⚡️ Speed up method BreaklessListsPreprocessor.run by 40%

[codeflash-ai]

📄 40% (0.40x) speedup for BreaklessListsPreprocessor.run in marimo/_output/md_extensions/breakless_lists.py

⏱️ Runtime : 2.80 milliseconds → 2.00 millisecond (best of 203 runs)

📝 Explanation and details

The optimized code achieves a 40% speedup through three key micro-optimizations that reduce Python's overhead in tight loops:

What optimizations were applied:

1. Localized method references: append = result_lines.append and match_list_start = self.LIST_START_PATTERN.match avoid repeated attribute lookups during iteration
2. Loop structure improvement: Replaced while i < len(lines) with for i in range(length) to eliminate redundant len() calls and manual index incrementing
3. Simplified conditional logic: Removed a redundant current_line.strip() check that was duplicated in the blank line insertion logic

Why these optimizations provide speedup:

• Attribute lookup elimination: Python's attribute resolution (self.LIST_START_PATTERN.match, result_lines.append) has overhead. Localizing these to variables makes them direct variable lookups, which are significantly faster in loops
• Loop efficiency: for loops with range() are more efficient than while loops with manual indexing because Python can optimize the iteration internally
• Reduced redundant operations: Eliminating the duplicate len(lines) calls and redundant .strip() checks reduces CPU cycles per iteration

Impact on workloads:
The optimizations are most beneficial for large-scale document processing, as shown by the test results where large documents (500-1000 lines) see 40-50% speedups, while small documents (2-5 lines) show minimal or slight slowdowns due to the overhead of variable assignment. This suggests the function processes substantial markdown documents in production, making these micro-optimizations valuable for real-world usage where document size matters more than single-line processing overhead.

✅ Correctness verification report:

Test	Status
⚙️ Existing Unit Tests	🔘 None Found
🌀 Generated Regression Tests	✅ 126 Passed
⏪ Replay Tests	🔘 None Found
🔎 Concolic Coverage Tests	🔘 None Found
📊 Tests Coverage	100.0%

🌀 Generated Regression Tests and Runtime

import re

imports

import pytest # used for our unit tests
from marimo._output.md_extensions.breakless_lists import
BreaklessListsPreprocessor
from markdown import Markdown, preprocessors

unit tests

class DummyMarkdown(Markdown):
"""Dummy Markdown class for instantiating the preprocessor."""
pass

@pytest.fixture
def preprocessor():
# Provide a fresh preprocessor for each test
return BreaklessListsPreprocessor(DummyMarkdown())

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

def test_empty_input(preprocessor):
# Should return empty list when input is empty
codeflash_output = preprocessor.run([]) # 414ns -> 462ns (10.4% slower)

def test_single_line_no_list(preprocessor):
# Single line that is not a list
codeflash_output = preprocessor.run(["Hello world"]) # 1.07μs -> 2.19μs (51.3% slower)

def test_single_list_line(preprocessor):
# Single list item; should not add blank lines
codeflash_output = preprocessor.run(["- item"]) # 1.03μs -> 1.93μs (46.5% slower)

def test_paragraph_followed_by_list(preprocessor):
# Paragraph directly followed by a list; should insert a blank line
lines = [
"This is a paragraph.",
"- item 1",
"- item 2"
]
expected = [
"This is a paragraph.",
"",
"- item 1",
"- item 2"
]
codeflash_output = preprocessor.run(lines) # 4.42μs -> 4.54μs (2.53% slower)

def test_paragraph_list_with_existing_blank_line(preprocessor):
# Paragraph, blank line, then list; should not add an extra blank line
lines = [
"This is a paragraph.",
"",
"- item 1"
]
expected = [
"This is a paragraph.",
"",
"- item 1"
]
codeflash_output = preprocessor.run(lines) # 2.50μs -> 2.94μs (15.0% slower)

def test_multiple_lists_interrupted_by_paragraphs(preprocessor):
# Paragraph, list, paragraph, list, all needing blank lines
lines = [
"Para1.",
"- item 1",
"Para2.",
"1. item 2"
]
expected = [
"Para1.",
"",
"- item 1",
"Para2.",
"",
"1. item 2"
]
codeflash_output = preprocessor.run(lines) # 4.83μs -> 5.06μs (4.70% slower)

