我们在处理小的文本文件时一般使用.read()、.readline() 和 .readlines()方法，但是当我们的文件有2个G，5个G甚至更大时，用这些方法内存就直接爆掉了。
对一般文件，如果文件很小，read()一次性读取最方便；如果不能确定文件大小，反复调用read(size)比较保险；如果是配置文件，调用readlines()最方便。
读取大文件方法：
一、Read In Chunks
把大文件分成小块来读
def read_in_chunks(filePath, chunk_size=1024*1024):
"""
Lazy function (generator) to read a file piece by piece.
Default chunk size: 1M
You can set your own chunk size
"""
file_object = open(filePath)
while True:
chunk_data = file_object.read(chunk_size)
if not chunk_data:
break
yield chunk_data
if __name__ == "__main__":
filePath = './path/filename'
for chunk in read_in_chunks(filePath):
process(chunk) # <do something with chunk>
二、Using with open()
with语句打开和关闭文件，包括抛出一个内部块异常。for line in f文件对象f视为一个迭代器，会自动的采用缓冲IO和内存管理，所以你不必担心大文件。
#If the file is line based
with open(...) as f:
for line in f:
process(line) # <do something with line>
三、fileinput处理
import fileinput
for line in fileinput.input(['sum.log']):
print line
参考：
http://www.zhidaow.com/post/python-read-big-file
https://www.cnblogs.com/wulaa/p/7852592.html
f = open(filename,'r')
f.read()
#1:
while True:
block = f.read(1024)
if not block:
break
#2:
while True:
line = f.readline()
if not line:
break
#3:
for line in f.readlines():
pass
#4:
with open(filename,'r') as file:
for line in file:
pass
#5:the second line
import linecache
txt = linecache.getline(filename,2)
