Sum of numbers from 0 to N in Python

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def show_sequence(n):
    if n == 0:
        return "0=0"
    elif n < 0:
        return str(n) + "<0"
    else:
        counter = sum(range(n+1))
        return '+'.join(map(str, range(n+1))) + " = " + str(counter)

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def show_sequence(n):
    s = '0'
    for i in range(1, n+1):
        s+='+'+str(i)
    return s+' = '+str(n*(n+1)//2) if n>0 else '0=0' if n==0 else str(n)+'<0'

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def show_sequence(n):

    if n < 0:
        return str(n) + "<0"
    if n == 0:
        return str(n) + "=0"
        
    summ = sum(range(n+1))
    numstr = "+".join(map(str, range(n+1)))
    
    return numstr + " = " + str(summ)

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import test
from solution import show_sequence

@test.describe("Fixed Tests")
def fixed_tests():
    @test.it('Basic Test Cases')
    def basic_test_cases():
        tests = (
            (6, "0+1+2+3+4+5+6 = 21"),
            (7, "0+1+2+3+4+5+6+7 = 28"),
            (0, "0=0"), 
            (-1, "-1<0"), 
            (-10, "-10<0"),
        )
        
        for inp, exp in tests:
            test.assert_equals(show_sequence(inp), exp)
            
@test.describe("Random Tests")
def _():
    
    from random import randint    
    
    def reference(n):
        if n < 0:
            return "%s<0" % n
        if n == 0:
            return "0=0"            
        return "%s = %s" % ("+".join(map(str, range(0, n+1))), n * (n + 1) // 2)    
    
    for _ in range(100):
        test_case = randint(-100, 101)
        @test.it(f"testing for show_sequence({test_case})")
        def test_case():
            test.assert_equals(show_sequence(test_case),reference(test_case))