Python → Synsema — the translation table

-- Doc example: the Python → Synsema divergences that bite the hardest.
-- Every claim in the translation-table page is asserted here (doctested).
intent: "doc example: python-to-synsema translation table"

task boom()
    raise("kaput")

task reraise_error()
    try
        boom()
    recover err
        raise(err)

task bad_return()
    return 5

task give_none()
    give None

task iterate_map_directly()
    each k in {"a": 1}
        print(k)

test "assignment is let/set, not = (x = 5 is a parse error)"
    let x be 1
    set x to 2
    assert_eq(x, 2)

test "Python 'return'/'None'/'True' PARSE but are undefined names — use give/nothing/true"
    assert_error(bad_return)
    assert_error(give_none)
    assert_eq(nothing == nothing, true)

test "try/recover (not try/except); recover SWALLOWS unless you raise(err)"
    let seen be ""
    try
        boom()
    recover err
        set seen to err
    assert(contains(seen, "kaput"))
    assert_error(reraise_error)

test "and/or do NOT short-circuit — guard with nested when, not with `and`"
    let m be {"a": 1}
    assert_error(() => contains(m, "b") and m["b"] == 1)
    when contains(m, "a")
        assert_eq(m["a"], 1)

test "append returns a NEW list (no mutating .append); reassign with set"
    let xs be [1, 2]
    let ys be append(xs, 3)
    assert_eq(xs, [1, 2])
    assert_eq(ys, [1, 2, 3])

test "f-string equivalent is the backtick string; quoted strings stay literal"
    assert_eq(`n={1 + 1}`, "n=2")
    assert(contains("n={1+1}", "{"))

test "each cannot iterate a map — go through keys()"
    assert_error(iterate_map_directly)
    let ks be []
    each k in keys({"x": 1, "y": 2})
        set ks to append(ks, k)
    assert_eq(ks, ["x", "y"])

test "comprehension equivalent: apply + where; slicing is slice()"
    assert_eq(apply((x) => x * 2, where([1, 2, 3], (x) => x > 1)), [4, 6])
    assert_eq(slice([10, 20, 30, 40], 1, 3), [20, 30])
    assert_eq(slice([10, 20, 30, 40], -2, 4), [30, 40])

test "the `in` operator is contains(); on maps it checks KEYS"
    assert(contains([1, 2], 2))
    assert(contains("abc", "b"))
    assert(contains({"a": 1}, "a"))

test "text + number CONCATENATES (unlike Python); text * n does not exist"
    assert_eq("a" + 1, "a1")
    assert_error(() => "ab" * 2)

test "len/str/sorted are length/text/sort_by; d.get(k, default) does not exist"
    assert_eq(length("abc"), 3)
    assert_eq(text(42), "42")
    assert_eq(sort_by([3, 1, 2], (x) => x), [1, 2, 3])
    assert_error(() => get({"a": 1}, "a"))
    assert_error(() => {"a": 1}["b"])