⚡️ Speed up method _Stack.back by 42%

[codeflash-ai]

📄 42% (0.42x) speedup for _Stack.back in lib/matplotlib/cbook.py

⏱️ Runtime : 1.27 milliseconds → 892 microseconds (best of 105 runs)

📝 Explanation and details

The optimization achieves a 42% speedup by eliminating the expensive self() method call and using local variables to reduce attribute lookups.

Key optimizations applied:

1. Eliminated method call overhead: The original code calls self() which involves method resolution and a separate function call. The optimized version directly accesses self._elements[self._pos] inline, avoiding this overhead entirely.

2. Reduced attribute lookups: Uses local variables (pos = self._pos, elements = self._elements) to minimize repeated attribute access, which is faster in Python's bytecode execution.

3. Simplified conditional logic: Replaces max(self._pos - 1, 0) with explicit if/else branches that are more efficient for the CPU's branch predictor.

Performance impact by test case:

• Empty stack operations: 87-106% faster due to eliminating the method call overhead
• Single element stacks: 83-88% faster from the same optimization
• Multi-element navigation: 40-50% faster, showing consistent gains across different stack sizes
• Large stack operations: 41-43% faster, demonstrating the optimization scales well

The line profiler shows the original return self() took 59.9% of execution time, while the optimized inline return takes only 18.1%. The optimization is particularly effective because stack navigation operations are typically called frequently in UI frameworks and data structure traversals, making this a worthwhile performance improvement for any code using matplotlib's internal stack operations.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 3797 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

from matplotlib.cbook import _Stack

# unit tests

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


def test_back_on_empty_stack_returns_none():
    # Test that back() on an empty stack returns None and does not error
    s = _Stack()
    codeflash_output = s.back()  # 1.41μs -> 752ns (87.0% faster)


def test_back_on_single_element_stack():
    # Test back() on a stack with one element
    s = _Stack()
    s._elements.append("a")
    s._pos = 0
    codeflash_output = s.back()  # 1.47μs -> 799ns (83.5% faster)


def test_back_moves_cursor_left():
    # Test that back() moves the cursor left and returns the correct element
    s = _Stack()
    s._elements = ["a", "b", "c"]
    s._pos = 2
    codeflash_output = s.back()  # 1.38μs -> 955ns (44.7% faster)


def test_multiple_back_calls():
    # Test calling back() multiple times does not move past 0
    s = _Stack()
    s._elements = ["x", "y", "z"]
    s._pos = 2
    codeflash_output = s.back()  # 1.38μs -> 917ns (50.6% faster)
    codeflash_output = s.back()  # 505ns -> 360ns (40.3% faster)
    # Further calls stay at position 0
    codeflash_output = s.back()  # 554ns -> 371ns (49.3% faster)
    codeflash_output = s.back()  # 317ns -> 219ns (44.7% faster)


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


def test_back_when_pos_is_already_zero():
    # If cursor is at 0, back() should not change position
    s = _Stack()
    s._elements = ["foo", "bar"]
    s._pos = 0
    codeflash_output = s.back()  # 1.38μs -> 731ns (88.2% faster)


def test_back_when_pos_is_negative():
    # If cursor is negative (should happen only for empty stack), back() should set to 0
    s = _Stack()
    s._elements = []
    s._pos = -1
    codeflash_output = s.back()  # 1.30μs -> 635ns (106% faster)


def test_back_on_stack_with_none_elements():
    # Stack can contain None as element
    s = _Stack()
    s._elements = [None, "a", None]
    s._pos = 2
    codeflash_output = s.back()  # 1.37μs -> 919ns (49.0% faster)
    codeflash_output = s.back()  # 535ns -> 367ns (45.8% faster)


def test_back_on_stack_with_various_types():
    # Stack with mixed types
    s = _Stack()
    obj = object()
    s._elements = [42, "hello", obj, 3.14]
    s._pos = 3
    codeflash_output = s.back()  # 1.31μs -> 927ns (41.6% faster)
    codeflash_output = s.back()  # 433ns -> 329ns (31.6% faster)
    codeflash_output = s.back()  # 462ns -> 282ns (63.8% faster)
    codeflash_output = s.back()  # 541ns -> 361ns (49.9% faster)


def test_back_does_not_modify_elements():
    # Ensure back() does not modify the stack's elements
    s = _Stack()
    s._elements = ["a", "b", "c"]
    s._pos = 2
    before = list(s._elements)
    s.back()  # 1.39μs -> 914ns (52.5% faster)


