如何在正确的 if 语句之前打印第 X 行

Question

我对 Python 还是很陌生，对我通过众多网页找到的内容只有零碎的知识。

话虽如此，我正在尝试在文件（约 10k 行）中搜索我编写的一组类似“过滤器”的条件，然后我希望它打印符合条件的行和一行X 之前的行数。

我创建了以下脚本来打开所述文件，逐行迭代，并将符合过滤条件的行打印到输出文件，但是我不知道如何将其合并到当前脚本中。

import os

output_file = 'Output.txt'
filename = 'BigFile.txt'                 

numLines = 0
numWords = 0
numChrs = 0
numMes = 0

f1 = open(output_file, 'w')
print 'Output File has been Opened'

with open(filename, 'r') as file:
   for line in file:
      wordsList = line.split()
      numLines += 1
      numWords += len(wordsList)
      numChrs += len(line)

      if "X" in line and "Y" not in line and "Z" in line:
          numMes += 1
          print >>f1, line
          print 'Object found and Catalogued in Output.txt'                          

print "Lines: %i\nWords: %i\nCharacters: %i" % (numLines, numWords, numChrs)
print >>f1, "Lines: %i\nWords: %i\nCharacters: %i" % (numLines, numWords, numChrs)

print "There are a total of %i thing in this file" % (numMes)
print >>f1, "There are a total of %i things in this file" % (numMes)

f1.close()

print 'Output Files have been Closed'

我的第一个猜测是使用line.enumeration，但我认为我不能只声明lines - 5 来打印lines 之前的第5 行：

lines = f1.enumeration()
if "blah blah" in line and "so so" not in line:
    print >>f1, lines
    print >>f1, [lines - 5]

最好的部分还没有到来，因为我必须获取 Output.txt 文件并与另一个文件进行比较以输出两个文件中的匹配条件......但一次一步，对吗？

-还可以随意添加“正确”技术的简介......我相信这个脚本可以写成更好的方式，所以请教育我做错了什么。

提前感谢您的帮助！

---

更新： 感谢以下帮助，已成功实施修复：

import os

output_file = 'Output.txt'
filename = 'BigFile.txt'                 

numLines = 0
numWords = 0
numChrs = 0

numMulMes = 0

last5 = []

f1 = open(output_file, 'w')
print 'Output Files have been Opened'

with open(filename, 'r') as file:
    for line in file:
        wordsList = line.split()
        numLines += 1
        numWords += len(wordsList)
        numChrs += len(line)
        last5[:] = last5[-5:]+[line] 
        if "X" in line and "Y" not in line and "Z" not in line:
            del last5[1:5]           ###the missing piece of the puzzle!
            numMulMes += 1
            print >>f1, last5
            print 'Object found and Catalogued in Output.txt'

print "Lines: %i\nWords: %i\nCharacters: %i" % (numLines, numWords, numChrs)
print >>f1, "Lines: %i\nWords: %i\nCharacters: %i" % (numLines, numWords, numChrs)

print "There are a total of %i messages in this file" % (numMulMes)
print >>f1, "There are a total of %i messages in this file" % (numMulMes)

f1.close()
f3.close()

print 'Output Files have been Closed'

我一直试图通过另一个单独的脚本来修改输出文件，并且在很长一段时间内我都在与 str 与 lst 操作和错误问题作斗争。只是决定回到原来的剧本，一时兴起把它扔进去，维奥拉。

感谢您将我推向正确的方向，从那里很容易弄清楚！

Answer 1

您自己解决了大部分问题（计算单词、行数、行号等） - 在浏览文件时，您可以简单地记住最后 n 行。

例子：

t = """"zero line
one line
two line
three line
four line 
five line 
six line
seven line 
eight line
""" 

last5 = [] # memory cell
for l in t.split("\n"):  # similar to your for line in file: 
    last5[:] = last5[-4:]+[l] # keep last 4 and add current line, inplace list mod 

    if "six" in l:
        print last5

您也可以查看deque并指定一个最大长度（您需要导入它）

from collections import deque

last5 = deque(maxlen=5)
for l in t.split("\n"): 
    last5.append(l) # will automatically only keep 5 (maxlen)

