3、pytest中文文件--編寫斷言
目錄
- 編寫斷言
- 使用assert編寫斷言
- 編寫觸發期望異常的斷言
- 特殊資料結構比較時的優化
- 為失敗斷言新增自定義的說明
- 關於斷言自省的細節
- 複寫快取檔案
- 去使能斷言自省
編寫斷言
使用assert編寫斷言
pytest允許你使用python標準的assert
表示式寫斷言;
例如,你可以這樣做:
# test_sample.py def func(x): return x + 1 def test_sample(): assert func(3) == 5
如果這個斷言失敗,你會看到func(3)
實際的返回值:
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2) λ pytest test_sample.py ================================================= test session starts ================================================= platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0 rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini collected 1 item test_sample.py F [100%] ====================================================== FAILURES ======================================================= _____________________________________________________ test_sample _____________________________________________________ def test_sample(): > assert func(3) == 5 E assert 4 == 5 E + where 4 = func(3) test_sample.py:28: AssertionError ================================================== 1 failed in 0.05s ==================================================
pytest支援顯示常見的python子表示式的值,包括:呼叫、屬性、比較、二進位制和一元運算子等(參考pytest支援的python失敗時報告的演示);
這允許你在沒有模版程式碼參考的情況下,可以使用的python的資料結構,而無須擔心丟失自省的問題;
同時,你也可以為斷言指定了一條說明資訊,用於失敗時的情況說明:
assert a % 2 == 0, "value was odd, should be even"
編寫觸發期望異常的斷言
你可以使用pytest.raises()
作為上下文管理器,來編寫一個觸發期望異常的斷言:
import pytest def myfunc(): raise ValueError("Exception 123 raised") def test_match(): with pytest.raises(ValueError): myfunc()
當用例沒有返回ValueError
或者沒有異常返回時,斷言判斷失敗;
如果你希望同時訪問異常的屬性,可以這樣:
import pytest
def myfunc():
raise ValueError("Exception 123 raised")
def test_match():
with pytest.raises(ValueError) as excinfo:
myfunc()
assert '123' in str(excinfo.value)
其中,excinfo
是ExceptionInfo
的一個例項,它封裝了異常的資訊;常用的屬性包括:.type
、.value
和.traceback
;
注意:在上下文管理器的作用域中,
raises
程式碼必須是最後一行,否則,其後面的程式碼將不會執行;所以,如果上述例子改成:def test_match(): with pytest.raises(ValueError) as excinfo: myfunc() assert '456' in str(excinfo.value)
則測試將永遠成功,因為
assert '456' in str(excinfo.value)
並不會執行;
你也可以給pytest.raises()
傳遞一個關鍵字引數match
,來測試異常的字串表示str(excinfo.value)
是否符合給定的正則表示式(和unittest中的TestCase.assertRaisesRegexp
方法類似):
import pytest
def myfunc():
raise ValueError("Exception 123 raised")
def test_match():
with pytest.raises((ValueError, RuntimeError), match=r'.* 123 .*'):
myfunc()
pytest實際呼叫的是re.search()
方法來做上述檢查;並且,pytest.raises()
也支援檢查多個期望異常(以元組的形式傳遞引數),我們只需要觸發其中任意一個;
pytest.raises
還有另外的一種使用形式:
首先,我們來看一下它在原始碼中的定義:
# _pytest/python_api.py
def raises( # noqa: F811
expected_exception: Union["Type[_E]", Tuple["Type[_E]", ...]],
*args: Any,
match: Optional[Union[str, "Pattern"]] = None,
**kwargs: Any
) -> Union["RaisesContext[_E]", Optional[_pytest._code.ExceptionInfo[_E]]]:
它接收一個位置引數expected_exception
,一組可變引數args
,一個關鍵字引數match
和一組關鍵字引數kwargs
;
接著往下看:
# _pytest/python_api.py
if not args:
if kwargs:
msg = "Unexpected keyword arguments passed to pytest.raises: "
msg += ", ".join(sorted(kwargs))
msg += "\nUse context-manager form instead?"
