Unit converter

Write the codeMedium 2 stages

Partner agencies send numbers in their own units: miles, pounds, psi, Fahrenheit. In 1999 a mix-up between pound-force seconds and newton-seconds cost NASA the Mars Climate Orbiter. The team wants one converter that everyone uses, driven by one table, and that refuses loudly when it doesn't know a unit.

Build it, then teach it temperatures.

Write convert(value, from_unit, to_unit). For a pair in CONVERSIONS, multiply by its factor; for the pair the other way round, divide; for the same unit twice, return the value. Raise ValueError for a unit the table doesn't know, unknown unit: ..., or for two known units with no factor between them. Then run the requests:

400 km = 248.55 mi
12.5 lb = 5.67 kg
7.8 km/h = 2.17 m/s
101.3 kPa = 14.69 psi
REFUSED 3 km/s -> km/h: unknown unit: km/s
REFUSED 5 kg -> kPa: no conversion from kg to kPa
42 N = 42.00 N

UNITS, every unit that appears in the table, is built for you at the top. Check both units against it first.

Seven requests

convert() for any pair

Finish stage 1 to unlock this stage.

Temperatures need more than a factor: Fahrenheit is Celsius times 1.8, plus 32. The table now holds (factor, offset) for every pair. Convert with value * factor + offset, and the other way round with (value - offset) / factor:

400 km = 248.55 mi
12.5 lb = 5.67 kg
-60 C = 213.15 K
77 K = -196.15 C
-40 F = -40.00 C
REFUSED 300 K -> F: no conversion from K to F
21 C = 21.00 C

The rest of convert() stays as it was. Kelvin to Fahrenheit has no pair in the table, so it is refused.

Temperatures and the rest

Offsets both ways

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Hints

Creating Dictionaries

creating-dictionaries-python.py

# A dictionary stores pairs: a key, then its value
webb = {
    "name": "James Webb Space Telescope",
    "mirror_metres": 6.5,
    "launched": 2021,
    "instruments": 4,
    "infrared": True,
}
print(webb)

# Square brackets look up a value by its key
print("Mirror width:", webb["mirror_metres"], "metres")

# get() returns a fallback instead of crashing on a missing key
print("Cost:", webb.get("cost", "not recorded"))

# The dict() function builds the same thing from keyword arguments
hubble = dict(name="Hubble", mirror_metres=2.4, launched=1990)
print("Hubble mirror:", hubble["mirror_metres"], "metres")

# Keys do not have to be text. Anything unchangeable works
launch_log = {
    1990: "Hubble",
    2021: "Webb",
    (2027, "planned"): "Roman",
}
print(launch_log[1990])
print("Missions logged:", len(launch_log))
print("Anything launched in 2021?", 2021 in launch_log)

Open the full example

script.py

PythonLn 1, Col 1Spaces: 4

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Command-line arguments (sys.argv)

Separate arguments with commas. They're passed on every Run as sys.argv[1:].