def test_multiple_paragraphs_and_lists(preprocessor):
# Paragraph, list, paragraph, list, with blank lines already present
lines = [
"Para1.",
"",
"- item 1",
"",
"Para2.",
"",
"1. item 2"
]
expected = [
"Para1.",
"",
"- item 1",
"",
"Para2.",
"",
"1. item 2"
]
codeflash_output = preprocessor.run(lines) # 3.28μs -> 3.50μs (6.34% slower)

def test_list_after_code_block(preprocessor):
# List after code block (code block lines start with 4 spaces)
lines = [
" code block",
"- item"
]
expected = [
" code block",
"",
"- item"
]
codeflash_output = preprocessor.run(lines) # 3.37μs -> 3.77μs (10.8% slower)

def test_list_with_indentation(preprocessor):
# List with indentation should be detected
lines = [
"Paragraph.",
" - indented item"
]
expected = [
"Paragraph.",
"",
" - indented item"
]
codeflash_output = preprocessor.run(lines) # 3.12μs -> 3.40μs (8.10% slower)

def test_ordered_list_followed_by_unordered_list(preprocessor):
# Ordered list followed by unordered list, both after paragraphs
lines = [
"Paragraph.",
"1. item one",
"Another paragraph.",
"* item two"
]
expected = [
"Paragraph.",
"",
"1. item one",
"Another paragraph.",
"",
"* item two"
]
codeflash_output = preprocessor.run(lines) # 4.59μs -> 4.87μs (5.63% slower)

def test_unordered_list_symbols(preprocessor):
# Test all unordered list symbols (, -, +)
lines = [
"Para.",
" star item",
"Para.",
"- dash item",
"Para.",
"+ plus item"
]
expected = [
"Para.",
"",
"* star item",
"Para.",
"",
"- dash item",
"Para.",
"",
"+ plus item"
]
codeflash_output = preprocessor.run(lines) # 4.96μs -> 4.96μs (0.101% slower)

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

def test_list_at_start_of_document(preprocessor):
# List at start should not have blank line inserted
lines = [
"- item 1",
"- item 2"
]
expected = [
"- item 1",
"- item 2"
]
codeflash_output = preprocessor.run(lines) # 2.92μs -> 3.38μs (13.5% slower)

def test_list_after_blank_line(preprocessor):
# List after blank line should not get extra blank line
lines = [
"",
"- item 1"
]
expected = [
"",
"- item 1"
]
codeflash_output = preprocessor.run(lines) # 1.50μs -> 2.17μs (30.8% slower)

def test_blank_line_between_paragraph_and_list(preprocessor):
# Paragraph, blank line, list; should not add extra blank line
lines = [
"Paragraph.",
"",
"- item"
]
expected = [
"Paragraph.",
"",
"- item"
]
codeflash_output = preprocessor.run(lines) # 2.46μs -> 3.01μs (18.1% slower)

def test_multiple_consecutive_lists(preprocessor):
# Two lists directly after each other; only the first after a paragraph gets a blank line
lines = [
"Paragraph.",
"- item 1",
"- item 2",
"1. item 3"
]
expected = [
"Paragraph.",
"",
"- item 1",
"- item 2",
"1. item 3"
]
codeflash_output = preprocessor.run(lines) # 4.80μs -> 5.03μs (4.51% slower)

def test_multiple_blank_lines_between_paragraph_and_list(preprocessor):
# Paragraph, multiple blank lines, list; should not add extra blank line
lines = [
"Paragraph.",
"",
"",
"- item"
]
expected = [
"Paragraph.",
"",
"",
"- item"
]
codeflash_output = preprocessor.run(lines) # 2.56μs -> 2.99μs (14.4% slower)

def test_list_with_multiple_spaces(preprocessor):
# List item with multiple spaces between marker and content
lines = [
"Para.",
"- spaced item"
]
expected = [
"Para.",
"",
"- spaced item"
]
codeflash_output = preprocessor.run(lines) # 3.15μs -> 3.79μs (16.9% slower)

