python读写文件操作的三大基本步骤

The default mode is 'rt' (open for reading text). For binary random access, the mode 'w+b' opens and truncates the file to 0 bytes, while 'r+b' opens the file without truncation. The 'x' mode implies 'w' and raises an `FileExistsError` if the file already exists.

close()函数

close() method of _io.TextIOWrapper instance

Flush and close the IO object.

This method has no effect if the file is already closed

打开文件，必须有对应的关闭，否则该未关闭的文件不能被其它的应用操作。

read()函数

read(size=-1, /) method of _io.TextIOWrapper instance

Read at most size characters from stream.

Read from underlying buffer until we have size characters or we hit EOF.

If size is negative or omitted, read until EOF.

read()可选参数size可以用于读取"size"大小的数据，返回的是字符串或字节对象，若是size的值没有填写，或者是个负值，那么read()函数将读取文件的所有内容，这也是用python开发pc软件中"复制黏贴"的功能比较常用的函数。

readlines()函数

Help on built-in function readlines:

readlines(hint=-1, /) method of _io.TextIOWrapper instance

Return a list of lines from the stream.

hint can be specified to control the number of lines read: no more

lines will be read if the total size (in bytes/characters) of all

lines so far exceeds hint.

readlines()函数是将文件当中的所有行，一行一行地读取，并逐一写入一个列表list内，最终返回这个列表。

readline()函数

Help on built-in function readline:

readline(size=-1, /) method of _io.TextIOWrapper instance

Read until newline or EOF.

Return an empty string if EOF is hit immediately.

If size is specified, at most size characters will be read.

readline()函数就是读取一行数据，用法除了size参数之外，就跟read()差不多，也是open()打开文件，readline()读取数据，close()关闭文件

write()函数

write(text, /) method of _io.TextIOWrapper instance

Write string s to stream.

Return the number of characters written

(which is always equal to the length of the string).

writelines()函数

writelines(lines, /) method of _io.TextIOWrapper instance

Write a list of lines to stream.

Line separators are not added, so it is usual for each of the

lines provided to have a line separator at the end.

with语句

通过with语句，不管读取还是写入文件操作都不用写对应的close()函数，语句块结束系统会自动关闭文件。

python 复制代码

with open('filename.txt', 'r', encoding='urf-8') as f:
    f.read()

读写操作的应用：

拷贝文件

python 复制代码

# 打开源文件以读取内容  
with open('source.txt', 'r') as source_file:  
    source_content = source_file.read()  
  
# 打开目标文件以写入内容  
with open('destination.txt', 'w') as destination_file:  
    destination_file.write(source_content)

with 语句的嵌套

以上两个with语句块还能嵌套写在一起：

python 复制代码

# 打开源文件以读取内容，并同时打开目标文件以写入内容  
with open('source.txt', 'r') as source_file:  
    with open('destination.txt', 'w') as destination_file:  
        # 读取源文件的内容  
        source_content = source_file.read()  
        # 将读取的内容写入到目标文件中  
        destination_file.write(source_content)

可以写在同一行：

python 复制代码

# 打开源文件以读取内容，并同时打开目标文件以写入内容  
with open('source.txt', 'r') as source_file, open('destination.txt', 'w') as destination_file:  
    # 读取源文件的内容  
    source_content = source_file.read()  
    # 将读取的内容写入到目标文件中  
    destination_file.write(source_content)

逐行拷贝

如果源文件很大，使用read()方法一次性读取所有内容可能会消耗较多的内存。对于大文件，更推荐的做法是使用文件对象的迭代器逐行读取和写入，这样可以减少内存的使用。

python 复制代码

with open('source.txt', 'r') as source_file, open('destination.txt', 'w') as destination_file:  
    # 逐行读取源文件并写入目标文件  
    for line in source_file:  
        destination_file.write(line)

完