Hi，大家好，我是编程小6，很荣幸遇见你，我把这些年在开发过程中遇到的问题或想法写出来，今天说一说
pytest断言技巧,希望能够帮助你!!!。

断言技巧

pytest的断言把Python语言简洁的优点发挥的淋漓尽致，因为它用的就是Python的标准断言assert。

assert基础

assert用法

assert_stmt ::= "assert" expression1 ["," expression2]

“assert” expression1，expression1和 if 的条件语句是一模一样的，if 能写什么这里就能写什么。

[“,” expression2]，是可选的，不写错误信息也可以。

assert断言等价于

if __debug__: if not expression1: raise AssertionError(expression2)

示例，只考了59分，满分100分，不及格

def test(): assert 59 == 100, "不及格"

结果，>指出了断言失败的代码行，E指出了具体的错误

a.py::test FAILED [100%] AssertionError: 不及格 59 != 100 Expected :100 Actual :59 <Click to see difference> def test(): > assert 59 == 100, "不及格" E AssertionError: 不及格 E assert 59 == 100 a.py:2: AssertionError ================================== FAILURES =================================== ____________________________________ test _____________________________________ def test(): > assert 59 == 100, "不及格" E AssertionError: 不及格 E assert 59 == 100 a.py:2: AssertionError =========================== short test summary info =========================== FAILED a.py::test - AssertionError: 不及格 ============================== 1 failed in 0.30s ==============================

把这个例子稍微抽象一下，assert 实际 == 预期，错误信息。记住把预期放在右边。

pytest.raises

pytest.raises用于断言异常。

示例，断言会抛出ZeroDivisionError，如果抛出了测试结果是pass，如果没抛出，测试结果是failed

import pytest def test_zero_division(): with pytest.raises(ZeroDivisionError): 1 / 0

同时可以断言异常信息。示例，定义f()，递归循环抛异常，assert断言这个异常信息包括“maximum recursion”

def test_recursion_depth(): with pytest.raises(RuntimeError) as excinfo: def f(): f() f() assert "maximum recursion" in str(excinfo.value)

可以添加match参数来用正则过滤异常信息。示例，匹配包括123的异常

import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises(ValueError, match=r".* 123 .*"): myfunc()

如果是对整个test异常断言，也可以加marker xfail

@pytest.mark.xfail(raises=IndexError)