def test_list_marker_in_paragraph(preprocessor):
# List marker in the middle of a paragraph should not be treated as a list
lines = [
"This is not a list: - just text",
"1. This is a list"
]
expected = [
"This is not a list: - just text",
"",
"1. This is a list"
]
codeflash_output = preprocessor.run(lines) # 3.32μs -> 3.72μs (10.8% slower)

def test_paragraph_with_whitespace(preprocessor):
# Paragraph with trailing whitespace before list
lines = [
"Paragraph. ",
"- item"
]
expected = [
"Paragraph. ",
"",
"- item"
]
codeflash_output = preprocessor.run(lines) # 3.40μs -> 3.77μs (9.99% slower)

def test_list_with_large_number_marker(preprocessor):
# Ordered list with large number marker
lines = [
"Para.",
"1234567890. big number"
]
expected = [
"Para.",
"",
"1234567890. big number"
]
codeflash_output = preprocessor.run(lines) # 3.37μs -> 3.64μs (7.34% slower)

def test_list_with_leading_tabs(preprocessor):
# List with leading tabs
lines = [
"Para.",
"\t- tabbed item"
]
expected = [
"Para.",
"",
"\t- tabbed item"
]
codeflash_output = preprocessor.run(lines) # 3.12μs -> 3.66μs (14.7% slower)

def test_list_with_nonstandard_whitespace(preprocessor):
# List with mixed whitespace (tabs and spaces)
lines = [
"Para.",
" \t - mixed whitespace"
]
expected = [
"Para.",
"",
" \t - mixed whitespace"
]
codeflash_output = preprocessor.run(lines) # 3.14μs -> 3.65μs (14.0% slower)

def test_list_marker_with_no_space(preprocessor):
# List marker with no space after marker is NOT a list
lines = [
"Para.",
"-item not a list"
]
expected = [
"Para.",
"-item not a list"
]
codeflash_output = preprocessor.run(lines) # 2.82μs -> 3.46μs (18.7% slower)

def test_list_marker_with_multiple_spaces_before_marker(preprocessor):
# List marker with multiple spaces before marker
lines = [
"Para.",
" - indented item"
]
expected = [
"Para.",
"",
" - indented item"
]
codeflash_output = preprocessor.run(lines) # 3.21μs -> 3.74μs (14.4% slower)

def test_list_marker_with_unicode_whitespace(preprocessor):
# List marker with unicode whitespace (should not match)
lines = [
"Para.",
"\u2003- unicode space item"
]
expected = [
"Para.",
"\u2003- unicode space item"
]
codeflash_output = preprocessor.run(lines) # 3.70μs -> 4.28μs (13.5% slower)

def test_list_marker_with_leading_blank_line(preprocessor):
# List preceded by a blank line, after paragraph
lines = [
"Para.",
"",
"1. item"
]
expected = [
"Para.",
"",
"1. item"
]
codeflash_output = preprocessor.run(lines) # 2.43μs -> 2.91μs (16.4% slower)

def test_list_marker_with_trailing_blank_line(preprocessor):
# List followed by blank line
lines = [
"Para.",
"- item",
""
]
expected = [
"Para.",
"",
"- item",
""
]
codeflash_output = preprocessor.run(lines) # 3.70μs -> 4.25μs (12.8% slower)

def test_list_marker_with_only_spaces(preprocessor):
# List marker with only spaces (should not match as a list)
lines = [
"Para.",
" "
]
expected = [
"Para.",
" "
]
codeflash_output = preprocessor.run(lines) # 3.21μs -> 3.61μs (11.1% slower)

