better-exceptions 可以在python异常发生时显示更多信息,帮助我们更好的发现问题.
简介
介绍这个库前,我们先看一个简单的例子(example.py),代码如下:
# -*- encoding: utf-8 -*-
def get_student_infos(logs):
student_infos = []
for log in logs:
name, catgory, grade = log.split(‘ ‘)
student_infos.append({
‘name’: name,
‘catgory’: catgory,
‘grade’: grade,
})
return student_infos
if __name__ == ‘__main__’:
exam_logs = [
‘zhangsan math 60’,
‘lisi english 80’,
‘wangwu chinese 90’,
‘qianliu music’
]
get_student_infos(exam_logs)
运行输出:
Traceback (most recent call last):
File “example.py”, line 23, in <module>
get_student_infos(exam_logs)
File “example.py”, line 7, in get_student_infos
name, catgory, grade = log.split(‘ ‘)
ValueError: not enough values to unpack (expected 3, got 2)
如果我们给example.py增加一行代码import better_exceptions,再执行,输入如下:
Traceback (most recent call last):
File “example.py”, line 24, in <module>
get_student_infos(exam_logs)
│ └ [‘zhangsan math 60’, ‘lisi english 80’, ‘wangwu chinese 90’, ‘qianliu music’]
└ <function get_student_infos at 0x7f147344ee18>
File “example.py”, line 8, in get_student_infos
name, catgory, grade = log.split(‘ ‘)
│ │ │ └ ‘qianliu music’
│ │ └ ’90’
│ └ ‘chinese’
└ ‘wangwu’
ValueError: not enough values to unpack (expected 3, got 2)
看到了吗,加上import better_exceptions后, 异常时会将调用栈每一层用的变量值打印出来, 和普通异常时输出有比有什么好处呢,然我们来回忆一下,
- 没有better_exceptions时是这样对待异常的:
- 看到一个异常, 能看到异常的类型, 但是无法直接给出错误原因
- 追踪异常时, 往往得修改代码,将关键变量打印出来. 如果是线上环境, 貌似没什么好办法,只能上线上改一改.
- 有better_exceptions时:
- 从异常时打印出来的信息可以清晰的看到异常发生时变量的值,可很容易定位问题.
是不是有小伙伴会想, 要是某些变量特别大(比如有几万个元素的列表),这样会造成日志很大的.确实存在这样的问题,不过这个库的作者已经够给解决方案了: 加上这句better_exceptions.MAX_LENGTH = 字符数控制字符的个数, 对于上面的例子加上better_exceptions.MAX_LENGTH = 20这句输出如下:
Traceback (most recent call last):
File “example.py”, line 27, in <module>
get_student_infos(exam_logs)
│ └ [‘zhangsan math 60’,…
└ <function get_studen…
File “example.py”, line 10, in get_student_infos
name, catgory, grade = log.split(‘ ‘)
│ │ │ └ ‘qianliu music’
│ │ └ ’90’
│ └ ‘chinese’
└ ‘wangwu’
ValueError: not enough values to unpack (expected 3, got 2)
看到了吗,只是简单的再代码上加上一句import better_exceptions就有如此神奇的效果. 但是, 这个库目前只会在控制台打印出这样的错误信息, 下面说一下怎么与logging、django集成.
logging接入
先简单说一下,:
- logging模块在输出一个异常的时候,会调用handler的formater的formatException函数来格式化异常.
- better_exceptions有个format_exception方法会将异常调用栈变量值输出,就是 上面例子的那种输出.
看代码:
# -*- encoding: utf-8 -*-
import logging
from better_exceptions import format_exception
logger = logging.getLogger()
formatter = logging.Formatter(‘%(asctime)s – %(name)s – %(levelname)s – %(message)s’)
formatter.formatException = lambda exc_info: format_exception(*exc_info)
file_handler = logging.FileHandler(“example.log”)
file_handler.setLevel(logging.DEBUG)
file_handler.setFormatter(formatter)
logger.addHandler(file_handler)
def get_student_infos(logs):
student_infos = []
for log in logs:
name, catgory, grade = log.split(‘ ‘)
student_infos.append({
‘name’: name,
‘catgory’: catgory,
‘grade’: grade,
})
return student_infos
if __name__ == ‘__main__’:
exam_logs = [
‘zhangsan math 60’,
‘lisi english 80’,
‘wangwu chinese 90’,
‘qianliu music’
]
try:
get_student_infos(exam_logs)
except Exception as e:
logger.exception(e)
查看example.log文件输出:
2017-09-03 22:07:47,121 – root – ERROR – not enough values to unpack (expected 3, got 2)
Traceback (most recent call last):
File “log_example.py”, line 39, in <module>
get_student_infos(exam_logs)
│ └ [‘zhangsan math 60’, ‘lisi english 80’, ‘wangwu chinese 90’, ‘qianliu music’]
└ <function get_student_infos at 0x7fd2e7fe46a8>
File “log_example.py”, line 22, in get_student_infos
name, catgory, grade = log.split(‘ ‘)
│ │ │ └ ‘qianliu music’
│ │ └ ’90’
│ └ ‘chinese’
└ ‘wangwu’
ValueError: not enough values to unpack (expected 3, got 2)
django接入
思路和logging接入一样的. 例如有如下django项目:
☁ test_better_exceptions_django tree
.
├── db.sqlite3
├── manage.py
└── test_better_exceptions_django
├── fm.py
├── __init__.py
├── settings.py
├── urls.py
├── views.py
└── wsgi.py
两处修改:
- 增加一个py文件:
- import logging
from better_exceptions import format_exception
class ExceptionFormatter(logging.Formatter):
def formatException(self, ei):
return format_exception(*ei) - 修改py的LOGGING配置:
- LOGGING = {
‘version’: 1,
‘disable_existing_loggers’: True,
‘formatters’: {
‘simple’: {
‘()’: ‘test_better_exceptions_django.fm.ExceptionFormatter’,
‘format’: ‘%(levelname)s %(message)s’
},
},
‘handlers’: {
‘console’: {
‘level’: ‘INFO’,
‘class’: ‘logging.StreamHandler’,
‘formatter’: ‘simple’
},
‘file’: {
‘level’: ‘ERROR’,
‘class’: ‘logging.FileHandler’,
‘filename’: os.path.join(BASE_DIR, “error.log”),
‘formatter’: ‘simple’}
},
‘loggers’: {
‘django’: {
‘handlers’: [‘console’, ‘file’],}
}
} - 这里主要是自定义了一个formatters.
我是在请求里故意出发了一个异常,代码如下:
- py:
- fromconf.urls import url
from django.contrib import admin
from . import viewsurlpatterns = [
url(r’^$’, views.index, name=’index’),
] - py:
- fromhttp import HttpResponse
import logging
logger = logging.getLogger(‘django’)
def index(request):
a, b = 2, 0
try:
a / b
except Exception as e:
logger.exception(e)
return HttpResponse(“You have an excepion, see console and log”)
接着打开浏览器,可以看到根目录下error.log文件内容:
☁ test_better_exceptions_django cat error.log
ERROR division by zero
Traceback (most recent call last):
File “/vagrant_data/better_exceptions/test_better_exceptions_django/test_better_exceptions_django/views.py”, line 12, in index
a / b
│ └ 0
└ 2
ZeroDivisionError: division by zero