readlines（）在使用readline（）后无法读取行

Question

The following simple code reads a CSV file and returns the number of lines of the file. As you can see in the output, the file has 501 lines.

>>> import codecs
>>> f = codecs.open("tmp.csv", "r", "utf_8")
>>> print len(f.readlines())
501

But if I insert a readline() before using readlines(), the latter does not reach at the end of the file.

>>> import codecs
>>> f = codecs.open("tmp.csv", "r", "utf_8")
>>> f.readline()
>>> print len(f.readlines())
1

Is there any basic mistake in my code? How can I mix readline() and readlines()? (actually I don't need to mix these two functions in my real program, but I am just curious...)

You can download the file at https://dl.dropboxusercontent.com/u/16653989/tmp/tmp.csv

Answer 1

This has something to do with the codecs module. Because when you do the same thing with the regular python open statement, it works as expected:

f = open('tmp.csv')
f.readline()
>>> print len(f.readlines())
500