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

def test_large_document_with_many_paragraphs_and_lists(preprocessor):
# Large document alternating paragraphs and lists
lines = []
expected = []
for i in range(500):
lines.append(f"Paragraph {i}")
lines.append(f"- item {i}")
expected.append(f"Paragraph {i}")
expected.append("")
expected.append(f"- item {i}")
codeflash_output = preprocessor.run(lines) # 308μs -> 206μs (49.5% faster)

def test_large_document_with_no_lists(preprocessor):
# Large document with no lists; should remain unchanged
lines = [f"Paragraph {i}" for i in range(1000)]
codeflash_output = preprocessor.run(lines) # 235μs -> 165μs (42.6% faster)

def test_large_document_with_all_lists(preprocessor):
# Large document with only lists; should remain unchanged
lines = [f"- item {i}" for i in range(1000)]
codeflash_output = preprocessor.run(lines) # 306μs -> 209μs (46.1% faster)

def test_large_document_with_lists_after_paragraphs_and_blank_lines(preprocessor):
# Large document with paragraphs, blank lines, and lists
lines = []
expected = []
for i in range(333):
lines.append(f"Paragraph {i}")
lines.append("")
lines.append(f"- item {i}")
expected.append(f"Paragraph {i}")
expected.append("")
expected.append(f"- item {i}")
codeflash_output = preprocessor.run(lines) # 195μs -> 131μs (48.6% faster)

def test_large_document_with_mixed_content(preprocessor):
# Large document with paragraphs, code blocks, lists, and blank lines
lines = []
expected = []
for i in range(250):
lines.append(f"Paragraph {i}")
lines.append(" code block")
lines.append(f"- item {i}")
expected.append(f"Paragraph {i}")
expected.append("")
expected.append(" code block")
expected.append("")
expected.append(f"- item {i}")
codeflash_output = preprocessor.run(lines) # 252μs -> 177μs (42.5% faster)

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

#------------------------------------------------
import re

imports

import pytest
from marimo._output.md_extensions.breakless_lists import
BreaklessListsPreprocessor
from markdown import Markdown, preprocessors

unit tests

class DummyMarkdown:
"""A dummy Markdown object for testing purposes."""
pass

@pytest.fixture
def preprocessor():
# Provides a fresh preprocessor for each test
return BreaklessListsPreprocessor(DummyMarkdown())

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

def test_empty_input_returns_empty(preprocessor):
# Should return an empty list when given an empty list
codeflash_output = preprocessor.run([]) # 399ns -> 393ns (1.53% faster)

def test_single_line_no_list(preprocessor):
# Single non-list line should be unchanged
codeflash_output = preprocessor.run(["Hello world"]) # 1.03μs -> 2.13μs (51.9% slower)

def test_single_list_item(preprocessor):
# Single list item line should be unchanged
codeflash_output = preprocessor.run(["- item"]) # 1.02μs -> 1.87μs (45.5% slower)

def test_paragraph_followed_by_list(preprocessor):
# Should insert a blank line between paragraph and list
lines = [
"This is a paragraph.",
"- list item 1",
"- list item 2"
]
expected = [
"This is a paragraph.",
"",
"- list item 1",
"- list item 2"
]
codeflash_output = preprocessor.run(lines) # 3.98μs -> 4.26μs (6.39% slower)

def test_paragraph_followed_by_ordered_list(preprocessor):
# Should insert a blank line before ordered list
lines = [
"Paragraph.",
"1. first",
"2. second"
]
expected = [
"Paragraph.",
"",
"1. first",
"2. second"
]
codeflash_output = preprocessor.run(lines) # 3.58μs -> 4.00μs (10.6% slower)

def test_multiple_paragraphs_and_lists(preprocessor):
# Should insert blank lines before each list that follows a paragraph
lines = [
"First paragraph.",
"- list1",
"",
"Second paragraph.",
"1. list2",
"",
"Third paragraph.",
"+ list3"
]
expected = [
"First paragraph.",
"",
"- list1",
"",
"Second paragraph.",
"",
"1. list2",
"",
"Third paragraph.",
"",
"+ list3"
]
codeflash_output = preprocessor.run(lines) # 5.00μs -> 5.17μs (3.21% slower)

def test_blank_line_before_list_is_preserved(preprocessor):
# Should not add extra blank line if one already exists
lines = [
"Paragraph.",
"",
"* item"
]
expected = [
"Paragraph.",
"",
"* item"
]
codeflash_output = preprocessor.run(lines) # 2.17μs -> 2.76μs (21.3% slower)