44个failure例子

每个test用大横线做了分隔

assertion $ pytest failure_demo.py =========================== test session starts ============================ platform linux -- Python 3.x.y, pytest-6.x.y, py-1.x.y, pluggy-0.x.y cachedir: $PYTHON_PREFIX/.pytest_cache rootdir: $REGENDOC_TMPDIR/assertion collected 44 items failure_demo.py FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF [100%] ================================= FAILURES ================================= ___________________________ test_generative[3-6] ___________________________ param1 = 3, param2 = 6 @pytest.mark.parametrize("param1, param2", [(3, 6)]) def test_generative(param1, param2): > assert param1 * 2 < param2 E assert (3 * 2) < 6 failure_demo.py:19: AssertionError _________________________ TestFailing.test_simple __________________________ self = <failure_demo.TestFailing object at 0xdeadbeef> def test_simple(self): def f(): return 42 def g(): return 43 > assert f() == g() E assert 42 == 43 E + where 42 = <function TestFailing.test_simple.<locals>.f at 0xdeadbeef>() E + and 43 = <function TestFailing.test_simple.<locals>.g at 0xdeadbeef>() failure_demo.py:30: AssertionError ____________________ TestFailing.test_simple_multiline _____________________ self = <failure_demo.TestFailing object at 0xdeadbeef> def test_simple_multiline(self): > otherfunc_multi(42, 6 * 9) failure_demo.py:33: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ a = 42, b = 54 def otherfunc_multi(a, b): > assert a == b E assert 42 == 54 failure_demo.py:14: AssertionError ___________________________ TestFailing.test_not ___________________________ self = <failure_demo.TestFailing object at 0xdeadbeef> def test_not(self): def f(): return 42 > assert not f() E assert not 42 E + where 42 = <function TestFailing.test_not.<locals>.f at 0xdeadbeef>() failure_demo.py:39: AssertionError _________________ TestSpecialisedExplanations.test_eq_text _________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_text(self): > assert "spam" == "eggs" E AssertionError: assert 'spam' == 'eggs' E - eggs E + spam failure_demo.py:44: AssertionError _____________ TestSpecialisedExplanations.test_eq_similar_text _____________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_similar_text(self): > assert "foo 1 bar" == "foo 2 bar" E AssertionError: assert 'foo 1 bar' == 'foo 2 bar' E - foo 2 bar E ? ^ E + foo 1 bar E ? ^ failure_demo.py:47: AssertionError ____________ TestSpecialisedExplanations.test_eq_multiline_text ____________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_multiline_text(self): > assert "foo\nspam\nbar" == "foo\neggs\nbar" E AssertionError: assert 'foo\nspam\nbar' == 'foo\neggs\nbar' E foo E - eggs E + spam E bar failure_demo.py:50: AssertionError ______________ TestSpecialisedExplanations.test_eq_long_text _______________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_long_text(self): a = "1" * 100 + "a" + "2" * 100 b = "1" * 100 + "b" + "2" * 100 > assert a == b E AssertionError: assert '1...22' == '1...22' E Skipping 90 identical leading characters in diff, use -v to show E Skipping 91 identical trailing characters in diff, use -v to show E - b E ? ^ E + a E ? ^ failure_demo.py:55: AssertionError _________ TestSpecialisedExplanations.test_eq_long_text_multiline __________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_long_text_multiline(self): a = "1\n" * 100 + "a" + "2\n" * 100 b = "1\n" * 100 + "b" + "2\n" * 100 > assert a == b E AssertionError: assert '1\n1\n1\n1\n...n2\n2\n2\n2\n' == '1\n1\n1\n1\n...n2\n2\n2\n2\n' E Skipping 190 identical leading characters in diff, use -v to show E Skipping 191 identical trailing characters in diff, use -v to show E 1 E 1 E 1 E 1 E 1... E E ...Full output truncated (7 lines hidden), use '-vv' to show failure_demo.py:60: AssertionError _________________ TestSpecialisedExplanations.test_eq_list _________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_list(self): > assert [0, 1, 2] == [0, 1, 3] E assert [0, 1, 2] == [0, 1, 3] E At index 2 diff: 2 != 3 E Use -v to get the full diff failure_demo.py:63: AssertionError ______________ TestSpecialisedExplanations.test_eq_list_long _______________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_list_long(self): a = [0] * 100 + [1] + [3] * 100 b = [0] * 100 + [2] + [3] * 100 > assert a == b E assert [0, 0, 0, 0, 0, 0, ...] == [0, 0, 0, 0, 0, 0, ...] E At index 100 diff: 1 != 2 E Use -v to get the full diff failure_demo.py:68: AssertionError _________________ TestSpecialisedExplanations.test_eq_dict _________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_dict(self): > assert {"a": 0, "b": 1, "c": 0} == {"a": 0, "b": 2, "d": 0} E AssertionError: assert {'a': 0, 'b': 1, 'c': 0} == {'a': 0, 'b': 2, 'd': 0} E Omitting 1 identical items, use -vv to show E Differing items: E {'b': 1} != {'b': 2} E Left contains 1 more item: E {'c': 0} E Right contains 1 more item: E {'d': 0}... E E ...Full output truncated (2 lines hidden), use '-vv' to show failure_demo.py:71: AssertionError _________________ TestSpecialisedExplanations.test_eq_set __________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_set(self): > assert {0, 10, 11, 12} == {0, 20, 21} E AssertionError: assert {0, 10, 11, 12} == {0, 20, 21} E Extra items in the left set: E 10 E 11 E 12 E Extra items in the right set: E 20 E 21... E E ...Full output truncated (2 lines hidden), use '-vv' to show failure_demo.py:74: AssertionError _____________ TestSpecialisedExplanations.test_eq_longer_list ______________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_longer_list(self): > assert [1, 2] == [1, 2, 3] E assert [1, 2] == [1, 2, 3] E Right contains one more item: 3 E Use -v to get the full diff failure_demo.py:77: AssertionError _________________ TestSpecialisedExplanations.test_in_list _________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_in_list(self): > assert 1 in [0, 2, 3, 4, 5] E assert 1 in [0, 2, 3, 4, 5] failure_demo.py:80: AssertionError __________ TestSpecialisedExplanations.test_not_in_text_multiline __________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_not_in_text_multiline(self): text = "some multiline\ntext\nwhich\nincludes foo\nand a\ntail" > assert "foo" not in text E AssertionError: assert 'foo' not in 'some multil...nand a\ntail' E 'foo' is contained here: E some multiline E text E which E includes foo E ? +++ E and a... E E ...Full output truncated (2 lines hidden), use '-vv' to show failure_demo.py:84: AssertionError ___________ TestSpecialisedExplanations.test_not_in_text_single ____________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_not_in_text_single(self): text = "single foo line" > assert "foo" not in text E AssertionError: assert 'foo' not in 'single foo line' E 'foo' is contained here: E single foo line E ? +++ failure_demo.py:88: AssertionError _________ TestSpecialisedExplanations.test_not_in_text_single_long _________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_not_in_text_single_long(self): text = "head " * 50 + "foo " + "tail " * 20 > assert "foo" not in text E AssertionError: assert 'foo' not in 'head head h...l tail tail ' E 'foo' is contained here: E head head foo tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail E ? +++ failure_demo.py:92: AssertionError ______ TestSpecialisedExplanations.test_not_in_text_single_long_term _______ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_not_in_text_single_long_term(self): text = "head " * 50 + "f" * 70 + "tail " * 20 > assert "f" * 70 not in text E AssertionError: assert 'fffffffffff...ffffffffffff' not in 'head head h...ltail tail ' E 'ffffffffffffffffff...fffffffffffffffffff' is contained here: E head head fffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffftail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail tail E ? ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ failure_demo.py:96: AssertionError ______________ TestSpecialisedExplanations.test_eq_dataclass _______________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_dataclass(self): from dataclasses import dataclass @dataclass class Foo: a: int b: str left = Foo(1, "b") right = Foo(1, "c") > assert left == right E AssertionError: assert TestSpecialis...oo(a=1, b='b') == TestSpecialis...oo(a=1, b='c') E E Omitting 1 identical items, use -vv to show E Differing attributes: E ['b'] E E Drill down into differing attribute b: E b: 'b' != 'c'... E E ...Full output truncated (3 lines hidden), use '-vv' to show failure_demo.py:108: AssertionError ________________ TestSpecialisedExplanations.test_eq_attrs _________________ self = <failure_demo.TestSpecialisedExplanations object at 0xdeadbeef> def test_eq_attrs(self): import attr @attr.s class Foo: a = attr.ib() b = attr.ib() left = Foo(1, "b") right = Foo(1, "c") > assert left == right E AssertionError: assert Foo(a=1, b='b') == Foo(a=1, b='c') E E Omitting 1 identical items, use -vv to show E Differing attributes: E ['b'] E E Drill down into differing attribute b: E b: 'b' != 'c'... E E ...Full output truncated (3 lines hidden), use '-vv' to show failure_demo.py:120: AssertionError ______________________________ test_attribute ______________________________ def test_attribute(): class Foo: b = 1 i = Foo() > assert i.b == 2 E assert 1 == 2 E + where 1 = <failure_demo.test_attribute.