作者：某某白米飯

來源：Python 技術(shù)

在寫算法的時候，總是要每行每個變量一個個的 debug，有時候還要多寫幾個 print，一道算法題要花好長時間才能理解。pysnooper 模塊可以把在運行中變量值都給打印出來。

模塊安裝

pip3 install pysnooper

簡單例子

下面是道簡單的力扣算法題作為一個簡單的例子

import pysnooper

@pysnooper.snoop()
def longestCommonPrefix(strs):
    res = ''
    for i in zip(*strs):
        print(i)
        if len(set(i)) == 1:
            res += i[0]
        else
            break
    return res
 
if __name__ == 'main':
    longestCommonPrefix(["flower","flow","flight"])

結(jié)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[0]
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[0]
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

我們可以看到 pysnooper 把整個執(zhí)行程序都記錄了下來，其中包括行號， 行內(nèi)容，變量的結(jié)果等情況，我們很容易的就看懂了這個算法的真實情況。并且不需要再使用 debug 和 print 調(diào)試代碼。很是省時省力，只需要在方法上面加一行 @pysnooper.snoop()。

復雜使用

pysnooper 包含了多個參數(shù)，一起來看看吧

output

output 默認輸出到控制臺，設置后輸出到文件，在服務器中運行的時候，特定的時間出現(xiàn)代碼問題就很容易定位錯誤了，不然容易抓瞎。小編在實際中已經(jīng)被這種問題困擾了好幾次，每次都掉好多頭發(fā)。

@pysnooper.snoop('D:pysnooper.log')
def longestCommonPrefix(strs):

示例結(jié)果:

watch 和 watch_explode

watch 用來設置跟蹤的非局部變量，watch_explode 表示設置的變量都不監(jiān)控，只監(jiān)控沒設置的變量，正好和 watch 相反。

index = 1
@pysnooper.snoop(watch=('index'))
def longestCommonPrefix(strs):

示例結(jié)果

沒有加 watch 參數(shù)

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 參數(shù)，就會有一個 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 監(jiān)控函數(shù)的深度

@pysnooper.snoop(depth=2)
def longestCommonPrefix(strs):
    otherMethod()

示例結(jié)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 = ''

監(jiān)控的結(jié)果顯示，當監(jiān)控到調(diào)用的函數(shù)的時候，記錄上會加上縮進，并將它的局部變量和返回值打印處理。

prefix

prefix 輸出內(nèi)容的前綴

@pysnooper.snoop(prefix='-------------')
def longestCommonPrefix(strs):

示例結(jié)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 代碼運行的時間

@pysnooper.snoop(relative_time=True)
def longestCommonPrefix(strs):

示例結(jié)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 輸出的變量和異常的最大長度，默認是 100 個字符，超過 100 個字符就會被截斷，可以設置為 max_variable_length=None 不截斷輸出

@pysnooper.snoop(max_variable_length=5)
def longestCommonPrefix(strs):

示例結(jié)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 = (...)

總結(jié)

本文介紹了怎么使用 pysnooper 工具，pysnooper 不僅可以少一些 debug 和 print，更能幫助理解算法題。