⚡️ Speed up function _merge_with_dialect_properties by 17%

[codeflash-ai]

📄 17% (0.17x) speedup for _merge_with_dialect_properties in pandas/io/parsers/readers.py

⏱️ Runtime : 401 microseconds → 342 microseconds (best of 90 runs)

📝 Explanation and details

The optimization achieves a 17% speedup by replacing a costly function call with direct attribute access in the hot path of stack inspection.

Key Optimization:

• Replaced inspect.getfile(frame) with frame.f_code.co_filename in find_stack_level()
• This eliminates an expensive function call that internally does file path resolution and validation

Why This Matters:
The line profiler shows inspect.getfile(frame) was consuming 41% of execution time (241,236ns out of 587,853ns total) in the original code. The optimized version reduces this to just 4.6% (17,314ns out of 379,930ns), representing a 93% reduction in that specific operation's cost.

Performance Impact Context:
Based on the function references, _merge_with_dialect_properties is called during CSV reader initialization (TextFileReader.__init__), which can happen frequently when processing multiple files or in data pipeline scenarios. The find_stack_level() function is invoked when warnings are issued for conflicting dialect parameters.

Test Results Analysis:
The optimization shows strongest benefits (17-35% faster) in test cases that trigger warnings due to parameter conflicts, where find_stack_level() is actually called. Tests without conflicts show minimal impact since the optimized function isn't invoked, confirming the targeted nature of this improvement.

The optimization maintains identical functionality while significantly reducing overhead in warning scenarios, making CSV parsing more efficient when dialect conflicts occur.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 32 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import csv
import warnings

# imports
import pytest
from pandas.io.parsers.readers import _merge_with_dialect_properties

# --- Function under test (copied from above, with dependencies mocked as needed) ---

# Mock parser_defaults as in pandas.io.parsers.base_parser
parser_defaults = {
    "delimiter": ",",
    "doublequote": True,
    "escapechar": None,
    "skipinitialspace": False,
    "quotechar": '"',
    "quoting": csv.QUOTE_MINIMAL,
}

# MANDATORY_DIALECT_ATTRS as in the source
MANDATORY_DIALECT_ATTRS = (
    "delimiter",
    "doublequote",
    "escapechar",
    "skipinitialspace",
    "quotechar",
    "quoting",
)


# Minimal ParserWarning class for testing
class ParserWarning(Warning):
    pass


# --- Helper: Custom dialects for testing ---


class DummyDialect:
    # Allows us to easily create dialects with arbitrary values
    def __init__(
        self,
        delimiter=",",
        doublequote=True,
        escapechar=None,
        skipinitialspace=False,
        quotechar='"',
        quoting=csv.QUOTE_MINIMAL,
    ):
        self.delimiter = delimiter
        self.doublequote = doublequote
        self.escapechar = escapechar
        self.skipinitialspace = skipinitialspace
        self.quotechar = quotechar
        self.quoting = quoting


# --- Test Suite ---

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


def test_basic_no_conflict_defaults_used():
    """If defaults match dialect, result should be identical and no warnings."""
    dialect = DummyDialect()
    defaults = {}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 2.79μs -> 2.99μs (6.82% slower)
        # All params should match the dialect
        for param in MANDATORY_DIALECT_ATTRS:
            pass


def test_basic_explicit_defaults_match_dialect():
    """If explicit defaults match dialect, no warning and values preserved."""
    dialect = DummyDialect(delimiter=";", doublequote=False)
    defaults = {
        "delimiter": ";",
        "doublequote": False,
        "escapechar": None,
        "skipinitialspace": False,
        "quotechar": '"',
        "quoting": csv.QUOTE_MINIMAL,
    }
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 2.73μs -> 2.97μs (8.21% slower)
        for param in MANDATORY_DIALECT_ATTRS:
            pass


