pytest 测试框架学习（3）：pytest.approx

前言

经过 API 我们已经了解到 pytest 中包括的 API 大致内容，接下来我们详细看看 Functions 中的 pytest.approx。

含义

approx：在一定误差范围内断言两个数字（或两组数字）相等。
源码如图：

使用

我们知道计算机运算浮点数的复杂性，我们直观认为相等的而实际上并不相等；

单数

0.1 + 0.2 == 0.3
# 我们认为上方计算出来结果进行比对返回应该是 True，而实际返回
False

而 approx 可以帮我们解决这个问题：

from pytest import approx
0.1 + 0.2 == approx(0.3)

元祖

(0.1 + 0.2, 0.2 + 0.4) == approx((0.3, 0.6))

字典

{'a': 0.1 + 0.2, 'b': 0.2 + 0.4} == approx({'a': 0.3, 'b': 0.6})

numpy 数组:

numpy 介绍和使用

import numpy as np
np.array([0.1, 0.2]) + np.array([0.2, 0.4]) == approx(np.array([0.3, 0.6]))

如果 numpy 数组中结果一致，则可以直接填入一个标量，如：

import numpy as np
np.array([0.1, 0.2]) + np.array([0.2, 0.1]) == approx(0.3)

相对误差 和 绝对误差

默认情况，approx() 使用的为 相对误差，误差的范围是 1e-6 也就是百万分之一。

1. 指定相对误差：
   
2. 只指定绝对误差，不指定相对误差，则根据绝对误差来进行比较：
   
3. 指定两个，只要满足其中一个，则认为是相等的：
   

进阶

主要包括：

1. 其他方法的浮点数比较与 pytest.approx() 的区别：
   技术能力有限，这里不做细致说明，贴出官网源码。

math.isclose(a, b, rel_tol=1e-9, abs_tol=0.0)
"""
- ``math.isclose(a, b, rel_tol=1e-9, abs_tol=0.0)``:  True if the relativetolerance is met w.r.t. either ``a`` or ``b`` or if the absolutetolerance is met.  Because the relative tolerance is calculated w.r.t.both ``a`` and ``b``, this test is symmetric (i.e.  neither ``a`` nor``b`` is a "reference value").  You have to specify an absolute toleranceif you want to compare to ``0.0`` because there is no tolerance bydefault.  Only available in python>=3.5.  `More information...`____ https://docs.python.org/3/library/math.html#math.isclose
"""numpy.isclose(a, b, rtol=1e-5, atol=1e-8)
"""
- ``numpy.isclose(a, b, rtol=1e-5, atol=1e-8)``: True if the differencebetween ``a`` and ``b`` is less that the sum of the relative tolerancew.r.t. ``b`` and the absolute tolerance.  Because the relative toleranceis only calculated w.r.t. ``b``, this test is asymmetric and you canthink of ``b`` as the reference value.  Support for comparing sequencesis provided by ``numpy.allclose``.  `More information...`____ http://docs.scipy.org/doc/numpy-1.10.0/reference/generated/numpy.isclose.html
"""unittest.TestCase.assertAlmostEqual(a, b)
"""
- ``unittest.TestCase.assertAlmostEqual(a, b)``: True if ``a`` and ``b``are within an absolute tolerance of ``1e-7``.  No relative tolerance isconsidered and the absolute tolerance cannot be changed, so this functionis not appropriate for very large or very small numbers.  Also, it's onlyavailable in subclasses of ``unittest.TestCase`` and it's ugly because itdoesn't follow PEP8.  `More information...`____ https://docs.python.org/3/library/unittest.html#unittest.TestCase.assertAlmostEqual
"""a == pytest.approx(b, rel=1e-6, abs=1e-12)
"""
- ``a == pytest.approx(b, rel=1e-6, abs=1e-12)``: True if the relativetolerance is met w.r.t. ``b`` or if the absolute tolerance is met.Because the relative tolerance is only calculated w.r.t. ``b``, this testis asymmetric and you can think of ``b`` as the reference value.  In thespecial case that you explicitly specify an absolute tolerance but not arelative tolerance, only the absolute tolerance is considered.
"""

2. 运算符比较

"""
In order to avoid inconsistent behavior, ``TypeError`` israised for ``>``, ``>=``, ``<`` and ``<=`` comparisons.The example below illustrates the problem::assert approx(0.1) > 0.1 + 1e-10  # calls approx(0.1).__gt__(0.1 + 1e-10)assert 0.1 + 1e-10 > approx(0.1)  # calls approx(0.1).__lt__(0.1 + 1e-10)In the second example one expects ``approx(0.1).__le__(0.1 + 1e-10)``to be called. But instead, ``approx(0.1).__lt__(0.1 + 1e-10)`` is used tocomparison. This is because the call hierarchy of rich comparisonsfollows a fixed behavior. `More information...`____ https://docs.python.org/3/reference/datamodel.html#object.__ge__
"""

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