What Does // Do in Python? Floor Division with Negative Values

Start with a small, explicit task

Assign synthetic signed positions to bins of width four. Positions immediately below zero should belong to the bin below the zero bin. Predict the results for -1 and -5 before running them; these are the cases that differ from truncating a division result toward zero.

Run the complete example

This original example was checked with Python 3.12.14. Save it as python_floor_division.py and run it with Python 3.12. It uses only the standard library. The assertions check the stated results.

offsets = [-5, -1, 0, 3, 4, 8]
bins = [offset // 4 for offset in offsets]
assert bins == [-2, -1, 0, 0, 1, 2]
assert -1 // 4 == -1
assert int(-1 / 4) == 0
floated = 9.0 // 4
assert floated == 2.0 and isinstance(floated, float)
try:
    4 // 0
except ZeroDivisionError:
    zero_result = "zero divisor rejected"
else:
    raise AssertionError("expected division failure")
print(bins)
print(floated, type(floated).__name__)
print(zero_result)

Explain the result

With a width of four, bin zero covers positions 0 through 3. Position -1 must therefore lie in bin -1, while -5 lies in bin -2. Rounding the mathematical quotients downward gives those results. int(-1 / 4) instead produces zero here, so it does not implement the same signed-bin rule.

The float check returns 2.0, which compares numerically equal to 2 but has a different type. The code checks both value and type rather than relying on printed appearance alone. Division by zero is explicitly rejected. These examples use small exact integers; they are not a claim that every float division avoids approximation.

Avoid these mistakes

Do not replace floor division with int(a / b) when negative values are possible. Keep the divisor nonzero and define whether negative widths are meaningful for your task. Python comments begin with #; using // as a comment marker creates an arithmetic expression or a syntax error.