python · Python
The message
ZeroDivisionError: division by zero
What it means
Something was divided by zero — and in practice the divisor is rarely a literal zero, it is a count that came out zero. Averaging code where the list turned out empty, or a filter that matched nothing, is the classic source: it passes on your development data and first fails in production. Check the denominator before you divide and decide what zero means there; returning 0, returning None, and letting the exception propagate are three different decisions, and each is right in some situations.
The fix
There is no one-line command for this. The explanation says what to look at instead.
- Printed by
- python
- Python
- 18
A Python traceback splits the answer in two — the last line says what went wrong, the frames above it say where — so reading only the last line gives you the name and loses the place, and for the value-is-missing errors such as NoneType and KeyError the cause almost always sits in a frame above the one that crashed.
Reading an error message
- Read from the first line down. The lower you go the more it is about the tool’s internals; the cause is usually at the top.
- If there is a file and a line number, start there — not the top stack frame, but the topmost line that names a file you wrote.
- Search the message verbatim, but strip your own paths and variable names first; those are what stop the search from matching.
- The same condition is worded differently across tool versions. If results look wrong, add the version number to the query.
- Before pasting a fix, check what it throws away. Some of these cannot be undone.
Common questions
Q. What does “ZeroDivisionError: division by zero” mean?
Something was divided by zero — and in practice the divisor is rarely a literal zero, it is a count that came out zero. Averaging code where the list turned out empty, or a filter that matched nothing, is the classic source: it passes on your development data and first fails in production. Check the denominator before you divide and decide what zero means there; returning 0, returning None, and letting the exception propagate are three different decisions, and each is right in some situations.
Q. How do I fix it?
There is no one-line command. The explanation above says what to look at instead.
Q. Which tool prints this?
python. It sits under Python, and the message runs to 4 words.