blame

Cell-level "why" for pandas pipelines. No code changes.

Your report says 41,900 and it should say 38,400. Today you find that out by adding print statements down a thirty-step pipeline until something looks wrong. blame records the pipeline as it runs, so you can point at the number instead.

Install

pip install pandas-blame

Use

import blame, pandas as pd

with blame.trace():
    orders    = pd.read_csv("orders.csv")
    customers = pd.read_csv("customers.csv")
    clean     = orders[orders.qty > 0]
    joined    = clean.merge(customers, on="customer_id")
    report    = joined.groupby("region", as_index=False)["total"].sum()

print(blame.last_run().why(row=1, col="total"))

why(groupby.sum [f5] 3x2, row [1], column 'total')

  derivation:
    step   3 groupby.sum('region') [demo.py:9] <- f4:2 rows
    step   2 merge(on='customer_id') [demo.py:8] <- f3:1 row, f1:2 rows
    step   0 filter(<frame>) [demo.py:6] <- f0:1 row

  source rows:
    read_csv('customers.csv') [f1]: 2 row(s) -> [1, 4]
    read_csv('orders.csv') [f0]: 1 row(s) -> [1]

One order row, but two customer rows: customers.csv has customer_id 11 twice, so the merge duplicated that order and the group-by counted it twice. Two row numbers in two files, not "somewhere in these steps".

blame steps    # every operation the pipeline ran, with line numbers
blame ui       # the picture above: click a cell, watch its sources light up
blame diff     # what changed since the last run, and which input row did it

blame diff is the one that earns its keep in a test suite:

assert not before.diff(after, on="order_id").unexplained_rows()

Inputs that did not move should not produce outputs that did. When they do, blame names the input row responsible — or says plainly that none explains it, which means the pipeline changed rather than the data.

Honest limits

Lineage is exact for filters, slices, sorts, joins, group-bys, resample, concats, pivot, pivot_table, melt, unstack and the row-preserving transforms. It is approximate, and says so with a ~, for apply/map with a user function, and for operations it does not cover (transpose, stack, explode). Approximate answers are widened to every candidate row, never narrowed and never presented as exact — see docs/accuracy.md.

Capture costs about 2x wall time and 5 ms per step on a million rows (benchmarks). Tested on pandas 2.0.3, 2.2.3 and 3.0.5; the eight pandas documentation tutorials run under it with no exceptions, no changed results and no approximate steps (validation).

Status

Alpha, and honest about it. It works, it is tested against ground truth, and it has never been run on your pipeline. If it gets something wrong about a real one, that is the bug report worth filing. Roadmap.

Apache-2.0.