.stderr is the default sys.stderr (should be an open file object)
    """
    # Save the original sys.stderr
    orig_stderr = sys.stderr
    sys.stderr = sys.__stderr__
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_stderr_is_duplicate_of_stderr():
    """
    Basic: sys.stderr is a duplicate file object of sys.__stderr__
    """
    orig_stderr = sys.stderr
    # Open a new file object pointing to the same file descriptor as sys.__stderr__
    new_stderr = open(sys.__stderr__.fileno(), mode="w", closefd=False)
    sys.stderr = new_stderr
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr
        new_stderr.close()


# Edge Test Cases


def test_stderr_fileno_raises_attributeerror():
    """
    Edge: sys.stderr does not have a fileno() method (e.g., StringIO)
    """
    orig_stderr = sys.stderr
    sys.stderr = io.StringIO()
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_raises_valueerror():
    """
    Edge: sys.stderr.fileno() raises ValueError (e.g., closed file)
    """
    orig_stderr = sys.stderr
    temp_file = open("/dev/null", "w")
    temp_file.close()  # Now the file is closed
    sys.stderr = temp_file
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_returns_minus_one():
    """
    Edge: sys.stderr.fileno() returns -1 (Twisted Logger simulation)
    """

    class FakeStderr:
        def fileno(self):
            return -1

    orig_stderr = sys.stderr
    sys.stderr = FakeStderr()
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_stderr_fileno_raises_unexpected_exception():
    """
    Edge: sys.stderr.fileno() raises an unexpected exception (should not catch)
    """

    class FakeStderr:
        def fileno(self):
            raise RuntimeError("unexpected error")

    orig_stderr = sys.stderr
    sys.stderr = FakeStderr()
    try:
        with pytest.raises(RuntimeError):
            get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr


def test_sys_stderr_is_none():
    """
    Edge: sys.stderr is None (should raise AttributeError and fallback)
    """
    orig_stderr = sys.stderr
    sys.stderr = None
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_sys_stderr_is_object_without_fileno():
    """
    Edge: sys.stderr is an object without fileno attribute
    """

    class NoFileno:
        pass

    orig_stderr = sys.stderr
    sys.stderr = NoFileno()
    try:
        codeflash_output = get_stderr_fileno()
        result = codeflash_output
    finally:
        sys.stderr = orig_stderr


# Large Scale Test Cases


def test_multiple_calls_consistency():
    """
    Large Scale: Call get_stderr_fileno() many times to ensure consistency and no resource leaks.
    """
    orig_stderr = sys.stderr
    sys.stderr = sys.__stderr__
    try:
        results = [get_stderr_fileno() for _ in range(500)]
        # All results should be the same and match sys.__stderr__.fileno()
        expected = sys.__stderr__.fileno()
    finally:
        sys.stderr = orig_stderr


def test_large_number_of_fake_stderr_objects():
    """
    Large Scale: Test with a large number of different fake stderr objects.
    """

    class FakeStderr:
        def fileno(self):
            return -1

    orig_stderr = sys.stderr
    try:
        for _ in range(200):
            sys.stderr = FakeStderr()
            codeflash_output = get_stderr_fileno()
            result = codeflash_output
    finally:
        sys.stderr = orig_stderr


def test_performance_with_stringio():
    """
    Large Scale: Test performance and correctness when sys.stderr is StringIO many times.
    """
    orig_stderr = sys.stderr
    try:
        for _ in range(200):
            sys.stderr = io.StringIO()
            codeflash_output = get_stderr_fileno()
            result = codeflash_output
    finally:
        sys.stderr = orig_stderr


# Additional edge: sys.__stderr__ is missing (should raise AttributeError)
def test_sys___stderr___missing(monkeypatch):
    """
    Edge: sys.__stderr__ is missing (simulate by deleting and restoring)
    """
    orig_stderr = sys.stderr
    orig___stderr__ = sys.__stderr__
    sys.stderr = io.StringIO()
    monkeypatch.delattr(sys, "__stderr__")
    try:
        with pytest.raises(AttributeError):
            get_stderr_fileno()
    finally:
        sys.stderr = orig_stderr
        sys.__stderr__ = orig___stderr__


# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.

from _pytest.faulthandler import get_stderr_fileno


def test_get_stderr_fileno():
    get_stderr_fileno()

🔎 Concolic Coverage Tests and Runtime

Test File::Test Function	Original ⏱️	Optimized ⏱️	Speedup
codeflash_concolic__lsdxkww/tmp_7xn02l5/test_concolic_coverage.py::test_get_stderr_fileno	2.64μs	2.49μs	6.07%✅

To edit these changes git checkout codeflash/optimize-get_stderr_fileno-mi9karq2 and push.