Replace a Margins Spreadsheet With 20 Lines of Tested Python

Less than two of the free Kindle days are left, so if a margins spreadsheet has ever surprised you, today is a good day to read this one. Below is a typical workbook rewritten as about 20 lines of Python, with the test that proves it.

The spreadsheet problem

Most stress offices have one: a workbook that takes loads from one tab, applies the factors in another, divides by allowables pulled from a third and colours the negative margins red. It works until someone sorts a column without its neighbours, pastes over a formula, or copies a row from an old revision. None of those leave a trace, and the reviewer only sees the numbers.

The same thing in Python

Here is the core of that workbook. It reads a CSV of cases, each with a name, a limit load and an allowable, and reports the critical one.

import csv

ULTIMATE = 1.5   # 14 CFR 25.303
FITTING = 1.15   # 14 CFR 25.625

def margin(allowable, limit, fitting=FITTING):
    design = limit * ULTIMATE * fitting
    return allowable / design - 1.0

def critical_case(path):
    with open(path, newline="") as f:
        rows = list(csv.DictReader(f))
    results = [(r["case"], margin(float(r["allowable"]), float(r["limit"])))
               for r in rows]
    return min(results, key=lambda r: r[1])

def test_margin_matches_hand_check():
    assert round(margin(18562.5, 8000.0), 2) == 0.35

if __name__ == "__main__":
    name, ms = critical_case("cases.csv")
    print(f"Critical case {name}: MS = {ms:+.2f}")

Every formula is on one screen. The factors have names and references instead of living in a cell somewhere. And the last function is a test: it repeats the fitting margin worked by hand in Volume 1 of the stress books, 18,562.5 lb against a limit load of 8,000 lb, and fails loudly if anyone ever changes the method so that number stops coming out at +0.35.

Why the test matters

Run it with pytest and it either passes or tells you exactly what broke. That is the step most spreadsheets never get. A checker can read the method, see the hand check it reproduces, and rerun it on the next revision of the loads in seconds. The book builds from this kind of script to load cases, section properties, buckling and joint checks, plots, and a small reporting tool, always with tests beside the code.

What a reviewer gets from it

A spreadsheet review usually means clicking cell by cell to see which formula feeds which, and hoping nothing was overwritten. With a script the reviewer reads the method once, from top to bottom, in the order it runs. The change history lives in version control, so the question of which revision produced a margin has an answer. And because the input is a plain CSV, the loads team can hand over a new file and the margins are rerun without anyone retyping a number.

None of this needs a software background. The listing above uses only the standard library: no installs, no frameworks, nothing your IT department needs to approve.

How to use the free copy this week

Pick one small workbook you own and rewrite only its core formula first, with one test against a number you already trust. Do not try to replace the whole thing. Once that test passes, the rest of the workbook is just more rows. The early chapters of the book walk through exactly this first step, from reading a CSV to writing the first test, with stress examples rather than generic ones.

All four books in the promotion

• GD&T Professional Exam Guide Kindle ebook on Amazon
• Practical Airframe Stress Analysis, Vol. 1 Kindle ebook on Amazon
• Practical Airframe Stress Analysis, Vol. 2 Kindle ebook on Amazon
• Python for Aerospace Stress Engineers Kindle ebook on Amazon

Read more

The book’s card and paperback are on the books page, and readers’ code, data files and notebooks are in the bonus materials. All the posts of this promotion are on the announcements page.