def test_basic_conflicting_values_warns_and_overrides():
    """If a default conflicts with dialect, warning is issued and dialect wins."""
    dialect = DummyDialect(delimiter=";")
    defaults = {"delimiter": ","}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 25.4μs -> 20.9μs (21.5% faster)


def test_basic_multiple_conflicts_all_warned():
    """Multiple conflicting params should each raise a warning."""
    dialect = DummyDialect(
        delimiter=":", doublequote=False, escapechar="\\", skipinitialspace=True
    )
    defaults = {
        "delimiter": ",",
        "doublequote": True,
        "escapechar": None,
        "skipinitialspace": False,
    }
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 21.6μs -> 17.6μs (22.7% faster)
        for param in ["delimiter", "doublequote", "escapechar", "skipinitialspace"]:
            pass


def test_basic_no_defaults_all_dialect_used():
    """If no defaults provided, all values come from dialect."""
    dialect = DummyDialect(
        delimiter="|",
        doublequote=False,
        escapechar="\\",
        skipinitialspace=True,
        quotechar="'",
        quoting=csv.QUOTE_ALL,
    )
    defaults = {}
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 2.58μs -> 2.84μs (9.26% slower)
    for param in MANDATORY_DIALECT_ATTRS:
        pass


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


def test_edge_empty_defaults_and_dialect_unusual_values():
    """Dialect has unusual values (None, False, odd quoting), should propagate."""
    dialect = DummyDialect(
        delimiter=None,
        doublequote=False,
        escapechar="~",
        skipinitialspace=True,
        quotechar=None,
        quoting=csv.QUOTE_NONE,
    )
    defaults = {}
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 2.56μs -> 2.82μs (9.33% slower)


def test_edge_defaults_missing_some_keys():
    """Defaults dict missing some keys, should still merge properly."""
    dialect = DummyDialect(delimiter=";", doublequote=False)
    defaults = {"delimiter": ",", "quoting": csv.QUOTE_ALL}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 32.9μs -> 25.9μs (27.1% faster)


def test_edge_sep_override_suppresses_delimiter_warning():
    """sep_override disables delimiter conflict warning."""
    dialect = DummyDialect(delimiter=";")
    defaults = {"delimiter": ",", "sep_override": True}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 3.77μs -> 3.72μs (1.29% faster)


def test_edge_defaults_and_dialect_both_none():
    """If both default and dialect value are None, no warning and None used."""
    dialect = DummyDialect(escapechar=None)
    defaults = {"escapechar": None}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 2.84μs -> 2.95μs (3.79% slower)


def test_edge_defaults_has_extra_keys_ignored():
    """Keys in defaults not in MANDATORY_DIALECT_ATTRS are preserved."""
    dialect = DummyDialect()
    defaults = {"foo": "bar", "delimiter": "|"}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 22.5μs -> 18.4μs (22.8% faster)


def test_edge_dialect_missing_attribute_raises():
    """If dialect is missing a required attribute, should raise AttributeError."""

    class IncompleteDialect:
        def __init__(self):
            self.delimiter = ","
            # missing doublequote, etc.

    dialect = IncompleteDialect()
    defaults = {}
    with pytest.raises(AttributeError):
        _merge_with_dialect_properties(
            dialect, defaults
        )  # 2.66μs -> 2.80μs (4.94% slower)


def test_edge_warning_message_content():
    """Warning message should contain all required info about the conflict."""
    dialect = DummyDialect(delimiter=";", doublequote=False)
    defaults = {"delimiter": ",", "doublequote": True}
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        _merge_with_dialect_properties(
            dialect, defaults
        )  # 25.6μs -> 21.6μs (18.3% faster)
        messages = [str(warning.message) for warning in w]