    if "six" in l:
        print last5

输出：

 # list version
 ['two line', 'three line', 'four line ', 'five line ', 'six line'] 

 # deque version
 deque(['two line', 'three line', 'four line ', 'five line ', 'six line'], maxlen=5) 

Answer 2

这里与@PatricArtner 建议的解决方案相同，但使用了环形缓冲区。它可能（或可能不会，我没有检查过）处理大文件的速度更快。 这个想法很简单：我们可以创建一个具有所需大小（您应该保留的行数）的列表和当前记录位置的计数器cnt。对于每一行，我们应该将 cnt 增加一，并按缓冲区大小取模。因此cnt 在列表中循环。例如，如果列表大小为 5 cnt = (cnt+1)%5 将给出 0 1 2 3 4 0 1 2 等等。 cnt 的每一步都将指向我们列表中最旧的数据，这些数据将被新数据替换。下面是一个实现的例子。

t = """"zero line
six line - surprize 
one line
two line
three line
four line 
five line 
six line
seven line 
eight line
""" 


last5 = [None,None,None,None,None]
cnt = 0
for l in t.split("\n"):
  last5[cnt]=l
  if 'six' in l:
    print last5[(cnt+1)%5]
    print last5[(cnt+2)%5]
    print last5[(cnt+3)%5]
    print last5[(cnt+4)%5]
    print last5[(cnt+0)%5]
    print
  cnt = (cnt+1)%5

输出很简单：

None
None
None
"zero line
six line - surprize 

two line
three line
four line 
five line 
six line

注意：如果您从一个文件中读取，并且该文件很大并且您需要保留的字符串很大（例如，基因序列）并且您的情况不会经常触发，要聪明，不要在内存中保留字符串。在文件中创建最后一个字符串开始的位置列表，并在需要时重新读取它们。下面是一个如何让它变得非常快的例子......

from numpy import random as rnd

print "Creating the file ...."
DNA=["G","C","T","A"]
with open("bigdatafile","w") as fd:
    for i in xrange(5000):
        fd.write("".join([ DNA[rnd.randint(4)] for x in xrange(2000)])+"\n")
print "DONE"
print
print "SEARCHING GGGGGGGGGGG"
last5, cnt = [0,0,0,0,0], 1
with open("bigdatafile","r") as fd:
    for i,l in enumerate(fd.readlines()):
        last5[cnt] = last5[(cnt+4)%5]+len(l)
        if "GGGGGGGGGGG" in l:
            print "FIND!"
            fd.seek(last5[(cnt+1)%5])
            print fd.read(last5[cnt]-last5[(cnt+1)%5])
        cnt = (cnt+1)%5

Answer 3

我没有写入文件，而是将内容输出到字典。处理完整个文件后，摘要数据字典将以json 的形式转储到文件中。使用 Artner 的测试文件。

import os
import json

output_file = 'Output.txt'
filename = 'BigFile.txt'                 

#initiate output container
outDict = {}
for fields in ['numLines', 'numWords', 'numChrs', 'numMes']:
    outDict[fields] = 0

outDict['lineNum'] = []    

with open(filename, 'r') as file:
    for line in file:
      wordsList = line.strip().split("\s")
      outDict['numLines'] += 1
      outDict['numWords'] += len(wordsList)
      outDict['numChrs'] += len(line)

      #find items in the line
      if "t" in line:
          outDict['numMes'] += 1
          #save line number
          outDict['lineNum'].append(outDict['numLines']) 
          #save line content
          outDict['lineList'].append(line)

#record output          
with open(output_file, 'w') as f1:
    f1.write(json.dumps(outDict))    

##print lines of desire
#x number of lines before
x=5    
with open(filename, 'r') as file:
    for i, line in enumerate(file):
        #iterate over line numbers for which condition is met
        for j in range(0,len(outDict['lineNum'])):
            #if line number is between found line num and line num minus x, print
            if (outDict['lineNum'][j]-x) <= i <= outDict['lineNum'][j]:
                print(line)

Answer 4

既然我在comments 中提到过，下面是如何使用grep 的context line control 功能在*nix 机器上做同样的事情。

首先假设您有以下文本文件test.txt：

zero line
one line
two line
three line
four line 
five line 
six line
seven line 
eight line

如果您想在匹配之前获得N 行，您可以使用-B 选项。比如"six"之前的5行：

$ grep -B 5 six test.txt 
one line
two line
three line
four line 
five line 
six line

还有-A 选项可用于在匹配后获取N 行，-C 可用于获取N 之前和之后的行。