def test_list_followed_by_paragraph(preprocessor):
# Should not insert blank line after list if not followed by another list
lines = [
"- item",
"Paragraph."
]
expected = [
"- item",
"Paragraph."
]
codeflash_output = preprocessor.run(lines) # 2.42μs -> 3.00μs (19.2% slower)

def test_list_followed_by_list(preprocessor):
# Should not insert blank line between consecutive list items
lines = [
"- item1",
"- item2"
]
expected = [
"- item1",
"- item2"
]
codeflash_output = preprocessor.run(lines) # 2.65μs -> 3.39μs (22.0% slower)

def test_multiple_lists_with_intervening_paragraphs(preprocessor):
# Should insert blank lines only before lists that follow paragraphs
lines = [
"Para 1.",
"+ list1",
"",
"Para 2.",
"* list2"
]
expected = [
"Para 1.",
"",
"+ list1",
"",
"Para 2.",
"",
"* list2"
]
codeflash_output = preprocessor.run(lines) # 3.85μs -> 4.16μs (7.29% slower)

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

def test_leading_and_trailing_whitespace(preprocessor):
# Should handle lines with leading/trailing whitespace correctly
lines = [
" Paragraph with spaces. ",
" - list with indent",
"Next para",
" 1. ordered list"
]
expected = [
" Paragraph with spaces. ",
"",
" - list with indent",
"Next para",
"",
" 1. ordered list"
]
codeflash_output = preprocessor.run(lines) # 4.62μs -> 4.84μs (4.52% slower)

def test_list_marker_with_many_spaces(preprocessor):
# Should match list markers with multiple spaces after marker
lines = [
"Para.",
"- item with many spaces"
]
expected = [
"Para.",
"",
"- item with many spaces"
]
codeflash_output = preprocessor.run(lines) # 2.67μs -> 3.21μs (17.1% slower)

def test_list_marker_with_leading_spaces(preprocessor):
# Should match list markers with leading spaces before marker
lines = [
"Para.",
" * indented list"
]
expected = [
"Para.",
"",
" * indented list"
]
codeflash_output = preprocessor.run(lines) # 2.67μs -> 3.30μs (19.0% slower)

def test_list_marker_with_nonstandard_bullet(preprocessor):
# Should not match nonstandard list markers (e.g., not *, -, +, or number.)
lines = [
"Para.",
"# not a list"
]
expected = [
"Para.",
"# not a list"
]
codeflash_output = preprocessor.run(lines) # 2.48μs -> 2.97μs (16.7% slower)

def test_list_marker_with_dot_but_not_number(preprocessor):
# Should not match a dot not preceded by a number
lines = [
"Para.",
". not a list"
]
expected = [
"Para.",
". not a list"
]
codeflash_output = preprocessor.run(lines) # 2.42μs -> 3.02μs (19.9% slower)

def test_list_marker_with_number_and_no_dot(preprocessor):
# Should not match a number with no dot
lines = [
"Para.",
"1 not a list"
]
expected = [
"Para.",
"1 not a list"
]
codeflash_output = preprocessor.run(lines) # 2.42μs -> 3.05μs (20.8% slower)

def test_list_marker_with_number_and_dot_and_no_space(preprocessor):
# Should not match a number and dot with no space after
lines = [
"Para.",
"1.not a list"
]
expected = [
"Para.",
"1.not a list"
]
codeflash_output = preprocessor.run(lines) # 2.42μs -> 3.35μs (27.6% slower)

def test_only_blank_lines(preprocessor):
# Should return only blank lines unchanged
lines = ["", "", ""]
expected = ["", "", ""]
codeflash_output = preprocessor.run(lines) # 1.70μs -> 2.31μs (26.2% slower)

def test_list_at_start_of_document(preprocessor):
# Should not add blank line before list at start
lines = [
"* item1",
"* item2"
]
expected = [
"* item1",
"* item2"
]
codeflash_output = preprocessor.run(lines) # 2.79μs -> 3.45μs (19.3% slower)

