Seek() Function

seek() function?

Regarding seek() there's not too much to worry about.

First of all, it is useful when operating over an open file.

It's important to note that its syntax is as follows:

fp.seek(offset, from_what)

where fp is the file pointer you're working with; offset means how many positions you will move; from_what defines your point of reference:

• 0: means your reference point is the beginning of the file
• 1: means your reference point is the current file position
• 2: means your reference point is the end of the file

if omitted, from_what defaults to 0.

Never forget that when managing files, there'll always be a position inside that file where you are currently working on. When just open, that position is the beginning of the file, but as you work with it, you may advance.

seek will be useful to you when you need to walk along that open file, just as a path you are traveling into.

Seek function Python

When you open file in text mode, the file-like object it returns is io.TextIOWrapper (derived from io._TextIOBase). Their seek() with regard to whence works as follows:

The default value for whence is SEEK_SET.

SEEK_SET or 0: seek from the start of the stream (the default); offset must either be a number returned by TextIOBase.tell(), or zero. Any other offset value produces undefined behaviour.

SEEK_CUR or 1: “seek” to the current position; offset must be zero, which is a no-operation (all other values are unsupported).

SEEK_END or 2: seek to the end of the stream; offset must be zero (all other values are unsupported).

I.e. you can use whence values of 1 and 2 only with offset of 0.

seek function doesn't work to update a file in a specific position - python

Quoting from tutorialspoint.com,

Note that if the file is opened for appending using either 'a' or 'a+', any seek() operations will be undone at the next write.

"a" mode write operations append to the end of the file. What seek does is it sets the write/read pointer to a specific location in the file.

Therefore, when a write is called, it will write to the end of file, regardless of the read/write pointer.

However, because you've opened the file in a+b, you would be able to seek to a specific location and read it.

How can I use seek() in a binary file?

seek changes the file postion but doesn't read anything. It wouldn't know in general how much to read. After the seek you can read 1 byte. As a side note, don't open with more rights than you need - no need to create an unnecessary failure point in your code.

def openFile():
    itaFile = filedialog.askopenfilename(
        filetypes=[("ITA Files", ".ITA"), ("All Files", "*")])
    with open(itaFile, "rb") as itaOpened:
        a = itaOpened.seek(6)
        a = itaOpened.read(1)
    print(a)

How does Python's seek function work?

It is OS- and libc-specific. the file.seek() operation is delegated to the fseek(3) C call for actual OS-level files.

Python seek() not shifting pointer to right place

you need to flush() the buffer to disk, because write is happening in the buffer in memory not the actual file on the disk.

In the second scenario, before readline() you are calling f.tell() which is actually flushing the buffer to disk.

text = " " 
with open("input.txt", "r+") as f:   
  while text!="":
    text = f.readline()
    fp = f.tell()
    if text == 'b\n':
      print("writing at position", fp)
      f.seek(fp)
      f.write("-")
      f.flush() #------------->

---

Related Topics