In-depth understanding of python file reading and writing methods

1.open

After using open to open a file, you must remember to call the close() method of the fileobject. For example, you can use the try/finally statement to ensure that the file can be closed in the end.

file_object = open('thefile.txt')

try:

all_the_text = file_object.read( )

finally:

file_object.close( )

Note: You cannot put the open statement in the try block because it will appear when the file is opened. When an exception occurs, the file object file_object cannot execute the close() method.

2. Read file

Read text file

input = open('data', 'r')

#The second parameter defaults to r

input = open('data')

Read binary file

input = open('data', 'rb')

Read all content

file_object = open('thefile.txt')

try:

all_the_text = file_object.read( )

finally:

file_object.close( )

Read fixed bytes

file_object = open('abinfile', 'rb')

try:

while True:

chunk = file_object.read(100)

if not chunk:

                                                                                                                                                                                  ​

##Read each line

list

_of_all_the_lines = file_object.

readline

s( )

If the file is Text file, you can also directly traverse the file object to obtain each line:

for line in file_object: process line

3. Write files

Write text files

output = open('data', 'w')

Write binary files

output = open('data', 'wb')

Append writing file

output = open('data', 'w+')

Write data

file_object = open('thefile.txt', 'w')

file_object.write(all_the_text)

file_object .close( )

Write multiple lines

file_object.writelines(list_of_text_strings)

Note that calling writelines to write multiple lines will have a performance impact It is higher than using write to write once.

When processing log files, we often encounter such a situation: the log file is huge, and it is impossible to read the entire file into the memory for processing at one time. For example, it needs to be on a computer with a physical memory of 2GB. Processing a 2GB log file on the machine, we may want to process only 200MB of its content at a time.

In

Python

, the built-in File object directly provides a readlines(sizehint)

function

to accomplish such a thing. Take the following code as an example:

file = open('test.log', 'r')sizehint = 209715200 # 200M

position

= 0lines = file.readlines(sizehint)while not file.tell() - position < 0: position = file.tell() lines = file.readlines(sizehint)

Every time the readlines(sizehint) function is called, approximately 200MB of data will be returned, and all The returned data must be complete row data. In most cases, the number of bytes of the returned data will be slightly larger than the value specified by sizehint (except when the readlines(sizehint) function is called for the last time). Normally, Python will automatically adjust the user-specified sizehint value to an integer multiple of the internal cache size. file is a special type in python, which is used to operate external files in python programs. Everything in Python is an object, and file is no exception. File has file methods and attributes. Let's first look at how to create a file object:

file(name[, mode[, buffering]])

The file() function is used to create a file object, which has an alias open() may be more vivid, they are built-in functions

. Let’s take a look at its parameters. Its parameters are all passed in the form of

string

. name is the name of the file.

mode is the open mode. The optional values ​​are r w a U, which represent read (default) and write. Add modes that support various line breaks. If you open a file in w or a mode, if the file does not exist, it will be created automatically. In addition, when using w mode to open an existing file, the content of the original file will be cleared, because the initial file operation mark is at the beginning of the file. If you perform a write operation at this time, the original content will undoubtedly be deleted. Erase it. Due to historical reasons, the newline character has different modes in different systems. For example, in Unix it is a \n, but in Windows it is '\r\n'. Opening a file in U mode supports all newline modes. In other words, '\r' '\n' '\r\n' can represent a newline, and there will be a tuple used to store the newline characters used in this file. However, although there are many modes for line breaks, when reading python, they are always replaced by \n. After the mode character, you can also add the two signs + b t, which respectively indicate that the file can be read and written at the same time and the file can be opened in binary mode or text mode (default).

buffering If it is 0, it means no buffering; if it is 1, it means "line buffering"; if it is a number greater than 1, it means the size of the buffer, which should be in bytes.

The file object has its own properties and methods. Let’s first look at the attributes of file.

closed #Mark whether the file has been closed, rewritten by close()

encoding #File encoding

mode #Open mode

name #File name

newlines #The newline mode used in the file is a tuple

softspace #boolean type, usually 0, which is said to be used for print

File reading and writing methods:

F.read([size]) #size is the length of the read, in bytes.

F.readline([size ])

#Read a line. If size is defined, it is possible to return only part of a line

F.readlines([size])

#Read each line of the file Acts as a member of a list and returns the list. In fact, it is implemented internally by calling readline() in a loop. If the size parameter is provided, size represents the total length of the read content, which means that only a part of the file may be read.

F.write(str)

#Write str to the file, write() will not add a newline character after str

F.writelines( seq)

#Write all the contents of seq to the file. This function also just writes faithfully, without adding anything after each line.

Other methods of file:

F.close()

#Close the file. Python will automatically close a file after it is no longer used. However, this function is not guaranteed. It is best to develop the habit of closing it yourself. If a file is operated on after it is closed, a ValueError will be generated

F.flush()

#Write the contents of the buffer to the hard disk

F.fileno()

#Returns a long integer "file label"

F.isatty()

#Whether the file is a terminal device file ( In unix systems)

F.tell()

#Returns the current position of the file operation mark, with the beginning of the file as the origin

F.next()

#Return to the next line and move the file operation flag to the next line. When a file is used in a statement such as for ... in file, the next() function is called to implement traversal.

F.seek(offset[,whence])

#Move the file operation mark to the offset position. This offset is generally calculated relative to the beginning of the file, and is generally a positive number. But this is not necessarily the case if the whence parameter is provided. whence can be 0 to start calculation from the beginning, and 1 to calculate from the current position as the origin. 2 means the calculation is performed with the end of the file as the origin. It should be noted that if the file is opened in a or a+ mode, the file operation mark will automatically return to the end of the file every time a write operation is performed.

F.truncate([size])

#Cut the file to the specified size. The default is to cut to the position of the current file operation mark. If size is larger than the file size, depending on the system, the file may not be changed, the file may be padded to the corresponding size with 0, or some random content may be added.

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