文 | 某某白米饭
来源：Python 技术「ID: pythonall」
在写算法的时候，总是要每行每个变量一个个的 debug，有时候还要多写几个 print，一道算法题要花好长时间才能理解。pysnooper 模块可以把在运行中变量值都给打印出来。
模块安装
pip3 install pysnooper
简单例子
下面是道简单的力扣算法题作为一个简单的例子
import pysnooper
@pysnooper.snoop()
def longestCommonPrefix(strs):
res = ''
for i in zip(*strs):
print(i)
if len(set(i)) == 1:
res += i[]
else
break
return res
if __name__ == 'main':
longestCommonPrefix(["flower","flow","flight"])
结果：
3:38:25.863579 call 4 def longestCommonPrefix(strs):
23:38:25.864474 line 5 res = ''
New var:....... res = ''
23:38:25.864474 line 6 for i in zip(*strs):
New var:....... i = ('f', 'f', 'f')
23:38:25.865479 line 7 print(i)
('f', 'f', 'f')
23:38:25.866471 line 8 if len(set(i))==1:
23:38:25.866471 line 9 res+=i[]
Modified var:.. res = 'f'
23:38:25.866471 line 6 for i in zip(*strs):
Modified var:.. i = ('l', 'l', 'l')
23:38:25.866471 line 7 print(i)
('l', 'l', 'l')
23:38:25.867468 line 8 if len(set(i))==1:
23:38:25.867468 line 9 res+=i[]
Modified var:.. res = 'fl'
23:38:25.868476 line 6 for i in zip(*strs):
Modified var:.. i = ('o', 'o', 'i')
23:38:25.868476 line 7 print(i)
('o', 'o', 'i')
23:38:25.869463 line 8 if len(set(i))==1:
23:38:25.869463 line 11 break
23:38:25.869463 line 12 return res
23:38:25.869463 return 12 return res
Return value:.. 'fl'
Elapsed time: 00:00:00.008201
我们可以看到 pysnooper 把整个执行程序都记录了下来，其中包括行号， 行内容，变量的结果等情况，我们很容易的就看懂了这个算法的真实情况。并且不需要再使用 debug 和 print 调试代码。很是省时省力，只需要在方法上面加一行 @pysnooper.snoop()。
复杂使用
pysnooper 包含了多个参数，一起来看看吧
output
output 默认输出到控制台，设置后输出到文件，在服务器中运行的时候，特定的时间出现代码问题就很容易定位错误了，不然容易抓瞎。小编在实际中已经被这种问题困扰了好几次，每次都掉好多头发。
@pysnooper.snoop('D:\pysnooper.log')
def longestCommonPrefix(strs):
示例结果:
watch 和 watch_explode
watch 用来设置跟踪的非局部变量，watch_explode 表示设置的变量都不监控，只监控没设置的变量，正好和 watch 相反。
index = 1
@pysnooper.snoop(watch=('index'))
def longestCommonPrefix(strs):
示例结果
没有加 watch 参数
Starting var:.. strs = ['flower', 'flow', 'flight']
00:12:33.715367 call 5 def longestCommonPrefix(strs):
00:12:33.717324 line 7 res = ''
New var:....... res = ''
加了watch 参数，就会有一个 Starting var:.. index
Starting var:.. strs = ['flower', 'flow', 'flight']
Starting var:.. index = 1
00:10:35.151036 call 5 def longestCommonPrefix(strs):
00:10:35.151288 line 7 res = ''
New var:....... res = ''
depth
depth 监控函数的深度
@pysnooper.snoop(depth=2)
def longestCommonPrefix(strs):
otherMethod()
示例结果
Starting var:.. strs = ['flower', 'flow', 'flight']
00:20:54.059803 call 5 def longestCommonPrefix(strs):
00:20:54.059803 line 6 otherMethod()
00:20:54.060785 call 16 def otherMethod():
00:20:54.060785 line 17 x = 1
New var:....... x = 1
00:20:54.060785 line 18 x = x + 1
Modified var:.. x = 2
00:20:54.060785 return 18 x = x + 1
Return value:.. None
00:20:54.061782 line 7 res = ''
监控的结果显示，当监控到调用的函数的时候，记录上会加上缩进，并将它的局部变量和返回值打印处理。
prefix
prefix 输出内容的前缀
@pysnooper.snoop(prefix='-------------')
def longestCommonPrefix(strs):
示例结果
-------------Starting var:.. strs = ['flower', 'flow', 'flight']
-------------00:39:13.986741 call 5 def longestCommonPrefix(strs):
-------------00:39:13.987218 line 6 res = ''
relative_time
relative_time 代码运行的时间
@pysnooper.snoop(relative_time=True)
def longestCommonPrefix(strs):
示例结果
Starting var:.. strs = ['flower', 'flow', 'flight']
00:00:00.000000 call 5 def longestCommonPrefix(strs):
00:00:00.001998 line 6 res = ''
New var:....... res = ''
00:00:00.001998 line 7 for i in zip(*strs):
max_variable_length
max_variable_length 输出的变量和异常的大长度，默认是 100 个字符，超过 100 个字符就会被截断，可以设置为 max_variable_length=None 不截断输出
@pysnooper.snoop(max_variable_length=5)
def longestCommonPrefix(strs):
示例结果
Starting var:.. strs = [...]
00:56:44.343639 call 5 def longestCommonPrefix(strs):
00:56:44.344696 line 6 res = ''
New var:....... res = ''
00:56:44.344696 line 7 for i in zip(*strs):
New var:....... i = (...)
总结
本文介绍了怎么使用 pysnooper 工具，pysnooper 不仅可以少一些 debug 和 print，更能帮助理解算法题。