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


def test_back_on_large_stack():
    # Test back() on a large stack (1000 elements)
    s = _Stack()
    s._elements = list(range(1000))
    s._pos = 999
    # Move back 10 times and check position and value
    for i in range(10):
        expected_pos = 999 - (i + 1)
        expected_val = expected_pos
        codeflash_output = s.back()  # 4.74μs -> 3.35μs (41.6% faster)
    # Move all the way to the start
    for i in range(s._pos, 0, -1):
        codeflash_output = s.back()
        val = codeflash_output  # 325μs -> 228μs (42.1% faster)
    # Further back() calls stay at 0
    for _ in range(5):
        codeflash_output = s.back()  # 1.94μs -> 1.19μs (62.5% faster)


def test_back_performance_on_large_stack(monkeypatch):
    # This test ensures back() is efficient and does not do unnecessary work
    s = _Stack()
    s._elements = list(range(1000))
    s._pos = 999

    # Patch __call__ to count how many times it's called
    call_count = {"count": 0}
    orig_call = s.__call__

    def counting_call():
        call_count["count"] += 1
        return orig_call()

    s.__call__ = counting_call

    # Call back 100 times
    for _ in range(100):
        s.back()  # 34.5μs -> 24.2μs (42.9% faster)


def test_back_on_stack_with_duplicates():
    # Stack with duplicate elements
    s = _Stack()
    s._elements = ["a", "b", "a", "b", "a"]
    s._pos = 4
    codeflash_output = s.back()  # 1.36μs -> 933ns (45.7% faster)
    codeflash_output = s.back()  # 452ns -> 327ns (38.2% faster)
    codeflash_output = s.back()  # 373ns -> 272ns (37.1% faster)
    codeflash_output = s.back()  # 416ns -> 276ns (50.7% faster)
    codeflash_output = s.back()  # 513ns -> 373ns (37.5% faster)


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

from matplotlib.cbook import _Stack

# unit tests

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


def test_back_on_empty_stack_returns_none():
    """Test that calling back on an empty stack returns None and does not raise."""
    s = _Stack()
    codeflash_output = s.back()  # 1.35μs -> 694ns (94.8% faster)


def test_back_on_single_element_stack():
    """Test back on a stack with one element."""
    s = _Stack()
    s._elements.append("a")
    s._pos = 0
    codeflash_output = s.back()  # 1.42μs -> 758ns (86.7% faster)


def test_back_on_two_element_stack():
    """Test back on a stack with two elements."""
    s = _Stack()
    s._elements.extend(["a", "b"])
    s._pos = 1  # Pointing at 'b'
    codeflash_output = s.back()  # 1.32μs -> 914ns (44.7% faster)
    # Back again should stay at 'a'
    codeflash_output = s.back()  # 651ns -> 415ns (56.9% faster)


def test_back_on_three_element_stack():
    """Test back moves position and returns correct element."""
    s = _Stack()
    s._elements.extend(["a", "b", "c"])
    s._pos = 2  # Pointing at 'c'
    codeflash_output = s.back()  # 1.36μs -> 900ns (51.3% faster)
    codeflash_output = s.back()  # 531ns -> 365ns (45.5% faster)
    # Further back should stay at 'a'
    codeflash_output = s.back()  # 517ns -> 386ns (33.9% faster)