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


def test_large_scale_many_conflicts():
    """Test with 1000 conflicting keys (using extra keys), ensure only MANDATORY_DIALECT_ATTRS are handled."""
    dialect = DummyDialect(
        delimiter=";",
        doublequote=False,
        escapechar="\\",
        skipinitialspace=True,
        quotechar="'",
        quoting=csv.QUOTE_ALL,
    )
    # 1000 extra keys, all conflicting
    defaults = {f"extra{i}": i for i in range(1000)}
    # Also add conflicting values for all mandatory attrs
    for param in MANDATORY_DIALECT_ATTRS:
        defaults[param] = "conflict"
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 63.8μs -> 47.0μs (35.7% faster)
        # All mandatory attrs should be set to dialect's value
        for param in MANDATORY_DIALECT_ATTRS:
            pass
        # All extra keys should be preserved
        for i in range(1000):
            pass


def test_large_scale_defaults_only_extra_keys():
    """Defaults with many extra keys and no conflicts."""
    dialect = DummyDialect()
    defaults = {f"foo{i}": i for i in range(1000)}
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 5.95μs -> 6.04μs (1.41% slower)
    # All extra keys should be preserved
    for i in range(1000):
        pass
    # All mandatory attrs should be present and equal to dialect
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_large_scale_all_defaults_match_dialect():
    """All defaults match dialect, no warnings, performance check."""
    dialect = DummyDialect(
        delimiter=";",
        doublequote=False,
        escapechar="\\",
        skipinitialspace=True,
        quotechar="'",
        quoting=csv.QUOTE_ALL,
    )
    defaults = {param: getattr(dialect, param) for param in MANDATORY_DIALECT_ATTRS}
    # Add 1000 extra keys
    defaults.update({f"x{i}": i for i in range(1000)})
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 5.90μs -> 6.07μs (2.72% slower)
        for param in MANDATORY_DIALECT_ATTRS:
            pass
        for i in range(1000):
            pass


def test_large_scale_sep_override_among_many_keys():
    """sep_override disables delimiter warning even with many keys."""
    dialect = DummyDialect(delimiter=";")
    defaults = {f"k{i}": i for i in range(500)}
    defaults.update({"delimiter": ",", "sep_override": True})
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 5.62μs -> 5.49μs (2.44% faster)
        # All extra keys preserved
        for i in range(500):
            pass


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

import csv
import warnings

# imports
from pandas.io.parsers.readers import _merge_with_dialect_properties

# Function under test (copied from above)
MANDATORY_DIALECT_ATTRS = (
    "delimiter",
    "doublequote",
    "escapechar",
    "skipinitialspace",
    "quotechar",
    "quoting",
)

# Minimal parser_defaults for test purposes
parser_defaults = {
    "delimiter": ",",
    "doublequote": True,
    "escapechar": None,
    "skipinitialspace": False,
    "quotechar": '"',
    "quoting": csv.QUOTE_MINIMAL,
}


class ParserWarning(Warning):
    pass


# ---- Test Infrastructure ----


# Helper: make a custom dialect class for testing
def make_dialect(**kwargs):
    # Create a new dialect type with given attributes
    attrs = {
        "delimiter": ",",
        "doublequote": True,
        "escapechar": None,
        "skipinitialspace": False,
        "quotechar": '"',
        "quoting": csv.QUOTE_MINIMAL,
    }
    attrs.update(kwargs)
    return type("TestDialect", (object,), attrs)()


# ---- Basic Test Cases ----


def test_basic_no_conflicts_all_defaults():
    """If defaults match the dialect, output should be identical to defaults, but with dialect values."""
    dialect = make_dialect()
    defaults = parser_defaults.copy()
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 3.04μs -> 3.19μs (4.67% slower)
    # All mandatory params should match the dialect
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_basic_conflicting_one_param_warns_and_overwrites():
    """If a single param (e.g. delimiter) is set differently, warning is raised and value is overwritten."""
    dialect = make_dialect(delimiter=";")
    defaults = parser_defaults.copy()
    defaults["delimiter"] = "|"
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 24.8μs -> 20.8μs (19.1% faster)
    # Other params unchanged
    for param in MANDATORY_DIALECT_ATTRS:
        if param != "delimiter":
            pass


