Python readline - 只读第一行

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工作正常并显示第一行和第二行，但＃2打印第一行的两个副本，并且不打印与第一行相同的内容？

Answer 1

当您致电open时，您将重新打开文件并从第一行开始。每当您在已打开的文件上拨打readline时，它会将其内部“指针”移动到下一行的开头。但是，如果您重新打开文件，“指针”也会重新初始化 - 当您拨打readline时，它会再次读取第一行。

试想一下，open返回file对象是这样的：

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 

当你运行你的第一个例子中，你会得到类似以下的输出：

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) 

虽然输出的第二个例子看起来像这样：

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) 

Answer 2

第二个片段打开文件两次，每次读一行。由于该文件是重新打开的，每次它是第一行被读取。