Python readline - 只讀第一行

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() 

爲什麼＃1工作正常並顯示第一行和第二行，但＃2打印第一行的兩個副本，並且不打印與第一行相同的內容？

Answer 1

當您致電open時，您將重新打開文件並從第一行開始。每當您在已打開的文件上撥打readline時，它會將其內部「指針」移動到下一行的開頭。但是，如果您重新打開文件，「指針」也會重新初始化 - 當您撥打readline時，它會再次讀取第一行。

試想一下，open返回file對象是這樣的：

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 

當你運行你的第一個例子中，你會得到類似以下的輸出：

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) 

雖然輸出的第二個例子看起來像這樣：

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

第二個片段打開文件兩次，每次讀一行。由於該文件是重新打開的，每次它是第一行被讀取。