def test_basic_multiple_conflicts_warns_for_each():
    """If multiple params conflict, warning for each, and all are overwritten to dialect values."""
    dialect = make_dialect(delimiter=";", quotechar="'", doublequote=False)
    defaults = parser_defaults.copy()
    defaults["delimiter"] = "|"
    defaults["quotechar"] = '"'
    defaults["doublequote"] = True
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 20.7μs -> 17.4μs (19.0% faster)
    # All overwritten
    for param in ("delimiter", "quotechar", "doublequote"):
        pass


def test_basic_no_warning_when_provided_value_is_parser_default():
    """If the provided value is the parser default, no warning is raised."""
    dialect = make_dialect(delimiter="|")
    defaults = parser_defaults.copy()
    # Don't set delimiter, so it is parser_default
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 20.4μs -> 17.3μs (17.9% faster)


def test_basic_no_warning_when_provided_value_is_dialect_value():
    """If the provided value matches the dialect, no warning is raised."""
    dialect = make_dialect(quotechar="'")
    defaults = parser_defaults.copy()
    defaults["quotechar"] = "'"
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 2.77μs -> 3.02μs (8.40% slower)


# ---- Edge Test Cases ----


def test_edge_missing_some_defaults():
    """If defaults dict is missing some keys, function should still fill them from dialect."""
    dialect = make_dialect(delimiter=";", quotechar="'")
    defaults = {"delimiter": "|"}  # Only one param provided
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 22.3μs -> 18.8μs (18.7% faster)
    # All params present in result
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_edge_defaults_has_extra_keys():
    """Extra keys in defaults should be preserved in result."""
    dialect = make_dialect()
    defaults = parser_defaults.copy()
    defaults["extra1"] = 123
    defaults["extra2"] = "abc"
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 2.66μs -> 2.93μs (9.17% slower)
    # Mandatory params still set to dialect
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_edge_dialect_with_none_values():
    """If dialect has None for a param, it should overwrite defaults."""
    dialect = make_dialect(escapechar=None)
    defaults = parser_defaults.copy()
    defaults["escapechar"] = "\\"
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 22.5μs -> 19.3μs (17.0% faster)


def test_edge_sep_override_suppresses_delimiter_warning():
    """If sep_override is True in defaults, delimiter conflict should not warn."""
    dialect = make_dialect(delimiter=";")
    defaults = parser_defaults.copy()
    defaults["delimiter"] = "|"
    defaults["sep_override"] = True
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 3.57μs -> 3.63μs (1.54% slower)


def test_edge_empty_defaults_dict():
    """If defaults is empty, all values should be taken from dialect."""
    dialect = make_dialect(
        delimiter=";",
        quotechar="'",
        doublequote=False,
        escapechar="\\",
        skipinitialspace=True,
        quoting=csv.QUOTE_ALL,
    )
    defaults = {}
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 2.94μs -> 3.08μs (4.74% slower)
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_edge_multiple_conflicts_and_sep_override_only_affects_delimiter():
    """sep_override only suppresses delimiter warning, not others."""
    dialect = make_dialect(delimiter=";", quotechar="'")
    defaults = parser_defaults.copy()
    defaults["delimiter"] = "|"
    defaults["quotechar"] = '"'
    defaults["sep_override"] = True
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 4.11μs -> 4.18μs (1.72% slower)


