python如何判断一个文件的行数

#encoding=utf-8
#文件比较小
count=len(open(r"train.data",'rU').readlines())
print(count)
#文件比较大
count=-1
for count, line in enumerate(open(r"train.data",'rU')):
pass
count+=1
print(count)
#更好的方法
count=0
thefile=open("train.data")
while True:
buffer=thefile.read(1024*8192)
if not buffer:
break
count+=buffer.count('\n')
thefile.close()
print(count)