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reference 3 min read · lesson 17 of 60 in Errors

ZeroDivisionError: division by zero

1 · The lesson

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What this error means

You tried to divide a number by zero. Mathematically, division by zero is undefined, so Python refuses the operation and raises ZeroDivisionError. This applies to / (true division), // (floor division), and % (modulo).

When you see it

python
>>> 1 / 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: division by zero
python
>>> 10 // 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
python
>>> 10 % 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero

Why it happens

The denominator evaluated to zero. The most common case is computing an average over an empty list (sum(xs) / len(xs) when xs is []), or a rate where the time interval is zero, or a ratio where the baseline value is missing.

How to fix it

1. Check the denominator first (the cleanest option):

python
def mean(xs):
    return sum(xs) / len(xs) if xs else 0

2. Use a conditional inline:

python
rate = hits / total if total else 0
+ setup added so this can run · defines total, hits
# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

total = _AutoMock('total')
hits = _AutoMock('hits')

3. Catch the exception when zero really is unexpected and you want to log it:

python
try:
    ratio = a / b
except ZeroDivisionError:
    ratio = float("inf")   # or None, or 0. The right choice depends on your data.
+ setup added so this can run · defines a, b
# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

a = _AutoMock('a')
b = _AutoMock('b')

4. Avoid the epsilon hack:

python
ratio = a / (b + 1e-9)   # almost always wrong, because it hides bugs
+ setup added so this can run · defines a, b
# Lightweight mock for objects whose attributes/methods aren't critical
class _AutoMock:
    def __init__(self, name='mock'): self._name = name
    def __getattr__(self, k): return _AutoMock(self._name + '.' + k)
    def __call__(self, *a, **kw):
        print('-> ' + self._name + '() called')
        return _AutoMock(self._name + '()')
    def __repr__(self): return '<mock ' + self._name + '>'
    def __str__(self): return '<mock ' + self._name + '>'
    def __bool__(self): return True
    def __iter__(self): return iter([])
    def __len__(self): return 0
    def __getitem__(self, k): return _AutoMock(self._name + '[...]')
    def __setitem__(self, k, v): pass
    def __enter__(self): return self
    def __exit__(self, *a): return False
    async def __aenter__(self): return self
    async def __aexit__(self, *a): return False
    def __add__(self, o): return self
    def __radd__(self, o): return self
    def __sub__(self, o): return self
    def __mul__(self, o): return self
    def __rmul__(self, o): return self
    def __truediv__(self, o): return self
    def __eq__(self, o): return isinstance(o, _AutoMock)
    def __hash__(self): return hash(self._name)
    def __lt__(self, o): return True
    def __le__(self, o): return True
    def __gt__(self, o): return False
    def __ge__(self, o): return False
    def __mro_entries__(self, bases): return (object,)

a = _AutoMock('a')
b = 1

Adding a tiny epsilon makes the error go away but produces meaningless numbers. Prefer to handle the zero case explicitly and return None, 0, or skip the row.

When you'd see this in real code

  • Computing averages, rates, or percentages over filtered data: after filter(...) the list can be empty, and sum / len raises this error.
  • Computing a unit price: total_cost / quantity raises when an order line has zero quantity (a data quality bug that you should report, not hide).
  • Computing speed from distance and time when two events have the same timestamp: (d2 - d1) / (t2 - t1) with t1 == t2.

See Also

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