# ---- Large Scale Test Cases ----


def test_large_many_extra_keys():
    """Test with a large number of extra keys in defaults."""
    dialect = make_dialect()
    defaults = parser_defaults.copy()
    # Add 500 extra keys
    for i in range(500):
        defaults[f"extra_{i}"] = i
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 4.46μs -> 4.79μs (6.83% slower)
    # All extra keys preserved
    for i in range(500):
        pass
    # Mandatory params correct
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_large_conflicts_on_all_params():
    """All mandatory params conflict and should warn, be overwritten."""
    dialect = make_dialect(
        delimiter="|",
        doublequote=False,
        escapechar="\\",
        skipinitialspace=True,
        quotechar="'",
        quoting=csv.QUOTE_ALL,
    )
    defaults = {
        "delimiter": ";",
        "doublequote": True,
        "escapechar": None,
        "skipinitialspace": False,
        "quotechar": '"',
        "quoting": csv.QUOTE_MINIMAL,
    }
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 25.2μs -> 21.0μs (20.3% faster)
    # One warning per param
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_large_defaults_missing_all_mandatory():
    """Defaults dict has 900 keys, none are mandatory params."""
    dialect = make_dialect(delimiter=";", quotechar="'")
    defaults = {f"foo_{i}": i for i in range(900)}
    codeflash_output = _merge_with_dialect_properties(dialect, defaults)
    result = codeflash_output  # 5.95μs -> 5.90μs (0.916% faster)
    # All extra keys preserved
    for i in range(900):
        pass
    # All mandatory params filled from dialect
    for param in MANDATORY_DIALECT_ATTRS:
        pass


def test_large_randomized_conflicts_and_nonconflicts():
    """Randomly assign some conflicting, some matching, some missing mandatory params."""
    import random

    random.seed(42)
    # Randomly generate dialect values
    dialect_values = {
        "delimiter": random.choice([",", ";", "|"]),
        "doublequote": random.choice([True, False]),
        "escapechar": random.choice([None, "\\", "/"]),
        "skipinitialspace": random.choice([True, False]),
        "quotechar": random.choice(['"', "'"]),
        "quoting": random.choice([csv.QUOTE_MINIMAL, csv.QUOTE_ALL, csv.QUOTE_NONE]),
    }
    dialect = make_dialect(**dialect_values)
    defaults = {}
    # For each param, randomly: set to dialect, set to parser_default, set to something else, or omit
    for param in MANDATORY_DIALECT_ATTRS:
        r = random.random()
        if r < 0.25:
            # set to dialect value
            defaults[param] = dialect_values[param]
        elif r < 0.5:
            # set to parser_default
            defaults[param] = parser_defaults[param]
        elif r < 0.75:
            # set to something else
            if param == "delimiter":
                val = "x"
            elif param == "doublequote":
                val = not dialect_values[param]
            elif param == "escapechar":
                val = "/" if dialect_values[param] != "/" else "\\"
            elif param == "skipinitialspace":
                val = not dialect_values[param]
            elif param == "quotechar":
                val = "'" if dialect_values[param] != "'" else '"'
            elif param == "quoting":
                val = (
                    csv.QUOTE_NONE
                    if dialect_values[param] != csv.QUOTE_NONE
                    else csv.QUOTE_ALL
                )
            defaults[param] = val
        # else: omit param
    with warnings.catch_warnings(record=True) as w:
        warnings.simplefilter("always")
        codeflash_output = _merge_with_dialect_properties(dialect, defaults)
        result = codeflash_output  # 2.93μs -> 3.04μs (3.75% slower)
    # All mandatory params present and set to dialect
    for param in MANDATORY_DIALECT_ATTRS:
        pass
    # For conflicts, should have warning
    for param in MANDATORY_DIALECT_ATTRS:
        if param in defaults and defaults[param] not in (
            parser_defaults[param],
            dialect_values[param],
        ):
            pass


# ---- Determinism and No Side Effects ----


def test_no_side_effects_on_dialect():
    """Function should not mutate the dialect object."""

    class MutableDialect:
        def __init__(self):
            self.delimiter = ","
            self.doublequote = True
            self.escapechar = None
            self.skipinitialspace = False
            self.quotechar = '"'
            self.quoting = csv.QUOTE_MINIMAL

    dialect = MutableDialect()
    orig = dialect.__dict__.copy()
    defaults = parser_defaults.copy()
    _merge_with_dialect_properties(dialect, defaults)  # 3.02μs -> 3.24μs (6.79% slower)


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

To edit these changes git checkout codeflash/optimize-_merge_with_dialect_properties-mjaa58rt and push.