raise TypeError(msg)
return RaisesContext(expected_exception, message, match)
else:
func = args[0]
if not callable(func):
raise TypeError(
"{!r} object (type: {}) must be callable".format(func, type(func))
)
try:
func(*args[1:], **kwargs)
except expected_exception as e:
# We just caught the exception - there is a traceback.
assert e.__traceback__ is not None
return _pytest._code.ExceptionInfo.from_exc_info(
(type(e), e, e.__traceback__)
)
fail(message)
其中,args
如果存在,那麼它的第一個引數必須是一個可呼叫的物件,否則會報TypeError
異常;同時,它會把剩餘的args
引數和所有kwargs
引數傳遞給這個可呼叫物件,然後檢查這個物件執行之後是否觸發指定異常;
所以我們有了一種新的寫法:
pytest.raises(ZeroDivisionError, lambda x: 1/x, 0)
# 或者
pytest.raises(ZeroDivisionError, lambda x: 1/x, x=0)
這個時候如果你再傳遞match
引數,是不生效的,因為它只有在if not args:
的時候生效;
另外,pytest.mark.xfail()
也可以接收一個raises
引數,來判斷用例是否因為一個具體的異常而導致失敗:
@pytest.mark.xfail(raises=IndexError)
def test_f():
f()
如果f()
觸發一個IndexError
異常,則用例標記為xfailed;如果沒有,則正常執行f()
;
注意:如果
f()
測試成功,用例的結果是xpassed,而不是passed;
pytest.raises
適用於檢查由程式碼故意引發的異常;而@pytest.mark.xfail()
更適合用於記錄一些未修復的Bug;
特殊資料結構比較時的優化
# test_special_compare.py
def test_set_comparison():
set1 = set('1308')
set2 = set('8035')
assert set1 == set2
def test_long_str_comparison():
str1 = 'show me codes'
str2 = 'show me money'
assert str1 == str2
def test_dict_comparison():
dict1 = {
'x': 1,
'y': 2,
}
dict2 = {
'x': 1,
'y': 1,
}
assert dict1 == dict2
上面,我們檢查了三種資料結構的比較:集合、字串和字典;
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2)
λ pytest test_special_compare.py
================================================= test session starts =================================================
platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini
collected 3 items
test_special_compare.py FFF [100%]
====================================================== FAILURES ======================================================= _________________________________________________ test_set_comparison _________________________________________________
def test_set_comparison():
set1 = set('1308')
set2 = set('8035')
> assert set1 == set2
E AssertionError: assert {'0', '1', '3', '8'} == {'0', '3', '5', '8'}
E Extra items in the left set:
E '1'
E Extra items in the right set:
E '5'
E Use -v to get the full diff
test_special_compare.py:26: AssertionError
______________________________________________ test_long_str_comparison _______________________________________________
def test_long_str_comparison():
str1 = 'show me codes'
str2 = 'show me money'
> assert str1 == str2
E AssertionError: assert 'show me codes' == 'show me money'
E - show me codes
E ? ^ ^ ^
E + show me money
E ? ^ ^ ^
test_special_compare.py:32: AssertionError
________________________________________________ test_dict_comparison _________________________________________________
def test_dict_comparison():
dict1 = {
'x': 1,
'y': 2,
}
dict2 = {
'x': 1,
'y': 1,
}
> assert dict1 == dict2
E AssertionError: assert {'x': 1, 'y': 2} == {'x': 1, 'y': 1}
E Omitting 1 identical items, use -vv to show
E Differing items:
E {'y': 2} != {'y': 1}
E Use -v to get the full diff
test_special_compare.py:44: AssertionError
================================================== 3 failed in 0.09s ==================================================
針對一些特殊的資料結構間的比較,pytest對結果的顯示做了一些優化:
- 集合、列表等:標記出第一個不同的元素;
- 字串:標記出不同的部分;
- 字典:標記出不同的條目;
更多例子參考pytest支援的python失敗時報告的演示
為失敗斷言新增自定義的說明
# test_foo_compare.py
class Foo:
def __init__(self, val):
self.val = val
def __eq__(self, other):
return self.val == other.val
def test_foo_compare():
f1 = Foo(1)
f2 = Foo(2)
assert f1 == f2
我們定義了一個Foo
物件,也複寫了它的__eq__()
方法,但當我們執行這個用例時:
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2)
λ pytest test_foo_compare.py
================================================= test session starts =================================================
platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini
collected 1 item
test_foo_compare.py F [100%]
====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________
def test_foo_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E assert <src.test_foo_compare.Foo object at 0x0000020E90C4E978> == <src.test_foo_compare.Foo object at 0x0000020E90C4E630>
test_foo_compare.py:37: AssertionError
================================================== 1 failed in 0.04s ==================================================
並不能直觀的看出來失敗的原因;
在這種情況下,我們有兩種方法來解決:
- 複寫
Foo
的__repr__()
方法:
def __repr__(self):
return str(self.val)
我們再執行用例:
luyao@NJ-LUYAO-T460 /d/Personal Files/Python/pytest-chinese-doc/src (5.1.2)
λ pytest test_foo_compare.py
================================================= test session starts =================================================
platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini
collected 1 item
test_foo_compare.py F [100%]
====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________
def test_foo_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E assert 1 == 2
test_foo_compare.py:37: AssertionError
================================================== 1 failed in 0.06s ==================================================
這時,我們能看到失敗的原因是因為1 == 2
不成立;
至於
__str__()
和__repr__()
的區別,可以參考StackFlow上的這個問題中的回答:https://stackoverflow.com/questions/1436703/difference-between-str-and-repr
- 使用
pytest_assertrepr_compare
這個鉤子方法新增自定義的失敗說明
# conftest.py
from .test_foo_compare import Foo
def pytest_assertrepr_compare(op, left, right):
if isinstance(left, Foo) and isinstance(right, Foo) and op == "==":
return [
"比較兩個Foo例項:", # 頂頭寫概要
" 值: {} != {}".format(left.val, right.val), # 除了第一個行,其餘都可以縮排
]
再次執行:
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2)
λ pytest test_foo_compare.py
================================================= test session starts =================================================
platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini
collected 1 item
test_foo_compare.py F [100%]
====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________
def test_foo_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E assert 比較兩個Foo例項:
E 值: 1 != 2
test_foo_compare.py:37: AssertionError
================================================== 1 failed in 0.05s ==================================================
我們會看到一個更友好的失敗說明;
關於斷言自省的細節
當斷言失敗時,pytest為我們提供了非常人性化的失敗說明,中間往往夾雜著相應變數的自省資訊,這個我們稱為斷言的自省;
那麼,pytest是如何做到這樣的:
- pytest發現測試模組,並引入他們,與此同時,pytest會複寫斷言語句,新增自省資訊;但是,不是測試模組的斷言語句並不會被複寫;
複寫快取檔案
pytest會把被複寫的模組儲存到本地作為快取使用,你可以通過在測試用例的根資料夾中的conftest.py
裡新增如下配置:
import sys
sys.dont_write_bytecode = True
來禁止這種行為;
但是,它並不會妨礙你享受斷言自省的好處,只是不會在本地儲存.pyc
檔案了。
去使能斷言自省
你可以通過一下兩種方法:
- 在需要去使能模組的
docstring
中新增PYTEST_DONT_REWRITE
字串; - 執行pytest時,新增
--assert=plain
選項;
我們來看一下去使能後的效果:
/d/Personal Files/Python/pytest-chinese-doc/src (5.1.2)
λ pytest test_foo_compare.py --assert=plain
================================================= test session starts =================================================
platform win32 -- Python 3.7.3, pytest-5.1.2, py-1.8.0, pluggy-0.12.0
rootdir: D:\Personal Files\Python\pytest-chinese-doc\src, inifile: pytest.ini
collected 1 item
test_foo_compare.py F [100%]
====================================================== FAILURES ======================================================= __________________________________________________ test_foo_compare ___________________________________________________
def test_foo_compare():
f1 = Foo(1)
f2 = Foo(2)
> assert f1 == f2
E AssertionError
test_foo_compare.py:37: AssertionError
================================================== 1 failed in 0.05s ==================================================
斷言失敗時的資訊就非常的不完整了,我們幾乎看不出任何有用的Debug信