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


def test_back_when_pos_negative():
    """If pos is negative, back should set pos to 0 and return first element."""
    s = _Stack()
    s._elements.extend(["a", "b"])
    s._pos = -1
    codeflash_output = s.back()  # 1.41μs -> 792ns (77.7% faster)


def test_back_when_pos_zero():
    """If pos is already 0, back should not change pos."""
    s = _Stack()
    s._elements.extend(["a", "b"])
    s._pos = 0
    codeflash_output = s.back()  # 1.42μs -> 754ns (87.8% faster)


def test_back_multiple_times_on_empty_stack():
    """Repeated back calls on empty stack always return None and pos stays 0."""
    s = _Stack()
    for _ in range(10):
        codeflash_output = s.back()  # 4.14μs -> 2.59μs (59.7% faster)


def test_back_after_adding_elements_and_resetting_pos():
    """Test back after modifying _elements and resetting _pos."""
    s = _Stack()
    s._elements.extend(["x", "y", "z"])
    s._pos = 2
    codeflash_output = s.back()  # 1.38μs -> 873ns (57.5% faster)
    s._elements.append("w")  # Add new element
    s._pos = 3
    codeflash_output = s.back()  # 415ns -> 284ns (46.1% faster)


def test_back_with_non_string_elements():
    """Test back works with non-string elements (e.g., numbers, objects)."""
    s = _Stack()
    s._elements.extend([1, 2, 3])
    s._pos = 2
    codeflash_output = s.back()  # 1.35μs -> 892ns (51.1% faster)
    codeflash_output = s.back()  # 500ns -> 359ns (39.3% faster)
    codeflash_output = s.back()  # 524ns -> 391ns (34.0% faster)


def test_back_large_stack():
    """Test back on a large stack with 1000 elements."""
    s = _Stack()
    s._elements = list(range(1000))
    s._pos = 999
    # Move back 10 times, should get 989
    for i in range(10):
        expected = 999 - i - 1
        codeflash_output = s.back()  # 4.72μs -> 3.30μs (43.1% faster)
    # Move back to start
    for _ in range(1000):
        s.back()  # 327μs -> 231μs (41.1% faster)
    codeflash_output = s.back()  # 322ns -> 210ns (53.3% faster)


def test_back_large_stack_multiple_calls():
    """Test that repeated back calls on large stack always return first element after reaching start."""
    s = _Stack()
    s._elements = list(range(1000))
    s._pos = 500
    for _ in range(600):
        s.back()  # 196μs -> 137μs (42.9% faster)
    codeflash_output = s.back()  # 324ns -> 211ns (53.6% faster)


def test_back_performance_on_large_stack():
    """Test that back is O(1) by calling it repeatedly on a large stack."""
    s = _Stack()
    s._elements = list(range(1000))
    s._pos = 999
    for _ in range(1000):
        s.back()  # 330μs -> 233μs (41.9% faster)
    codeflash_output = s.back()  # 340ns -> 268ns (26.9% faster)


# ----------------------------
# Additional Edge Cases
# ----------------------------


def test_back_with_mutable_elements():
    """Test back with mutable elements in the stack."""
    s = _Stack()
    l1, l2, l3 = [1], [2], [3]
    s._elements.extend([l1, l2, l3])
    s._pos = 2
    codeflash_output = s.back()  # 1.44μs -> 922ns (55.9% faster)
    l2.append(99)
    codeflash_output = s.back()  # 509ns -> 365ns (39.5% faster)


def test_back_with_none_elements():
    """Test back with None as an element in the stack."""
    s = _Stack()
    s._elements.extend([None, "a", None])
    s._pos = 2
    codeflash_output = s.back()  # 1.38μs -> 900ns (52.8% faster)
    codeflash_output = s.back()  # 503ns -> 336ns (49.7% faster)


def test_back_does_not_modify_elements():
    """Test that back does not modify the elements themselves."""
    s = _Stack()
    s._elements.extend(["a", "b", "c"])
    s._pos = 2
    before = list(s._elements)
    s.back()  # 1.36μs -> 916ns (48.1% faster)
    s.back()  # 534ns -> 371ns (43.9% faster)
    after = list(s._elements)


def test_back_on_stack_after_clearing_elements():
    """Test back after clearing the stack elements."""
    s = _Stack()
    s._elements.extend(["a", "b", "c"])
    s._pos = 2
    s._elements.clear()
    codeflash_output = s.back()  # 1.23μs -> 806ns (52.1% faster)


# 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-_Stack.back-misd5yyq and push.