def test_list_marker_in_middle_of_line(preprocessor):
# Should not match list marker in the middle of a line
lines = [
"This is a * not a list",
"Another line"
]
expected = [
"This is a * not a list",
"Another line"
]
codeflash_output = preprocessor.run(lines) # 2.38μs -> 2.93μs (18.6% slower)

def test_line_with_only_spaces(preprocessor):
# Should treat lines with only spaces as blank
lines = [
"Paragraph.",
" ",
"- list"
]
expected = [
"Paragraph.",
" ",
"- list"
]
codeflash_output = preprocessor.run(lines) # 3.13μs -> 3.75μs (16.4% slower)

def test_multiple_lists_no_paragraphs(preprocessor):
# Should not add blank lines between lists when no paragraph present
lines = [
"- item1",
"",
"+ item2",
"",
"1. item3"
]
expected = [
"- item1",
"",
"+ item2",
"",
"1. item3"
]
codeflash_output = preprocessor.run(lines) # 2.74μs -> 3.18μs (13.8% slower)

def test_blank_line_between_paragraph_and_list(preprocessor):
# Should not add a blank line if one already exists between paragraph and list
lines = [
"Paragraph.",
"",
"- item"
]
expected = [
"Paragraph.",
"",
"- item"
]
codeflash_output = preprocessor.run(lines) # 2.11μs -> 2.70μs (21.6% slower)

def test_paragraph_followed_by_non_list(preprocessor):
# Should not add blank line before non-list line
lines = [
"Paragraph.",
"Not a list"
]
expected = [
"Paragraph.",
"Not a list"
]
codeflash_output = preprocessor.run(lines) # 2.42μs -> 3.09μs (21.6% slower)

def test_list_marker_with_tab(preprocessor):
# Should match list marker with tab after marker
lines = [
"Paragraph.",
"-\titem"
]
expected = [
"Paragraph.",
"",
"-\titem"
]
codeflash_output = preprocessor.run(lines) # 2.85μs -> 3.44μs (17.3% slower)

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

def test_large_document_with_many_paragraphs_and_lists(preprocessor):
# Should efficiently process a large document with many paragraphs and lists
lines = []
expected = []
for i in range(500):
lines.append(f"Paragraph {i}.")
lines.append(f"- list item {i}")
# The expected output should have a blank line inserted before each list item
for i in range(500):
expected.append(f"Paragraph {i}.")
expected.append("")
expected.append(f"- list item {i}")
codeflash_output = preprocessor.run(lines) # 287μs -> 198μs (44.7% faster)

def test_large_document_with_no_lists(preprocessor):
# Should efficiently process a large document with no lists
lines = [f"Paragraph {i}." for i in range(1000)]
expected = lines.copy()
codeflash_output = preprocessor.run(lines) # 233μs -> 165μs (41.3% faster)

def test_large_document_with_no_paragraphs(preprocessor):
# Should efficiently process a large document with only lists
lines = [f"- item {i}" for i in range(1000)]
expected = lines.copy()
codeflash_output = preprocessor.run(lines) # 311μs -> 218μs (42.4% faster)

def test_large_document_with_alternating_blank_and_list(preprocessor):
# Should not add extra blank lines when alternating blank and list
lines = []
expected = []
for i in range(500):
lines.append("")
lines.append(f"+ item {i}")
expected.append("")
expected.append(f"+ item {i}")
codeflash_output = preprocessor.run(lines) # 169μs -> 113μs (49.2% faster)

def test_large_document_with_complex_patterns(preprocessor):
# Mix paragraphs, blank lines, and lists in a complex pattern
lines = []
expected = []
for i in range(250):
lines.append(f"Paragraph {i}")
lines.append(f" - list {i}")
lines.append("")
lines.append(f"Another paragraph {i}")
lines.append(f"1. first ordered {i}")
expected.append(f"Paragraph {i}")
expected.append("")
expected.append(f" - list {i}")
expected.append("")
expected.append(f"Another paragraph {i}")
expected.append("")
expected.append(f"1. first ordered {i}")
codeflash_output = preprocessor.run(lines) # 350μs -> 232μs (50.5% 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-BreaklessListsPreprocessor.run-mhv5yi72 and push.