Python readline - 只读取第一行

Question

#1
input_file = 'my-textfile.txt'
current_file = open(input_file)
print current_file.readline()
print current_file.readline()

#2
input_file = 'my-textfile.txt'
print open(input_file).readline()
print open(input_file).readline()

为什么#1能正常工作，显示第一行和第二行，而#2却打印了两遍第一行，并且没有像#1那样打印？

Answer 1

第二段代码打开文件两次，每次读取一行。因为每次打开文件都是从头开始，所以每次读取的都是第一行。

Answer 2

当你调用 open 的时候，其实是在重新打开一个文件，并且是从第一行开始读的。每次你在已经打开的文件上调用 readline，它会把内部的“指针”移动到下一行的开头。不过，如果你重新打开这个文件，那个“指针”也会被重置——这时候再调用 readline 就会再次读取第一行。

想象一下，open 返回了一个看起来像这样的 file 对象：

class File(object):
    """Instances of this class are returned by `open` (pretend)"""

    def __init__(self, filesystem_handle):
        """Called when the file object is initialized by `open`"""

        print "Starting up a new file instance for {file} pointing at position 0.".format(...)

        self.position = 0
        self.handle = filesystem_handle


    def readline(self):
        """Read a line. Terribly naive. Do not use at home"

        i = self.position
        c = None
        line = ""
        while c != "\n":
            c = self.handle.read_a_byte()
            line += c

        print "Read line from {p} to {end} ({i} + {p})".format(...)

        self.position += i
        return line

当你运行第一个例子时，你会得到类似下面的输出：

Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)
Read line from 80 to 160 (80 + 80)

而你第二个例子的输出会像这样：

Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)
Starting up a new file instance for /my-textfile.txt pointing at position 0.
Read line from 0 to 80 (80 + 0)