⚡️ Speed up method BaseExpression._resolve_output_field by 8%

[codeflash-ai]

📄 8% (0.08x) speedup for BaseExpression._resolve_output_field in django/db/models/expressions.py

⏱️ Runtime : 323 microseconds → 299 microseconds (best of 366 runs)

📝 Explanation and details

The optimized code eliminates a costly double iteration pattern that was causing quadratic time complexity in the original implementation.

Key Optimization: Eliminate Nested Generator Iteration
The original code used a nested loop structure where sources_iter was iterated twice - once for the outer loop and once for each inner loop iteration. This created O(n²) behavior where each source field was compared against every other source field.

The optimized version:

1. Materializes the filtered list once - source_fields is computed upfront with a single list comprehension, avoiding repeated generator evaluation
2. Uses linear comparison - Instead of nested loops, it compares each field against only the first field's type using type(source) is not first_cls
3. Replaces isinstance() with type() comparison - Uses strict type equality (type() is not type()) which is faster than isinstance() for exact type matching

Performance Impact by Test Case:

• Large identical fields (500 items): 25.8% faster - Benefits most from eliminating O(n²) comparisons
• Multiple identical fields (2-3 items): 11-15% faster - Reduces overhead even for small datasets
• All None fields: 11-19% faster - Early return after list comprehension avoids any iteration
• Mixed types: Slight performance variation - Early detection of type mismatches can be faster or slower depending on when the mismatch occurs

The optimization is particularly effective for expressions with many source fields of the same type, which is a common Django ORM pattern when dealing with complex aggregations or field expressions.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 49 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import pytest  # used for our unit tests
# function to test
from django.core.exceptions import FieldError
from django.db.models.expressions import BaseExpression


# Helper classes for simulating output fields
class IntegerField:
    pass

class CharField:
    pass

class CustomField:
    pass

# Helper class to simulate source expressions
class SourceExpression:
    def __init__(self, output_field_or_none):
        self._output_field_or_none = output_field_or_none

# Helper subclass to allow us to inject source expressions
class TestExpression(BaseExpression):
    def __init__(self, source_expressions):
        super().__init__()
        self._source_expressions = source_expressions

    def get_source_expressions(self):
        return self._source_expressions

# Basic Test Cases

def test_single_output_field_basic():
    # Single source, output field is IntegerField
    expr = TestExpression([SourceExpression(IntegerField())])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 1.86μs -> 1.95μs (4.57% slower)

def test_multiple_identical_output_fields():
    # Multiple sources, all output fields are IntegerField
    expr = TestExpression([SourceExpression(IntegerField()), SourceExpression(IntegerField())])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 2.14μs -> 1.91μs (11.8% faster)

def test_multiple_identical_custom_output_fields():
    # Multiple sources, all output fields are CustomField
    expr = TestExpression([SourceExpression(CustomField()), SourceExpression(CustomField()), SourceExpression(CustomField())])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 2.26μs -> 1.96μs (15.3% faster)

# Edge Test Cases

def test_mixed_output_fields_raises_field_error():
    # Sources with different output field types should raise FieldError
    expr = TestExpression([SourceExpression(IntegerField()), SourceExpression(CharField())])
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.18μs -> 3.19μs (0.157% slower)

def test_some_none_output_fields():
    # Some sources have None, should ignore them if at least one non-None
    expr = TestExpression([SourceExpression(None), SourceExpression(IntegerField()), SourceExpression(None)])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 1.94μs -> 1.88μs (3.20% faster)

def test_all_none_output_fields():
    # All sources have None, should not return any output field (no iteration)
    expr = TestExpression([SourceExpression(None), SourceExpression(None)])
    # _resolve_output_field returns None because no output field is found
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 1.62μs -> 1.45μs (11.5% faster)

def test_no_source_expressions():
    # No sources, should return None
    expr = TestExpression([])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 1.44μs -> 1.23μs (17.1% faster)

def test_single_none_output_field():
    # Single source with None output field, should return None
    expr = TestExpression([SourceExpression(None)])
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 1.55μs -> 1.32μs (17.2% faster)

def test_mixed_types_with_none_should_raise():
    # Mixed types with one None, should still raise FieldError
    expr = TestExpression([SourceExpression(IntegerField()), SourceExpression(CharField()), SourceExpression(None)])
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.27μs -> 3.25μs (0.400% faster)

