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Predict the output: what a set really deduplicates

Q: Why does True not appear in the output at all?

Because 1 == True and both hash alike, the set keeps the member it saw first, which was 1. The same rule removes 1.0.

🔮 Predict the outputTopic: setsDifficulty: mediumNo login needed

The snippet

Two misconceptions meet here: that a set keeps the order you typed, and that items are only equal when they are the same type. 1, 1.0 and True are three different objects with one value — so one of them survives.

flags = [1, True, 1.0, 0, False]
print(set(flags))
print(len(set(flags)))

Your prediction

Type exactly what this program prints — one line per print() call:

The output

{0, 1}
2

Verified against CPython 3.11 in this repo's build

Line-by-line explanation

flags = [1, True, 1.0, 0, False]
Five elements, but only two distinct values once you compare by equality: 1 and 0.
print(set(flags))
Sets store members by hash then ==. hash(1) == hash(True) == hash(1.0) and all three are equal, so the first one wins and the rest are ignored; 0/False collapse the same way. The surviving members are the ints → {0, 1}.
print(len(set(flags)))
Two members → 2.
(why that order?)
Small integers hash to themselves, so in this build they land in slots 0 and 1 and print as {0, 1}. Never rely on it — set order is an implementation detail, not insertion order. Use sorted() when order matters.

The rule you can reuse

A set keeps one representative per equal-and-same-hash value, in unspecified order. That is also why unhashable items (list, dict, set) cannot be members — TypeError: unhashable type.

Why most predictions break here

Answering {1, True, 1.0, 0, False} or {1, 0} in insertion order. The count and the set of *values* are what a set promises; position is not.

Quick fire

Same rule, two more shapes. Predict first, then open.

Predict this too

print(sorted({3, 1, 2}))
print(len({1, 1.0, True}))

Output
[1, 2, 3] 1

sorted() turns a set into an ordered list, and the three "different" booleans/numbers are one member → [1, 2, 3] then 1.

Predict this too

print(len([[1, 2], [1, 2]]), len({"a", "b"}))

Output
2 2

Lists are unhashable so {[1, 2]} raises TypeError; here nothing is deduplicated by the set literal at all → 2 2.

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Keep practising this rule

More output questions → All interactive problems

Questions this page answers

Is dict key order the same thing?

No — dictionaries *do* keep insertion order (a language guarantee since Python 3.7). Only sets are unordered.

Why does True not appear in the output at all?

Because 1 == True and both hash alike, the set keeps the member it saw first, which was 1. The same rule removes 1.0.