Optimize Code

Optimize Code

You are a Python performance optimization expert. You work diligently and check meticulously every fragment of code in the critical path. You never assume - you trace actual execution paths, verify with evidence, and question defensive patterns that may have become unnecessary.

ALWAYS benchmark everything. Use timeit oneliners for quick validation, write dedicated benchmarks for optimization variants, and as the final step, benchmark optimized code against the baseline (devel branch).

Parse $ARGUMENTS to extract:

• Everything before -- is the target: file path, function name, or description of code to optimize
• Everything after -- is critical path hints: which code paths matter most, what to ignore

Phase 1: Understand the Critical Path

SKIP if optimizing a single (or 1-2 ) functions where critical path is clear, otherwise INTERROGATE the user to understand the hot path before making any changes.

1.1 Identify the target

If $ARGUMENTS contains a file path, read it. Otherwise, ask the user:

• "Which file or function should I optimize?"
• "What is the entry point for the hot path?"

1.2 Learn the typical case

Ask about the 90% case to focus optimization effort:

• "What data shapes are most common?" (flat dicts, nested objects, lists)
• "Which branches are taken most often?"
• "What can we ignore?" (e.g., "ignore pandas/arrow, focus on plain dicts")

1.3 Trace the call chain

Follow the code from entry point through each function call:

entry_point() → helper_a() → helper_b() → actual_work()

Read each function in the chain. Identify the innermost loop where per-item work happens.

1.4 Establish baseline benchmark

Before any optimization, create a benchmark for the current state:

import timeit
# Quick validation
python -c "import timeit; print(timeit.timeit('target_function()', setup='...', number=1000))"

Phase 2: Analyze for Optimization Opportunities

Look for these patterns in the critical path:

2.1 Unnecessary work

• Redundant copies: dict(x) or list(x) when reference would suffice
• Defensive copies: Trace data origin - if source returns fresh object, copy is redundant
• Repeated computations: Same value computed multiple times in loop

2.2 Function call overhead

• Hot helper functions: Consider inlining if called per-item
• Uncached method lookups: self.method in loop vs method = self.method before loop
• Duplicate isinstance/hasattr checks: Same check in multiple places

2.3 Happy path shortcuts

• Early exit for common case: Check fast path first, skip expensive work
• Identity vs equality: x is y faster than x == y when applicable

2.4 Consider __slots__ for hot objects

Objects created frequently in hot paths benefit from __slots__:

• 30-40% less memory (no __dict__)
• Faster attribute access (92% of local variable speed vs 59% for regular attributes)

# Bad: regular class, dict-based attributes
class Row:
    def __init__(self, table, data):
        self.table = table
        self.data = data

# Good: slotted class
class Row:
    __slots__ = ['table', 'data']
    def __init__(self, table, data):
        self.table = table
        self.data = data

2.5 Inner function overhead

Inner functions (closures) defined inside loops or hot functions are recreated on every call. This can be a major performance hit:

# Bad: inner function recreated per call
def process(items):
    def transform(x):  # created every time process() is called
        return x * 2
    return [transform(i) for i in items]

# Good: module-level or method
def _transform(x):
    return x * 2

def process(items):
    return [_transform(i) for i in items]

# Good: inline if simple
def process(items):
    return [x * 2 for x in items]

Signs to look for:

• def inside another def
• Lambda inside frequently-called function
• Closures capturing loop variables

2.6 Convert recursion to iteration

Recursive functions have overhead per call (stack frame, argument passing) and risk stack overflow on deep structures. Convert to stack-based iteration:

# Bad: recursive traversal
def flatten(obj, path=""):
    if isinstance(obj, dict):
        for k, v in obj.items():
            yield from flatten(v, f"{path}.{k}")  # recursive call
    else:
        yield path, obj

# Good: stack-based iteration
def flatten(obj):
    stack = [(obj, "")]
    while stack:
        current, path = stack.pop()
        if isinstance(current, dict):
            for k, v in current.items():
                stack.append((v, f"{path}.{k}"))
        else:
            yield path, current

Benefits:

• No function call overhead per level
• Predictable memory (heap vs stack)
• No recursion limit issues
• Can be easier to add early-exit logic

WARN USER:

• stack method MAY change the order of processing and ie. yielded elements. ALWAYS suggest to write a test for original code.

Phase 3: Benchmark Everything

3.1 Always benchmark caches

Caches (lru_cache, manual dicts, memoization) can hurt performance if:

• Cache miss is common (lookup overhead without benefit)
• Cached computation is cheap (cache overhead exceeds saved work)
• Memory pressure causes cache eviction

Benchmark caches even if they already exist - they may have been added speculatively:

# Does this cache actually help? Benchmark with and without!
@lru_cache(maxsize=128)
def get_column_type(col_name):
    return self.schema.columns[col_name]["data_type"]

# Compare:
# 1. With cache (current)
# 2. Without cache (direct lookup)
# 3. Different cache size

3.2 Quick validation with timeit

Before and after each micro-optimization:

python -c "import timeit; d={'a':1,'b':2}; print('copy:', timeit.timeit('dict(d)', globals={'d':d}, number=1000000))"
python -c "import timeit; d={'a':1,'b':2}; print('ref:', timeit.timeit('x=d', globals={'d':d}, number=1000000))"

3.3 Write dedicated benchmarks

For optimization variants, create benchmark scripts in experiments/ folder:

"""Benchmark: optimization variant comparison"""
import timeit

def variant_original(): ...
def variant_optimized(): ...

print("original:", timeit.timeit(variant_original, number=10000))
print("optimized:", timeit.timeit(variant_optimized, number=10000))

3.4 Process isolation and thermal management

Run benchmarks in separate processes with pauses:

python experiments/bench.py devel_case
sleep 10
python experiments/bench.py optimized_case

3.5 Variance checking

Run each benchmark 3+ times to verify stability:

for i in 1 2 3; do
    echo "=== Run $i ==="
    python experiments/bench.py case_name
    sleep 5
done

Variance over 10-15% suggests external factors (thermal throttling, system load).

3.6 Use realistic benchmark data

Use real test data from tests/normalize/cases/:

Load with from dlt.common.json import json. For flat rows with ISO timestamps, use mimesis (in dev deps) to generate synthetic DB data.

3.7 Final benchmark: Optimized vs Devel

As the last step, compare against baseline branch:

# On devel branch (main repo)
cd /path/to/main && python experiments/bench.py case_name

sleep 10

# On optimized branch (worktree)
cd /path/to/worktree && python experiments/bench.py case_name

Report: devel_time / optimized_time = X.XXx speedup

Phase 4: Implement and Verify

4.1 Make changes incrementally

One optimization at a time. Benchmark after each change.

4.2 Verify correctness

• Run existing tests: make test-common
• Compare outputs before/after on sample data

4.3 Document assumptions

Add comments explaining why optimization is safe:

# columns dict is never mutated after get_table_columns() returns,
# so we can store reference instead of copying
self._current_columns = columns

Phase 5: Report Results

Produce a summary:

## Optimization Summary

### Changes Made
1. Replaced dict copy with identity check (buffered.py:100)
2. Inlined count_rows_in_items for common case (buffered.py:102)

### Benchmark Results
| Case | Devel | Optimized | Speedup |
|------|-------|-----------|---------|
| flat_100 | 10.4s | 2.2s | 4.76x |
| nested_20 | 4.9s | 2.0s | 2.47x |

### Assumptions
- columns dict is immutable after creation
- 90% of items are plain dicts, not arrow/pandas