# Large Scale Test Cases

def test_large_identical_output_fields():
    # Large number of sources, all output fields are IntegerField
    sources = [SourceExpression(IntegerField()) for _ in range(500)]
    expr = TestExpression(sources)
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 34.8μs -> 28.9μs (20.6% faster)

def test_large_mixed_output_fields_should_raise():
    # Large number of sources, half IntegerField, half CharField
    sources = [SourceExpression(IntegerField()) for _ in range(250)] + [SourceExpression(CharField()) for _ in range(250)]
    expr = TestExpression(sources)
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 23.9μs -> 25.7μs (6.94% slower)

def test_large_some_none_output_fields():
    # Large number of sources, some are None, rest are IntegerField
    sources = [SourceExpression(None) for _ in range(200)] + [SourceExpression(IntegerField()) for _ in range(300)]
    expr = TestExpression(sources)
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 27.8μs -> 23.5μs (18.4% faster)

def test_large_all_none_output_fields():
    # Large number of sources, all None
    sources = [SourceExpression(None) for _ in range(500)]
    expr = TestExpression(sources)
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 15.2μs -> 14.5μs (4.55% faster)

def test_large_one_non_none_output_field():
    # Large number of sources, only one is non-None
    sources = [SourceExpression(None) for _ in range(499)] + [SourceExpression(CharField())]
    expr = TestExpression(sources)
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 15.5μs -> 15.3μs (1.49% faster)

# Edge: Custom field type equality
def test_custom_field_type_equality():
    # Two different instances of CustomField, should be considered same type
    sources = [SourceExpression(CustomField()), SourceExpression(CustomField())]
    expr = TestExpression(sources)
    codeflash_output = expr._resolve_output_field(); result = codeflash_output # 2.13μs -> 1.84μs (15.5% faster)

# Edge: Output fields are subclasses
class SubIntegerField(IntegerField):
    pass

def test_subclass_output_fields():
    # Output fields are subclass and parent class, should raise FieldError (different classes)
    sources = [SourceExpression(IntegerField()), SourceExpression(SubIntegerField())]
    expr = TestExpression(sources)
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.20μs -> 3.15μs (1.55% faster)
# codeflash_output is used to check that the output of the original code is the same as that of the optimized code.
#------------------------------------------------
import pytest  # used for our unit tests
# function to test
from django.core.exceptions import FieldError
from django.db.models.expressions import BaseExpression


# Helper classes to simulate fields and expressions
class FakeFieldA:
    pass

class FakeFieldB:
    pass

class FakeExpr:
    """Simulate an expression with an output field."""
    def __init__(self, output_field):
        self._output_field_or_none = output_field

class CustomExpression(BaseExpression):
    """Allows custom source expressions for testing."""
    def __init__(self, source_expressions):
        self._source_expressions = source_expressions

    def get_source_expressions(self):
        return self._source_expressions

# unit tests

# --- BASIC TEST CASES ---

def test_single_source_field():
    """Test with a single source field (should return that field)."""
    field = FakeFieldA()
    expr = CustomExpression([FakeExpr(field)])
    codeflash_output = expr._resolve_output_field() # 1.85μs -> 1.89μs (1.64% slower)

def test_multiple_same_type_fields():
    """Test with multiple fields of the same type (should return the first)."""
    field1 = FakeFieldA()
    field2 = FakeFieldA()
    expr = CustomExpression([FakeExpr(field1), FakeExpr(field2)])
    codeflash_output = expr._resolve_output_field() # 2.11μs -> 1.82μs (15.6% faster)

def test_multiple_fields_all_none():
    """Test with all source fields being None (should not return, error raised elsewhere)."""
    expr = CustomExpression([FakeExpr(None), FakeExpr(None)])
    # _resolve_output_field returns None (no iteration), so no error here
    codeflash_output = expr._resolve_output_field() # 1.65μs -> 1.38μs (19.2% faster)

def test_mixed_fields_types_raises():
    """Test with mixed field types (should raise FieldError)."""
    field1 = FakeFieldA()
    field2 = FakeFieldB()
    expr = CustomExpression([FakeExpr(field1), FakeExpr(field2)])
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.16μs -> 3.32μs (4.76% slower)

def test_some_none_some_same_type():
    """Test with some None fields and some same type fields (should ignore None, return first)."""
    field1 = FakeFieldA()
    expr = CustomExpression([FakeExpr(None), FakeExpr(field1), FakeExpr(field1)])
    codeflash_output = expr._resolve_output_field() # 2.26μs -> 1.96μs (15.4% faster)

def test_some_none_some_mixed_types():
    """Test with some None fields and some mixed type fields (should raise FieldError)."""
    field1 = FakeFieldA()
    field2 = FakeFieldB()
    expr = CustomExpression([FakeExpr(None), FakeExpr(field1), FakeExpr(field2)])
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.15μs -> 3.31μs (4.89% slower)