<locals>.Foo object at 0xdeadbeef>.b failure_demo.py:128: AssertionError _________________________ test_attribute_instance __________________________ def test_attribute_instance(): class Foo: b = 1 > assert Foo().b == 2 E AssertionError: assert 1 == 2 E + where 1 = <failure_demo.test_attribute_instance.<locals>.Foo object at 0xdeadbeef>.b E + where <failure_demo.test_attribute_instance.<locals>.Foo object at 0xdeadbeef> = <class 'failure_demo.test_attribute_instance.<locals>.Foo'>() failure_demo.py:135: AssertionError __________________________ test_attribute_failure __________________________ def test_attribute_failure(): class Foo: def _get_b(self): raise Exception("Failed to get attrib") b = property(_get_b) i = Foo() > assert i.b == 2 failure_demo.py:146: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ self = <failure_demo.test_attribute_failure.<locals>.Foo object at 0xdeadbeef> def _get_b(self): > raise Exception("Failed to get attrib") E Exception: Failed to get attrib failure_demo.py:141: Exception _________________________ test_attribute_multiple __________________________ def test_attribute_multiple(): class Foo: b = 1 class Bar: b = 2 > assert Foo().b == Bar().b E AssertionError: assert 1 == 2 E + where 1 = <failure_demo.test_attribute_multiple.<locals>.Foo object at 0xdeadbeef>.b E + where <failure_demo.test_attribute_multiple.<locals>.Foo object at 0xdeadbeef> = <class 'failure_demo.test_attribute_multiple.<locals>.Foo'>() E + and 2 = <failure_demo.test_attribute_multiple.<locals>.Bar object at 0xdeadbeef>.b E + where <failure_demo.test_attribute_multiple.<locals>.Bar object at 0xdeadbeef> = <class 'failure_demo.test_attribute_multiple.<locals>.Bar'>() failure_demo.py:156: AssertionError __________________________ TestRaises.test_raises __________________________ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_raises(self): s = "qwe" > raises(TypeError, int, s) E ValueError: invalid literal for int() with base 10: 'qwe' failure_demo.py:166: ValueError ______________________ TestRaises.test_raises_doesnt _______________________ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_raises_doesnt(self): > raises(OSError, int, "3") E Failed: DID NOT RAISE <class 'OSError'> failure_demo.py:169: Failed __________________________ TestRaises.test_raise ___________________________ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_raise(self): > raise ValueError("demo error") E ValueError: demo error failure_demo.py:172: ValueError ________________________ TestRaises.test_tupleerror ________________________ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_tupleerror(self): > a, b = [1] # NOQA E ValueError: not enough values to unpack (expected 2, got 1) failure_demo.py:175: ValueError ______ TestRaises.test_reinterpret_fails_with_print_for_the_fun_of_it ______ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_reinterpret_fails_with_print_for_the_fun_of_it(self): items = [1, 2, 3] print("items is {!r}".format(items)) > a, b = items.pop() E TypeError: cannot unpack non-iterable int object failure_demo.py:180: TypeError --------------------------- Captured stdout call --------------------------- items is [1, 2, 3] ________________________ TestRaises.test_some_error ________________________ self = <failure_demo.TestRaises object at 0xdeadbeef> def test_some_error(self): > if namenotexi: # NOQA E NameError: name 'namenotexi' is not defined failure_demo.py:183: NameError ____________________ test_dynamic_compile_shows_nicely _____________________ def test_dynamic_compile_shows_nicely(): import importlib.util import sys src = "def foo():\n assert 1 == 0\n" name = "abc-123" spec = importlib.util.spec_from_loader(name, loader=None) module = importlib.util.module_from_spec(spec) code = compile(src, name, "exec") exec(code, module.