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take()/drop(): raise a clear ValueError for negative n #628

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take()/drop(): raise a clear ValueError for negative n #628

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67ba32c
drop: raise a clear ValueError for negative n
Jul 1, 2026
0a1188d
take: raise a clear ValueError for negative n
Jul 1, 2026
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4 changes: 4 additions & 0 deletions toolz/itertoolz.py
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Original file line number Diff line number Diff line change
Expand Up @@ -319,6 +319,8 @@ def take(n, seq):
drop
tail
"""
if n < 0:
raise ValueError('take: n must be a non-negative integer, got %r' % (n,))
return itertools.islice(seq, n)


Expand Down Expand Up @@ -348,6 +350,8 @@ def drop(n, seq):
take
tail
"""
if n < 0:
raise ValueError('drop: n must be a non-negative integer, got %r' % (n,))
return itertools.islice(seq, n, None)


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18 changes: 18 additions & 0 deletions toolz/tests/test_itertoolz.py
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Original file line number Diff line number Diff line change
Expand Up @@ -185,6 +185,15 @@ def test_take():
assert list(take(2, (3, 2, 1))) == list((3, 2))


def test_take_negative_n():
try:
list(take(-1, [1, 2, 3]))
assert False, 'expected ValueError'
except ValueError as e:
assert 'non-negative' in str(e)
assert list(take(0, [1, 2, 3])) == [] # n == 0 boundary still returns, not rejected


def test_tail():
assert list(tail(3, 'ABCDE')) == list('CDE')
assert list(tail(3, iter('ABCDE'))) == list('CDE')
Expand All @@ -196,6 +205,15 @@ def test_drop():
assert list(drop(1, (3, 2, 1))) == list((2, 1))


def test_drop_negative_n():
try:
list(drop(-1, [1, 2, 3]))
assert False, 'expected ValueError'
except ValueError as e:
assert 'non-negative' in str(e)
assert list(drop(0, [1, 2, 3])) == [1, 2, 3]


def test_take_nth():
assert list(take_nth(2, 'ABCDE')) == list('ACE')

Expand Down