# --- EDGE TEST CASES ---

def test_no_source_expressions():
    """Test with no source expressions (should return None)."""
    expr = CustomExpression([])
    codeflash_output = expr._resolve_output_field() # 1.48μs -> 1.23μs (20.4% faster)

def test_one_none_one_valid():
    """Test with one None and one valid field (should return the valid field)."""
    field = FakeFieldA()
    expr = CustomExpression([FakeExpr(None), FakeExpr(field)])
    codeflash_output = expr._resolve_output_field() # 1.73μs -> 1.84μs (6.19% slower)

def test_all_none_fields():
    """Test with all fields None (should return None)."""
    expr = CustomExpression([FakeExpr(None), FakeExpr(None), FakeExpr(None)])
    codeflash_output = expr._resolve_output_field() # 1.77μs -> 1.53μs (15.8% faster)

def test_only_one_valid_among_many_nones():
    """Test with many None fields and one valid (should return the valid)."""
    field = FakeFieldA()
    expr = CustomExpression([FakeExpr(None)] * 5 + [FakeExpr(field)] + [FakeExpr(None)] * 5)
    codeflash_output = expr._resolve_output_field() # 2.19μs -> 2.23μs (1.71% slower)

def test_field_subclass_type():
    """Test with fields that are subclasses of each other (should not raise if same type)."""
    class SubFieldA(FakeFieldA): pass
    field1 = FakeFieldA()
    field2 = SubFieldA()
    expr = CustomExpression([FakeExpr(field1), FakeExpr(field2)])
    # Should raise because output_field and source are not same class
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 3.36μs -> 3.47μs (3.06% slower)

def test_field_exact_type_match():
    """Test with exactly same instance (should pass)."""
    field = FakeFieldA()
    expr = CustomExpression([FakeExpr(field), FakeExpr(field)])
    codeflash_output = expr._resolve_output_field() # 2.16μs -> 1.96μs (10.1% faster)

# --- LARGE SCALE TEST CASES ---

def test_large_same_type_fields():
    """Test with a large number of same type fields (should return the first)."""
    fields = [FakeFieldA() for _ in range(500)]
    exprs = [FakeExpr(f) for f in fields]
    expr = CustomExpression(exprs)
    codeflash_output = expr._resolve_output_field() # 35.2μs -> 28.0μs (25.8% faster)

def test_large_mixed_type_fields_raises():
    """Test with a large number of fields, one of a different type (should raise FieldError)."""
    fields = [FakeFieldA() for _ in range(499)] + [FakeFieldB()]
    exprs = [FakeExpr(f) for f in fields]
    expr = CustomExpression(exprs)
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 36.3μs -> 29.8μs (21.7% faster)

def test_large_number_of_none_fields():
    """Test with a large number of None fields (should return None)."""
    exprs = [FakeExpr(None) for _ in range(1000)]
    expr = CustomExpression(exprs)
    codeflash_output = expr._resolve_output_field() # 26.7μs -> 26.6μs (0.493% faster)

def test_large_number_some_none_some_valid():
    """Test with a large number of None fields and a few valid (should return first valid)."""
    valid_field = FakeFieldA()
    exprs = [FakeExpr(None) for _ in range(998)] + [FakeExpr(valid_field), FakeExpr(valid_field)]
    expr = CustomExpression(exprs)
    codeflash_output = expr._resolve_output_field() # 28.3μs -> 28.4μs (0.437% slower)

def test_large_number_some_none_some_mixed_types():
    """Test with a large number of None fields and two valid of different types (should raise)."""
    field_a = FakeFieldA()
    field_b = FakeFieldB()
    exprs = [FakeExpr(None) for _ in range(998)] + [FakeExpr(field_a), FakeExpr(field_b)]
    expr = CustomExpression(exprs)
    with pytest.raises(FieldError):
        expr._resolve_output_field() # 27.4μs -> 29.3μs (6.35% 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-BaseExpression._resolve_output_field-mh2xphzs and push.