__dict__) sys.modules[name] = module > module.foo() failure_demo.py:202: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ > ??? E AssertionError abc-123:2: AssertionError ____________________ TestMoreErrors.test_complex_error _____________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_complex_error(self): def f(): return 44 def g(): return 43 > somefunc(f(), g()) failure_demo.py:213: _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ failure_demo.py:10: in somefunc otherfunc(x, y) _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ a = 44, b = 43 def otherfunc(a, b): > assert a == b E assert 44 == 43 failure_demo.py:6: AssertionError ___________________ TestMoreErrors.test_z1_unpack_error ____________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_z1_unpack_error(self): items = [] > a, b = items E ValueError: not enough values to unpack (expected 2, got 0) failure_demo.py:217: ValueError ____________________ TestMoreErrors.test_z2_type_error _____________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_z2_type_error(self): items = 3 > a, b = items E TypeError: cannot unpack non-iterable int object failure_demo.py:221: TypeError ______________________ TestMoreErrors.test_startswith ______________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_startswith(self): s = "123" g = "456" > assert s.startswith(g) E AssertionError: assert False E + where False = <built-in method startswith of str object at 0xdeadbeef>('456') E + where <built-in method startswith of str object at 0xdeadbeef> = '123'.startswith failure_demo.py:226: AssertionError __________________ TestMoreErrors.test_startswith_nested ___________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_startswith_nested(self): def f(): return "123" def g(): return "456" > assert f().startswith(g()) E AssertionError: assert False E + where False = <built-in method startswith of str object at 0xdeadbeef>('456') E + where <built-in method startswith of str object at 0xdeadbeef> = '123'.startswith E + where '123' = <function TestMoreErrors.test_startswith_nested.<locals>.f at 0xdeadbeef>() E + and '456' = <function TestMoreErrors.test_startswith_nested.<locals>.g at 0xdeadbeef>() failure_demo.py:235: AssertionError _____________________ TestMoreErrors.test_global_func ______________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_global_func(self): > assert isinstance(globf(42), float) E assert False E + where False = isinstance(43, float) E + where 43 = globf(42) failure_demo.py:238: AssertionError _______________________ TestMoreErrors.test_instance _______________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_instance(self): self.x = 6 * 7 > assert self.x != 42 E assert 42 != 42 E + where 42 = <failure_demo.TestMoreErrors object at 0xdeadbeef>.x failure_demo.py:242: AssertionError _______________________ TestMoreErrors.test_compare ________________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_compare(self): > assert globf(10) < 5 E assert 11 < 5 E + where 11 = globf(10) failure_demo.py:245: AssertionError _____________________ TestMoreErrors.test_try_finally ______________________ self = <failure_demo.TestMoreErrors object at 0xdeadbeef> def test_try_finally(self): x = 1 try: > assert x == 0 E assert 1 == 0 failure_demo.py:250: AssertionError ___________________ TestCustomAssertMsg.test_single_line ___________________ self = <failure_demo.TestCustomAssertMsg object at 0xdeadbeef> def test_single_line(self): class A: a = 1 b = 2 > assert A.a == b, "A.a appears not to be b" E AssertionError: A.a appears not to be b E assert 1 == 2 E + where 1 = <class 'failure_demo.TestCustomAssertMsg.test_single_line.<locals>.A'>.a failure_demo.py:261: AssertionError ____________________ TestCustomAssertMsg.test_multiline ____________________ self = <failure_demo.TestCustomAssertMsg object at 0xdeadbeef> def test_multiline(self): class A: a = 1 b = 2 > assert ( A.a == b ), "A.a appears not to be b\nor does not appear to be b\none of those" E AssertionError: A.a appears not to be b E or does not appear to be b E one of those E assert 1 == 2 E + where 1 = <class 'failure_demo.TestCustomAssertMsg.test_multiline.<locals>.A'>.a failure_demo.py:268: AssertionError ___________________ TestCustomAssertMsg.test_custom_repr ___________________ self = <failure_demo.TestCustomAssertMsg object at 0xdeadbeef> def test_custom_repr(self): class JSON: a = 1 def __repr__(self): return "This is JSON\n{\n 'foo': 'bar'\n}" a = JSON() b = 2 > assert a.a == b, a E AssertionError: This is JSON E { E 'foo': 'bar' E } E assert 1 == 2 E + where 1 = This is JSON\n{\n 'foo': 'bar'\n}.a failure_demo.py:281: AssertionError ============================ 44 failed in 0.12s ============================

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