PERF: Optimise to_long_format by eliminating intermediate DataFrame allocations - #1675
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CamDavidsonPilon merged 1 commit intoMar 7, 2026
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Replaced chained `assign` and `drop` methods with a single `.copy()` and an in-place `.pop()`. This prevents pandas from allocating memory for intermediate DataFrames, resulting in faster execution and reduced memory usage on large survival datasets.
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👍 I love perf wins like this! Have any numbers to share? |
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Here are the performance numbers comparing the existing ⏱️ Performance
Numbers obtained after running each function 10x |
CamDavidsonPilon
merged commit Mar 7, 2026
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Description
This PR improves the performance of
lifelines.utils.to_long_format.The previous implementation relied on method chaining (
df.assign(...).drop(...)), which inadvertently triggered two full DataFrame allocations in memory:assigncreated the first copy to append the new columns.dropcreated a second copy to remove theduration_col.By refactoring this to perform a single explicit
.copy()followed by fast, in-place operations (.pop()and direct assignment), we effectively halve the memory overhead and noticeably speed up execution on large DataFrames.This change introduces no breaking changes and is fully backward-compatible.
Type of change