Advanced application files and modified in two ways

Advanced application files and modified in two ways

First, the advanced application file

Readable, writable

• r + t: readable, writable
• w + t: write, read
• a + t: appendable readable

# wt
with open('36w.txt', 'wt', encoding='utf-8') as fw:
    print(fw.readable())
    print(fw.writable())
False
True
# w+t
with open('36w.txt', 'w+t', encoding='utf-8') as fw:
    print(fw.readable())
    print(fw.writable())
True
True
# r+t
with open('36w.txt', 'r+t', encoding='utf-8') as fr:
    print(fr.readable())
    print(fr.writable())
True
True

File pointer is moved within

Suppose we need to add the contents of a file in the middle row of content, if using a basic r / w / a pattern is very difficult to achieve, so we need to move the pointer within the file.

with open('36r.txt', 'r+t', encoding='utf-8') as fr:
    fr.readline()
    fr.write('nick 真衰呀')  # 写在文件的最后一行

The hard disk has never modify a say on the hard disk only cover that new content covering the new content.

1.seek (offset, whence): offset pointer offset represents the file offset in units of the number of bytes

# seek()
with open('36r.txt', 'rb') as fr:
    print(f"fr.seek(4, 0): {fr.seek(4, 0)}")  # 0相当于文件头开始；1相当于当前文件所在位置；2相当于文件末尾
    # fr.seek(0,2)  # 切换到文件末尾
fr.seek(4, 0): 3

2.tell (): Every time statistics are from the beginning of the file to the current position pointer is located

# tell()
with open('36r.txt', 'rb') as fr:
    fr.seek(4, 0)
    print(f"fr.tell(): {fr.tell()}")
fr.tell(): 4

3.read (n): only read (n) in the mode, n is the number of characters represents, in addition, provided that the other is directed to the number of bytes of the file pointer

# read()
with open('36r.txt', 'rt', encoding='utf-8') as fr:
    print(f"fr.read(3): {fr.read(3)}")
fr.read(3): sdf

4.truncate (n): truncate (n) is truncated file, Open file must be written, but can not be opened with w or w +, etc., as directly as empty files, so truncate () To r + or a or under the effect of a + test modes. It's always a reference file header. And truncate () without parameters, corresponding to empty it.

# truncate()
with open('36r.txt', 'ab') as fr:
    fr.truncate(2) # 截断2个字节后的所有字符，如果3个字节一个字符，只能截断2/3个字符，还会遗留1/3个字符，会造成乱码